TheStructure of ScientificRevolutions
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TheStructure of ScientificRevolutions
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ThomasS. Kuhn
TheStructure of Scientific Revolutions Third Edition
The Universityof ChicagoPress Chicagoand Lordon
The Universityof ChicagoPress,Chicago60637 The Universityof ChicagoPress,Ltd., London @ 1962,1970,1996by The Universityof Chicago All rightsreserved. Third edition 1996 Printedin the United Statesof America 050403020100 (cloth) ISBN: 0-226-45807-5 (paPer) ISBN: 0-226-45808-3
345
Data Library of CongressCataloging-in-Publication Kuhn,ThomasS. The structurcof scientificrevolutions/ ThomasS. Kuhn.- 3rd ed' p.cm. andindex. Includesbibliographicalreferences ISBN 0-22645807-5(cloth : alk. paper)' ISBN 0-22645808-3(pbk' : alk'paper) L science--Philosophy.2. Science--History.I. Title. Ql7s.K95 1996 96-13195 501-dc20 CIP of the @ fhe paperusedin this publicationmeetsthe minimum requirements for of Paper Sciences-Permanence for Information Standard National American PrintedLibrary Materials,ANSI 239.48-1992.
Contents Preface vii I. Introduction: A Rolefor History I U. The Routeto NormalScience I0 m. The Natureof Normal Science 23 fV. NormalScienceasPuzzle-solving 35 V. The Priority of Paradigms 43 VI. AnomalyandtheEmergence of ScientificDiscoveries 52 VII. Crisisandthe Emergence of ScientificTheories 66 Vm. TheResponse to Crisis 77 I)(. The NatureandNecessityof ScientificRevolutions 92 X. RevolutionsasChangesof World View I I I XI. The Invisibility of Revolutions 136 )(II. The Resolutionsof Revolutions 144 )(III. ProgressthroughRevolutions 160 Postscript-1969174 Index 2I I
Prefoce The essaythat follows is the first full published report on a project originally conceived almost fffteen years ago. At that time I was a graduate student in theoretical physics already within sight of the end of my dissertation.A fortunate involvement with an experimental college course treating physical sciencefor the non-scientistprovided my ffrst exposureto the history of science.To my complete suqprise,that exposureto out-of-date scientiffc theory and practice radically undermined some of my basic conceptionsabout the nature of scienceand the reasonsfor its specialsuccess. Those conceptionswere ones I had previously drawn partly from scientiffctraining itself and partly from a long-standing avocational interest in the philosophy of science.Somehow, whatever their pedagogicutility and their abstract plausibility, thosenotionsdid not at all fft the enterprisethat historicalstudy displayed. Yet they were and are fundamental to many diicussionsof science,and their failuresof verisimilitudetherefore seemedthoroughly worth pursuing. The result was a drastic shift in my_career plans, a shift from physics to history of science and then, gradually, from relatively straightforward historical problemsback to the more philosophicalconcernsthat had initially led me to history. Except foi a few articles, this essayis the ff-rstof my published works in which these early concernsare dominant.In_somepart it is an attempt to explain to myself and_to friends how I happened to be dt"*tt ito* scienceto its history in the first place. - Yr fi1stopportunity to pursuein depth someof the ideasset forth below was provided by three y"atr as a Junior Fellow of the society of Fellows of Harvard uttiversity. without that period of freedom ihe transition to a new ffeld of study would have beenfar more difficult and might not have beenalhieved. Part of !/ time in thoseyearswas devoted to history of science proper. In particular I continued to study the writings of AlexYll
Prefoce andre Koyr6 and ffrst encounteredthose of Emile Meyerson, H6ldne Metzger, and AnnelieseMaier.r More clearly than most other recent scholars,this group has shown what it was like to think scientiffcallyin a period when the canons of scientiffc thought were very different from those current today. Though I increasinglyquestiona few of their particular historicalinterpretations, their works, together with A. O. Loveioy's Great Chain of Being, have been secondonly to primary sourcematerialsin shapingmy conceptionof what the history of scientiffc ideascan be. Much of my time in thoseyears,however,was spent exploring fields without apparentrelation to history of sciencebut in which researchnow disclosesproblemslike the oneshistory was bringing to my attention. A footnote encounteredby chance led me to the experimentsby which JeanPiagethas illuminated both the various worlds of the growing child and the process of transitionfrom one to the next.2One of my colleaguesset me to reading papersin the psychologyof perception,particularly the Gestalt psychologists;another introduced me to B. L. Whorf's speculationsabout the effect of language on world view; and W. V. O. Quine openedfor me the philosophical puzzlesof the analytic-syntheticdistinction.sThat is the sort of random exploration that the Society of Fellows permits, and only through it could I haveencounteredLudwik Fleck'salmost unknown monograph,Entstehung und. Entu;icklung einer usis1 Particularly infuential were Alexandre Koyr6, Etud.es Galll4ennes (8 raols.; Paris, 1939); Emile Meyerson, Identity ard Reality, trans. Kate Loewenberg ( New York, 1980 ); H6l0ne Metzger, Lei dnarines chlmiqtns en Frarrce du illbit du XVlle d la fin du )(Vllle stdcle (Pans, 1923), and Nerotoa, Stalil, Boeilwaoe a Ia doailrc chimiquc (Paris, 1930); and Anneliese Maier, Db Vorhufet GaIh ("Studien zur Nahrrphilosophie der Spltscholastik"; Leis im 74. Iahrhutderf Rome, f949). 2 Because they &splayed concepts and processesthat also emerge directly from the history of science, two sets of Piaget's investigations proved particularly important: ihe Childs Cotrceptbn of C"ausotity, Ea"ns. Mar'j,orie Cibain (Loidoru (Paris, f946). 1930), and Les rwtions de moutsement et de oltesse clvzfenlont 8 Whorfs papers have since been collected by B. Langtnge, Carroll, John (New-Yoik, Thouglt, atd, Realitg-Seleaed Lee Wlwt Wfitings of Beniaiin f 956). Quine has presented his views in "Two Dogmas of Empiricisrn," reprinted in his From a Logical Pokrt ol Viruu: (Cambridge, Mass., l95g), pp. 20-,1O.
Yiii
Preloce settsclwftlichen Tatsache (Basel, 1935), an essay that antici-
ences to either these works or conversationsbelow, I am debted to them in more ways than I can now reconstruct or evaluate. During my last year as a Junior Fellow, an invitation to lecture for the Lowell Institute in Boston provided a first chance to try out my still developing notion of science.The result was a seriesof eight publie lectures,deliveredduring March, 1951, on "The Quest for PhysicalTheory." h the next year I began to teach history of scienceproper, and for almost a decadethe problems of instructing in a field I had never systematically studied left little time for explicit articulation of the ideas that had first brought me to it. Fortunately, however, those ideas proved a sourceof implicit orientation and of some problemstructure for much of my more advancedteaching.I therefore have my studentsto thank for invaluable lessonsboth about the viability of my views and about the techniquesappropriate to their effectivecommunieation.The sameproblemsand orientation give unity to most of the dominantly historical, and apparently diverse,studiesI have published since the end of my fellowship. Severalof them deal with the integral part played by one or another metaphysic in creative scientific research. Others examinethe way in which the experimentalbasesof a new theoryarc accumulatedand assimilated by men committed to an incompatibleolder theory. In the processthey describe the type of developmentthat I have below called the "emergence"of a new theory or discovery.There are other such ties besides. The ffnal stage in the development of this essay began with an invitation to spend the yeir 1958-59 at the cinter for Advancedstudiesin the Behavioralsciences.once again I was able to give undivided attention to the problems discussed below.Even more important,spendingthe year in a community tx
Prefoce composed predominantly of social scientists confronted me with unanticipated problems about the differences between such communities and those of the natural scientists among whom I had been trained. Particularly, I was struck by the number and extent of the overt disagreementsbetween social scientistsabout the nature of legitimatescientificproblemsand methods.Both history and acquaintancemade me doubt that practitioners of the natural sciencespossessfirmer or more perm_anentanswersto such questions than their colleaguesin socialscience.Yet, somehow,the practiceof astronomy,physics, chemistry, or biology normally fails to evoke the controversies
time provide model problems and solutions to a community of practitioners. Onee that piece of my puzzle fell into place, a draft of this essayemergedrapidly. The subsequenthistory of that draft need not be recounted here, but a few words must be said about the form that it has preservedthrough revisions.Until a ffrst versionhad been com-
much indebted to them, particularly to Charles Morris, for wielding the essentialgoad and for advising me about the
an essayrather than the full-scale book my subiect will ultimately demand. Sincemy most fundamentalobiective is to urge a changein x
Prefoce the perception and evaluationof familiar data, the schematic characterof this first presentationneed be no drawback. On the contrary,readerswhoseown researchhaspreparedthem for the sort of reorientationhere advocatedmay find the essayform both more suggestiveand easierto assimilate.But it has disadvantagesas well, and thesemay iustify ^y illustrating at the very start the sorts of extensionin both scopeand depth that I hope ultimately to include in a longer version.Far more historical evidenceis available than I have had spaceto exploit below. Furthermore, that evidencecomesfrom the history of biological as well as of physical science.My decisionto deal here exclusively with the latter was made partly to increasethis essay's coherenceand partly on grounds of present competence.In addition, the view of scienceto be developedhere suggeststhe potential fruitfulness of a number of new sortsof research,both historical and sociological.For example,the manner in which anomalies,or violations of expectation, attract the increasing attention of a scientiffc community needs detailed study, as does the emergenceof the crises that may be induced by repeatedfailure to make an anomalyconform. Or again, if I am right that eachscientific revolution alters the historical perspective of the community that experiencesit, then that change of perspective should afrect the structure of postrevolutionary textbooksand researchpublications. One such effect-a shift in the distribution of the technical literature cited in the footnotes to researchreports-ought to be studied as a possibleindex to the occurrenceof revolutions. The needfor drasticcondensationhas alsoforced me to forego discussionof a number of maior problems. My distinction betweenthe pre- and the post-paradigmperiodsin the development of a scienceis, for example,much too schematic.Each of the schoolswhose competition characterizesthe earlier period is guided by somethingmuch like a paradigm;there are circumstances,though I think them rare, under which two paradigms can coexistpeacefully in the later period. Mere possessionof a paradigm is not quite a sufficient criterion for the developmental transition discussedin SectionII. More important, exxl
Prefoce cept in occasionalbrief asides,I have said nothing about the role of technological advance or of external social, economic, and intellectual conditions in the development of the sciences. One need, however, look no further than Copernicus and the calendarto discoverthat external conditions may help to transform a mere anomaly into a source of acute crisis. The same examplewould illustrate the way in which conditions outside the sciencesmay influencethe range of alternativesavailable to the man who seeksto end a crisis by proposing one or another revolutionary reform.r Explicit consideration of effects like these would not, I think, modify the main thesesdevelopedin this essay,but it would zurely add an analytic dimension of ffrst-rateimportancefor the understandingof scientific advance. Finally, and perhaps most important of all, limitations of spacehave drastically affected my treatment of the philosophical implications of this essay'shistorically oriented view of science.Clearly, there are such implications, and I have tried both to point out and to document the main ones.But in doing so I have usually refrained from detailed discussion of the various positions taken by contemporary philosophers on the correspondingissues.Where I have indicated skepticism,it has more often been directed to a philosophical attitude than to any one of its fully articulated expressions.As a result, someof thosewho know and work within one of trhosearticulated positions may feel that I have missedtheir point. I think they will be wrong, but this essayis not calculated to convince them. To attempt that would have required a far longer and very different sort of book. The autobiographical fragments with which this preface r Thesefactorsare discussedin T. S. Kuhn, The CopemlcanReoohnbn:Phtpy AfiotwmV tury Astronomgin the Deoelopment Deoelopment_of of Western Westen firougl* flwugl* (Cambridge, Mass., 1957), pp. 12?-32, 27|.l-^71. Other effectsof external intellectual and-economic cundifio-ni upon condiuons condiuorxr upon substaDtive substantivescieDtrtrc development evelopment are are ruusrated illustrated in illusEated scientiffc development in my mv Dalrers. mv DaDers. Daners.
"Consenratioln of Energy as an Example rle of Simultaneous Simultaneous Discovery," Discovery," er*;/lcol er*;bol koblemt ln koblems lnthe the Hfrtory HMor{'olof Science, Science, ed. ed.-trlarshall Marshall Clagett Clagett ((Madison,liris., Madison, lg59), "E-ngineering Precedent pp. 821-56; 821-5-6; "Engineering kecedent for the Work o[ o{ Sadi Carnot," Carnot,' Archloes l* 'Sadi tenatUnules thi*oire d,asccbtwes, XIII ( 1960), 247-5li and Carnot and
the CagnardEngine," Isis, LII ( 196l ), 567:l4.It is, therefore,only with lespect to the problens iliscussedin tl'is essaythat I take the role of externil factors t6 be rninor.
xii
Prefoce opens will serve to acknowledgewhat I can recognize of my main debt both to the works of scholarship and to the instihrtions that have helped give form to my thought. the remainder of that debt I shall try to dischargeby citation in the pagesthat follow. Nothing said above or below, however, will more than hint at the number and nature of my personalobligationsto the many individuals whose suggestionsand criticisms have at one time or another sustainedand directed my intellectual development. Too much time has elapsedsince the ideas in this essay began to take shape; a list of all those who may properly ffnd some signs of their infuence in its pageswould be almost coextensivewith a list of my friends and acquaintances.Under the circumstances,I must restrict myseUto the few most significant infuences that even a faulty memory will never entirely suPPress. It was |ames B. Conant, trhenpresident of Harvard University, who ffrst introduced me to the history of scienceand thus initiated the transformation in my conception of the nature of scientiffc advance.Ever since that processbegan, he has been generousof his ideas, criticisms, and time-including the time required to read and suggestimportant changesin the draft of my manuscript. Leonard K. Nash, with whom for ffve years I taught the historically oriented c€urse that Dr. Conant had started, was an even more active collaborator during the years when my ideas ffrst began to take shape,and he has been much missedduring the later stagesof their development.Fortunately, however, after my departure from Cambridge, his place as creative soundingboard and more was assumedby my Berkeley colleague, Stanley Cavell. That Cavell, a philosopher mainly concernedwith ethics and aesthetics,should have reachedconclusions quite so congruent to my own has been a constant sourceof stimulation and enoouragementto me. He is, furthermore, the only person with whom I have ever been able to explore my ideas in incomplete sentences.That mode of communication attests an understanding that has enabled him to point me the way through or around severalmaior barriers encourtered while preparing my first manuscript. xill
Prefoce Since that version was drafted, many other friends have helped with its reformulation. They will, I think, forgive me if I name only the four whose contributions proved most farreachingand decisive:Paul K. Feyerabendof Berkeley,Ernest Nagel of Columbia,H. PierreNoyesof the LawrenceRadiation Laborator/, and my student,John L. Heilbron, who has often worked closelywith me in preparing a ffnal versionfor the press. I have found all their reservationsand suggestionsextremely helpful, but I have no reasonto believe (and somereasonto doubt) that either they or the othersmentioned above approve in its entirety the manuscriptthat results. My ffnal acknowledgments, to my parents,wife, and children, must be of a rather different sort. In ways which I shall probably be the last to recognize,eachof them, too, has contributed intellectualingredientsto my work. But they havealso,in varying degrees,done somethingmore important. They have, that is, let it go on and evenencouragedmy devotionto it. Anyone who haswrestledwith a project like mine will recognizewhat it has occasionallycost them. I do not know how to give them thanks. T. S. K. lBxlrrr.gv, Cer.rronxr.l February 1962
xtY
A Rolefor History l. Introduclion;
from which eachnew scientiffcgenerationleams to practice its trade. Inevitably, however, the aim of such books is persuasive and pedagogc; a concept of science drawn fiom them is no more likely to fft the enteqprisethat produced them than an image of a national culture drawn from a tourist brochure or a language text. This essayattempts to show that we have been misled by them in fundamental ways. Its aim is a sketchof the quite different concept of science that can emerge from the historical record of the researchactivity itseU. Even from history, however, trhat new concept will not be forthcoming if historical data continue to be sought and scrutinized mainly to answer questions posed by the unhistorical stereotype drawn from science texts. Those texts have, for example,often seemedto imply that the content of scienceis uniquely exemplified by the observations,laws, and theories described in their pages.Almost as regularly, the same books have been read as saying that scientific methods are simply the onesillustrated by the manipulative techniquesused in gathering textbook data, together with the logical operations employed when relating those data to the textbook's theoretical generalizations.The result has been a concept of sciencewith profound implications about its nature and development. If scienceis the constellationof facts, theories,and methods collected in current texts, then scientistsare the men who, successfully or not, have striven to contribute one or another element to that particular cunstellation.Scientiffcdevelopmentbecomes the piecemealprocessby which these items have been
fhe Sfrucfureof ScientificRevofutions added,singly and in combination,to the ever growing stockpile that constitutesscientifictechnique and knowledge.And history of science becomes the discipline that chronicles both these successiveincrements and the obstaclesthat have inhibited their accumulation.Concernedwith scientiffcdevelopment,the historian then appearsto have two main tasks.On the one hand, he must determineby what man and at what point in time each contemporaryscientiffcfact, law, and theory was discoveredor invented. On the other, he must deseribeand explain the congeries of error, myth, and superstition that have inhibited the more rapid accumulation of the constituents of the modern sciencetext. Much researchhasbeendirectedto theseends,and somestill is. In recent years, however, a few historians of science have been finding it more and more difficult to fulffl the functions assignsto that the concept of development-by-accumulation them. As chroniclers of an incremental proctss, they discover that additional researchmakesit harder, not easier,to answer questionslike: When was oxygen discovered?Who first conceived of energy conservation?Increasingly,a few of them suspect that these are simply the wrong sorts of questionsto ask. Perhapssciencedoesnot developby the accumulationof individual discoveriesand inventions.Simultaneously,these same historians confront growing difffculties in distinguishing the "scientific"componentof past observationand belief from what their predecessorshad readily labeled "elTor" and "superstition." The more carefullythey study, say,Aristoteliandynamics, phlogistic chemistry, or caloric thermodynamics,the more certain they feel that thoseonce current views of nature were, as a whole, neither less scientific nor more the product of human idiosyncrasythan those current today. If theseout-of-datebeliefs are to be called myths, then myths can be produced by the samesorts of methodsand held for the samesorts of reasons that now lead to scientific knowledge.If, on the other hand, they trre to be called science,then sciencehas included bodies of belief quite incompatiblewith the oneswe hold today. Given thesealternatives,the historian must choosethe latter. Out-of-
2
A Rolefor HistorY lnlroduction: date theoriesare not in principleunscientificbecaurytley have been discarded.That c[oice, however,makesit difficult to see scientificdevelopmentas a Processof accretion.The samehistorical researchthat displays the difficulties in isolating individual inventions and discoveriesgives ground for profound doubts about the cumulativeprocessthrough which theseindividual contributionsto sciencewere thoughtto havebeencompounded. The result of all these doubts and difficulties is a historiothough one that js graphic revolution in the study of scie_nce, ititl i.t its early stages.Gradually, and often without entirely realizing they are doing so,historiansof sciencehav-eb-egunto asknew-sortsof questionsand to tracedifferent,and often less than cumulative, developmentallines for the sciences.Rather than seekingthe Permanentcontributionsof an older scienceto dur presentvantage,they attempt to display the historical integrily of that sciencein its own time. They ask, for -example, no"t a'bout the relation of Galileo'sviews to those of modern science,but rather about the relationshipbetweenhis viewsand and immethoseof his group,i.e.,his teachers,contemporaries, diate srrccesiotsin the sciences.Furthermore,they insist uPon other similar onesfrom studying the opinionsof that grouP ""qfrom that of modern scivery difierent ttte "ieripointlusually ence-th at givesthose opinions th e m aximum internal-cnherp-Bglland the clJsestpossiblefit to nature. Seenthrough the works that result, worfs perhapsbest exemplifiedin the writings of the same Ale&pdre_K'g6, icience does not seem altogether older historioin the writers by discussed one tie as enteryrise historical these least, at implication, By tradition. g.uplii" Jt,tii"r suggestthe possibility of a new image of science.This essayaims"fodelineatethat image by making explicit someof implications. the new historiography's What aspectsof science will emerge !o prgminence in the courseof this eflort? First, at least in order of presentation,is theinsufficiencyof qtrSgdgJ-o-g,ry-e$g9S!ry9t-bythemselves,to .+---L
a*-*'
--''"Y_'
conclusion to many sorts of sciendic@stantive tific questionJ. Instructed to examine electrical or chemical Ph"-
TheSfruclureol ScienliffcRevolutions nomena,the man who is ignorant of theseffeldsbut who knows what it is to be scientiftc may legitimately reach any one of a number of incompatible conclusions.Among those legitimate , possibilities, the particular conclusionshe does arrive at are J probably determined by his prior experiencein other ffelds, by / I the accidents of his investigation, and by his own individual makeup. What beliefs about the stars, for example, does he bring to the study of chemistry or electricity? Which of the many conceivableexperimentsrelevant to the new ffeld doeshe elect to perform ffrstPAnd what aspectsof the complexphenomenon that then results strike him as particularly relevant to an elucidation of the nature of chemical change or of electrical affinity? For the individual, at least, and sometimesfor the scientific community as well, answersto questionslike theseare i of scientiffcdevelopment.We shall rn II that the early developmental re been characterizedby continual ber of distinct views of nature,each rll roughly compatiblewith, the dicrn and method. What differentiated thesevarious schoolswas not one or anotherfailure of methodthey were all "scientiffc"-but what we shall come to call their incommensurableways of seeing the world and of practicing sciencein it. Observationand experiencecan and must drastically restrict the range of admissiblescientiftcbelief, elsethere would be no science.But they cannot alone determine a particular bo_dyof such belief. An apparently arbitrary element, compounded of_personal and historical accident, ii always a formative ingredient of the beliefs espousedby . given scientific community at a given time. That elementof arbitrarinessdoesnot, however,indicatethat any scientiffcgroup could practiceits trade without someset of received beliefs. Nor does it make less consequentialthe particular constellation to which the group, at a given time, ii in fact committed. Effective reseatch scarcely Legins before a scientific community thinks it has acquired ffrm answers to questionslike the following: What are the fundamental entities 1
Introduction:A Rolefor History of which the universeis composed?How do theseinteract with What questionsmay legitimateeachother and with the senses? ly be askedabout such entities and what techniquesemployed in seeking solutions?At least in the mature sciences'answers (or full iubstitutes for answers) to questionslike these are ffrmly embeddedin the educationalinitiation that preparesand Iicenies the student for professionalpractice. Becausethat edu-
historic origins and, occasionally,in their subsequentdevelopment. Yet that elementof arbitrarinessis present,and it too has an important effect on scientificdevelopment,one which will be examined in detail in SectionsVI, VII, and VI[. Normal sci-
novelties becausethey are necessarilysubversive of its basic commitments.Nevertheless,so long as those commitmentsretain an element of the arbitrary, the very nature of normal research ensuresthat novelty shall not be suPPressedfor very
fhe Sfructureof ScientificRevolutions to perform in the anticipated manner, revealing an anomaly that cannot, despite repeated effort, be aligned with professional expectation.In these and other ways besides,normal sciencerepeatedlygoesastray.And when it does-when, that is, the professioncan no longer evadeanomaliesthat subvertthe existingtradition of scientificpractice-then begin the extraordinary investigations that lead the professionat last to a new set of commitments,a new basisfor the practice of science.The extraordinaryepisodesin which that shift of professionalcommitments occursare the onesknown in this essavas scientific revolutions.They are the tradition-shatteringcomplementsto the tradition-boundactivity of normal science] The most obviousexamplesof scientificrevolutionsare those famousepisodesin scientiftcdevelopmentthat have often been labeled revelutions before. Therefore, in SectionsIX and X, where the nature of scientificrevolutionsis ffrst directly scrutinized, we shall deal repeatedlywith the major turning points in scientificdevelopmentassociatedwith the namesof Copernicus, Newton, Lavoisier,and Einstein. More clearly than most other episodesin the histoqy of at least the physical sciences,these display what all scientiftcrevolutionsare about. Each of them necessitatedthe community's rejection of one time-honored scientifictheory in favor of another incompatiblewith it. Each produceda consequentshift in the problemsavailablefor scientiffc scrutiny and in the standardsby which the professiondetermined what should count as an admissibleproblem or as a Iegitimate problem-solution.And each transformedthe scientific imagination in ways that we shall ultimately need to describe as a transformationof the world within which scientific work was done. Such changes,together with the controversies that almostalwaysaccompanythem, are the deffningcharacteristicsof scientiftcrevolutions. These characteristicsemerge with particular clarity from a study of, say, the Newtonian or the chemicalrevolution. It is, however,a fundamentalthesisof this essaythat they can also be retrieved from the study of many other episodesthat were not so obviouslyrevolutionary.For the far smallerprofessional 6
lnlrodvcliontA Rolefor HistorY
an isolatedevent. only scientiftcevents that have revolutionary impact upon the specialistsin whose domain they occur. The commitments that govern normal science specify not only what sorts of entities the universe does contain,but also,by implication, those that it doesnot. It follows, though the point will require extendeddiscussion,that a discoverylike that of oxygenor X-rays doesnot simply add one more item to the populatibn of the scientist'sworld. Ultimately it has that efiect, but not until the professionalcommunity has re-evaluated traditional experimental procedures, altered its conceptionof entities with which it has long been familiar, and, in the process,shifted the network of theory through which it dealswith the world. Scientiffcfact and theory arcnot categorically separable,except perhapswithin a single tradition of normal-scientiffcpractice. That is why the unexpecteddiscoveryis not simply factual in its import and why the scientist'sworld is qualitatively transformed as well as quantitatively enriched by fundamental noveltiesof either fact or theory. This extended conception of the nature of scientiffc revolutions is the one delineated in the pagesthat follow. Admittedly the extensionstrainsctrstomaryusage.Nevertheless,I shall conNor are new inventions
fhe Struclureof ScienliffcRevolufions tinue to speakeven of discoveriesas revolutionary, becauseit is iust the pbssibilityof relating their structureto that of, say,the Copernican revolution that makes the extended conception seem to me so important. The preceding discussionindicates how the complementarynotionsbf normal scienceand of scien-
revolutionary competition between the proponents of the old normal-scientifictradition and the adherentsof the new one. It
mies is available to suggestthat it cannot properly do so. Hist_ory,we too often say, is a purely descriptive discipline. The theses suggestedabove are, however, often interpietive and
8
lntroduction:A Rolefor HidorY tion.' Can anything more than profound confusionbe indicated by this admixture of diverseffelds and concerns? Having been weaned intellectually on these distinctions and others[k1 them, I could scarcelybe more aware of their impor! and force. For many yearsI took them to be about the nature of knowledge, and I ;till suPPosethat, appropriately recast, they have ronr'"thitrgimportattt to tell us. Yet my attempts to apply them, even grooi mado, to the actual situations in which knowledgeis gained,accepted,and assimilatedhavemade them seemextraordinarily problematic.Rather than being elementary logical or methodological distinctions, which would thus be priot to the analysis of scientific knowledge, they now t"9m integral parts of a traditional set of substantiveanswersto the very q,restionsupon which they have been deployed. That_circularily doesnot at all invalidate them. But it doesmake them parts of a theory and, by doing so, subiectsthem to the same icrutiny regularly applied to theoriesin other fields. If they are to have more than pure abstraction as trheir content, then that content must be discoveredby observingthem in application to the data they are meant to elucidate. How could history of sciencefail to be a sourceof phenomenato which theoriesabout knowledgemay legitimately be askedto apply?
ll. The Route lo Normol Science In this essay,'normal science'meansresearchfirmly based upon one or more past scientific achievements,achievements that someparticular scientific community acknowledgesfor a time as supplyingthe foundationfor its further practice.Today such achievementsare recounted,though seldomin their original form, by science textbooks, elementary and advanced. Thesetextbooksexpoundthe body of acceptedtheory, illustrate many or all of its successfulapplications,and compare these applicationswith exemplaryobservationsand experiments.Before suchbooksbecamepopular early in the nineteenthcentury ( and until even more recently in the newly matured sciences ), many of the famousclassicsof sciencefulfflled a similar function. Aristotle's Physica, Ptolemy's Alrnagest,Newton's Principia and Opticks, Franklin's Electricity, Lavoisier'sChemistry, and Lyell's Geolagy-theseand many other works servedfor a time implicitly to define the legitimate problemsand methods of a researchfteld for succeedinggenerationsof practitioners. They were able to do sobecausethey sharedtwo essentialcharacteristics.Their achievementwassufficientlyunprecedentedto attract an enduring group of adherentsaway from competing modes of scientific activity. Simultaneously,it was sufficiently open-endedto leave all sorts of problems for the redefined group of practitionersto resolve. Achievementsthat share these two characteristicsI shall henceforthrefer to as'paradigms,'aterm that relatescloselyto 'normal science.'Bychoosingit, I mean to suggestthat some acceptedexamplesof actualscientificpractice-exampleswhich include law, theory,application,and instrumentationtogetherprovide modelsfrom which springparticular coherenttraditions of scientific research.These are the traditions which the historiandescribesundersuchrubricsas'Ptolemaicastronomy'(or 'Copernic"r'),'Aristotelian dynamics'(or'Newtonian'),'corpuscularoptics'(or'wave optics'),and so on. The study of l0
fhe Routefo Normol Science paradigms, including many that are far more specializedthan those named illustratively above, is what mainly preparesthe student for membershipin the particular scientificcommunity with which he will later practice. Becausehe there joins men who learned the basesof their ffeld from the same concrete models,his subsequentpractice will seldom evoke overt disagreementover fundamentals.Men whoseresearchis basedon sharedparadigmsare committed to the samerules and standards for scientificpractice.That commitmentand the apparent consensusit producesare prerequisitesfor normal science,i.e., for the genesisand continuationof a particular researchtradition. Becausein this essaythe concept of a paradigm will often substitutefor a variety of familiar notions,more will need to be said about the reasonsfor its introduction.\Mhy is the concrete scientificachievement,as a locus of professionalcommitment, prior to the variousconcepts,Iaws,theories,and points of view that may be abstractedfrom it? In what senseis the shared paradigm a fundamentalunit for the student of scientific development, a unit that cannot be fully reduced to logically atomic componentswhich might function in its stead?When \Meencounterthem in SectionV, answersto thesequestionsand to otherslike them will prove basicto an understandingboth of normal scienceand of the associatedconcept of paradigms. That more abstract discussionwill depend, however, upon a previous exposureto examplesof normal scienceor of paradigms in operation.In particular, both these related concepts will be clarified by noting that there can be a sort of scientific researchwithout paradigms,or at least without any so unequivocaland so binding as the onesnamedabove.Acquisition of a paradigm and of the more esoterictype of researchit permits is a sign of maturity in the developmentof any given scientific field. If the historiantracesthe scientificknowledgeof any selected group of related phenomenabackward in time, he is likely to encountersomeminor variant of a pattern here illustratedfrom the history of physicaloptics.Today'sphysicstextbookstell the
1l
fhe Sfruclureof ScienfificRevolufions student that light is photons, i.e., quantum-mechanicalentities that exhibit soire chiracteristics of walnesand someof particles. accordinglYtot rather accordingto $e.more Researchproceeds ^and mathematical characterization from which this elaborate usual verbalization is derived. That characterizationof light is, however, scarcelyhalf a century old. Before it was developed by Planck, Einstein, and otheri early in this century, physics texts taught that light was transversewave motion, a concePtion roo6d in a p-aradigmthat derived ultimatgly from th3 optical writings of Yo.to! and Fresnel in the early nineteenth by cintury. Nor ivas the wa-vetheory the first t9 be emb_raced During science. optical gi_gltof al-ort all practitioners !\e eenth centuiy the paradigm for this field was Provided by N:*ton's Opticki, which taught that light was material coqput-d"t. At that time physicistssought evidence,as the early -wavetheorists had notlof the pressureexertedby light particlesimpinging on solid bodies.l th.rc transformationsof the paradigmsof physicaloptics are scientiffc revolutions, and the successivetransition from one usual developmental paradigm 'pattern'ofto another via revolution is the the P-attern.charhowever, is not, 'acteristic mature science.It and that is the work, Newton's of the period before remote anbetween No period here. contrast that corrcernsus a exhibjted century the sevenGenth tiquity and the end of Illight. of nature the about sitigle generally accepted_view -n,rmber of competing schools and substeid i"h.r, *br. a schools,most of them espousingone viriant or anothe-r-ofEpiAristotelian, or PiatoniJtheory. One group togk light to ",rr""rr, be pariicles emanatingfrom m-aterialbodies; for another it was -lodifi"ation of the riedium that intervenedbetween the body "and the eye; still another explainedlight in-terms of an inter-"tt 'the from the eye; and medium with action of "*atation besides.Each modifications and there were other combinations its relation strength-from derived of the colrespondingschools as Paraemphasized, each and to someparticular metaphysic, r loseoh Priestley, The Htslallr? atd Prcse* State of Dlscooerbc RelAlng to Liglrt, ardColours (London, 17721, pp. 88L90' Visi;,
12
fhe Roufelo Normol Science digmatic observations,the particular cluster of optical phenomena that its own theory could do most to explain.Other observations were dealt with by d hoc elaborations,or they remained as outstanding problemsfor further research.2 At various times all these schoolsmade signiffcant contributions to the body of concepts,phenomena,and techniquesfrom which Newton drew the ffrst nearly uniformly acceptedparadig* for physicaloptics.Any deffnition of the scientistthat excludes at least the more creative members of these various schoolswill excludetheir modern successors as well. Thosemen were scientists.Yet anyone examining a survey of physical optics before Newton may well conclude that, though the ffeld's practitioners were scientists,the net result of their activity was something less than science. Being able to take no common
schoolsas it was to nature. That pattern is not unfamiliar in a
The history of electrical researchin the ffrst half of the eighteenth centuqy provides a more concrete and better known exampleof the way a sciencedevelopsbefore it acquiresits ffrst universally received paradigm. During that period there were almost as many views about the nafure of electricity as there were important electrical experimenters,men like Hauksbee, Gray, Desaguliers, Du Fay, Nollett, Watson, Franklin, and others. All their numerous concepts of electricity had something in common-they were partially derived from one or an2 vasco Ronchi, Hlstohede k.luml*'e, trans. Iean Tatoa (paris, 1956), chaps. l-lv.
t3
fhe Sfruclureof ScienfiffcRevolulions other version of the mechanico-corpuscular philosophy that guided all scientiffcresearchof the day. In addition, all were componentsof real scientiffctheories,of theoriesthat had been drawn in part from experimentand observationand that partially determined the choice and interpretation of additional problems undertaken in research.Yet though all the experiments were electrical and though most of the experimenters read eachother'sworks, their theorieshad no more than a family resemblance.s One early group of theories, following seventeenth-century practice, regarded attraction and frictional generationas the fundamentalelectricalphenomena.This group tended to treat repulsionas a secondaryeffect due to somesort of mechanical rebounding and also to postponefor as long as possibleboth discussionand systematicresearchon Gray'snewly discovered effect, electrical conduction.Other "electricians" (the term is their olvn ) took attraction and repulsion to be equally elementary manifestationsof electricity and modified their theoriesand researchaccordingly.(Actually, this group is remarkably small-even Franklin's theory never quite accounted for the mutual repulsion of two negatively charged bodies.) But they had as much difficulty as the first group in accounting simultaneouslyfor any but the simplest conduction effects. Those effects, however, provided the starting point for still a third group,one which tendedto speakof electricity as a "fuid" that could run through conductorsrather than as an "effiuvium" This group,in its turn, had that emanatedfrom non-conductors. difficulty reconciling its theory with a number of attractive and
r1
fhe Roufelo Normof Science repulsive effects. Only through the work of Franklin and his did a theory arisethat could accountwith immediate successors somethinglike equal facility for very nearly all theseeffectsand that therefore could and did provide a subsequentgenerationof "electricians"with a commonparadigm for its research. Excluding those ffelds, like mathematics and astronomy, in which the ffrst ffrm paradig*r date from prehistory and also those,like biochemistry, that aroseby division and recombination of specialties already matured, the situations outlined above are historically typical. Though it involvesmy continuing to employ the unfortunate simpliffcation that tags an extended historical episodewith a single and somewhatarbitrarily chosen name (e.8., Newton or Franklin), I suggestthat similar fundamental disagreementscharacterized,for example,the shrdy of motion before Aristotle and of statics before Archimedes,the study of heat before Black, of chemistry before Boyle and Boerhaave,and of historicalgeologybeforeHutton. In parts of biology-the study of heredity, for example-the ffrst universally receivedparadigmsare still more recent;and it remainsan open question what parts of social sciencehave yet acquired such paradigms at all. History suggeststhat the road to a ffrm researchconsensus is extraordinarilyarduous. History also suggests,however,somereasonsfor the difficul-
developmentmakesfamiliar. Furthermore, in the absenceof a reasonfor seekingsomeparticularform of more reconditeinformation, early fact-gathering is usually restricted to the wealth of data that lie ready to hand. The resultingpool of facts contains trhoseaccessibleto casualobservationand experiment together with some of the more esoteric data retrievable from establishedcrafts like medicine, calendarmaking, and metallurgy. Becausethe crafts are one readily accessiblesotrtce of facts that could not have been casually discovered,technology
I5
fhe Slruclureol ScienliffcRevolulions has often played a vital role in the emergenceof new sciences. But though this sort of fact-collecting has been essentialto the origin of many signiffcant sciences,anyone who examines, for example,Pliny's encyclopedicwritings or the Baconiannatural histories of the seventeenthcentury will discover that it producesa morass.One somehowhesitatesto call the literature that results scientiffc.The Baconian"histories"of heat, color, wind, mining, and so on, are fflled with information,someof it recondite. But they iuxtaposefacts that will later prove revealirg (e.g.,heatingby mixture) with others(..g., the warmth of dung heaps) that will for sometime remain too complexto be integratedwith theory at all.' In addition, sinceany description must be partial, the typical natural history often omits from its immenselycircumstantialaccountsiust thosedetails that later scientistswill find sourcesof important illumination. Almost noneof the early "histories"of electricity,for example,mention that chaff, attracted to a rubbed glassrod, bouncesoff again. That eftect seemedmechanical,not electrical.bMoreover,since the casualfact-gathererseldompossesses the time or the tools to be critical, the natural historiesoften iuxtaposedescriptions Iike the abovewith others,say,heating by antiperistasis(or by cooling), that we are now quite unable to conftrm.oOnly very occasionally,as in the easesof ancient statics,dynamics,and geometricaloptics, do facts collected with so little guidance from pre-establishedtheory speakwith sufficientclarity to permit the emergenceof a ffrst paradip. This is the situation that createsthe schoolscharacteristicof the early stagesof a science'sdevelopment.No natural history can be interpretedin the absenceof at leastsomeimplicit body _ 1 C_g1p-e th-esketch for a natural history of heat in Bacon's Notum Organum, Vol. VIII of Tfu Works of Frarcis Bacon, ed. J. Spedding, R. L. El[s, aod D. D. Heath (New York, 1869), pp. 17$203. 6 Roller and Roller, op. cit., pp. 14, 22, 28,43. Onlv after the worlc recorded in the last of these citations do-rtpulsive efiects gain leneral recognition as unequivocally electrical. 6 Bacon, op. clt., pp. 235, 337, says, "Water slightly warm is more easily frozen than quite cold." For a partial account of the earliei history of this strange observation, see Marshall Clagett, Giooanni Marltuni atd Ldte Medb:al Fhysics ( New York, l94l ), chap. iv.
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fhe Roulefo Normol Science of intertwined theoretical and methodologicalbelief that permits selection,evaluation,and criticism. If that body of belief is not already implicit in the collection of facts-in which case more than "mere facts" are at hand-it must be externallysup-
difierent ways. What is surprising,and perhapsalso unique in its degreeto the fields we call science,is that such initial divergencesshouldever largely disappear. For they do disappearto a very considerableextentand then is apparentlyonceand for all. Furthelrnore,their disappearan_ce usually caused by the triumph of one of the pre-paradigrn schools,which, becauseof its own characteristicbeliefsand pre-
been discoveredby a man exploringnature casuallyor at random, but which was in fact independentlydevelopedby at least two investigatorsin the eatly 1740's.?Almost from the start of Franklin was particularly concernedto his electricalresearches,
paradigm,a theory mrtst seembetter than its competitot's,bttt ? Roller and Roller, op. clt., pp. 5f-54. 8 The troublesomecasewas the mutual repulsionof negativelychargedbodies, for which seeCohen,ry. cit.,pp. 491-94,531-43.
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fhe Structureof ScienfiffcRevolulions it need not, and in fact never does,explain all the facts with which it can be confronted. What the fluid theory of electricity did for the subgroupthat held it, the Franklinian paradigm later did for the entire group of electricians.It suggestedwhich experimentswould be worth performing and which, becausedirected to secondaryor to overly complex manifestationsof electricity, would not. Only the paradigm did the job far more effectively,partly because the end of interschooldebateended the constantreiteration of fundamentalsand partly becausethe confidencethat they were on the right track encouragedscientiststo undertakemore precise, esoteric,and consumingsorts of work.8 Freed from the concern with any and all electrical phenomena,the united group of electricianscould pursue selectedphenomenain far moredetail, designingmuch specialequipmentfor the task and employing it more stubbornly and systematicallythan electricians had ever done before. Both fact collection and theory articulationbecamehighly directed activities.The efiectiveness and efficiencyof electricalresearchincreasedaccordingly,providing evidencefor a societalversionof Francis Bacon'sacute methodological dictum: "Truth emerges more readily from error than from confusion."lo We shall be examiningthe nature of this highly directed or paradigm-basedresearchin the next section,but must first note briefly how the emergenceof a paradigm affectsthe structure of the group that practicesthe fteld. When, in the development of a natural science,an individual or group first producesa synthesisable to attract most of the next generation'spractitioners, the older schoolsgradually disappear.In part their disappear-
1oBacon, op. cit., p. 2f0.
l8
fhe Roulefo Normol Science ance is causedby their members'conversionto the new paradigo. But there are always somemen who cling to one or another of the older views, and they are simply read out of the profession,which thereafter ignores their work. The new paradig- implies a new and more rigid definition of the field. Those unwilling or unable to accommodatetheir work to it must proceed in isolation or attach themselvesto some other group.lr Historically, they have often simply stayed in the departments of philosophy from which so many of the special scienceshave been spawned.-As these indicationshint, it is sometimesjust its reception of a paradigm that transforms a group previously interestedmerely in the study of nature into i professionor, at least,a discipline. In the sciences(though nof in ffelds like medicine, technology, and law, of which the principal raison d'atre is an externalsocialneed), the formation of specialized iournals, the foundation of specialists'societies,and the claim for a _specialplace in the curriculum have usually been associated-with a group's first reception of a single paradigm. At Ieast this was the casebetween the time, a cJntury an{a half lgo, -whe1 the institutional pattern of scientiffc specialization first developedand the vgry recent time when the piraphernalia of specializationacquired a prestigeof their own. The more rigid deftnition of the scientific soup has other consequences. the individual scientist cin take a para{hen dign{o-r-Stqt-r$, h" needno longer,inhis maiorworks,att-empt to build his field anew,startingfrom first principlesand iustifyrr The rhe _hi historv tory of electricitv electricity provides an excellent example which could be duplicated from t}e careers of Priesdey, Kelvin, and othe?s. Franklin reoorts that l\o[er, rnar Nollet, wno who ar at nuo-cenrury mid-century was tne ihe most influential of the continental Continintal electricians, "lived to see himseli the last 9f -hi-sSect, except Mr. B.-his Eleve - J
and immediate Disciple" (Max Farrand led.l, Beniamin'Frunkfhts u*riti Calif.. l9-40J, [Berkeley. f9401. pp. op.384-86). Mor" inilresUng, interccriic h^*o.,o, ia +],[Berkeley,Calif., S8L86). Uor" ho*"u"r, is the ^-J..enduianceof ance ot whole whole schools schoolsin in increasingisolation isolationfrom from professional profelsionalscience. science.Consider, Consider. for example,the caseof astrology,-which*as on""'"r, integral part of astronomy. 3ral Or ur Or consider considerthe consider "rL";;;;:centhe continuation continuation in continuation in the i6 the late late eighteenth eishteenth and-earlv arr,l""^"|i, .i-"r..-tt'^-iearly nineteenth nineteenth centuries of a previously respected tradition of "romantic" cheriistrv. This is the tradition discussed by bi, Charles C. C. Gillispie Gillispie in "The Etrcuclmtddlc in "The oi.l th. Ercgclop&d,b_arid the rqrnhi-
Jacobin Philosophyof science: A study in Idelasand consequ"encls,"c;niA-p;;ii;r* in the \*g:V of Scietrce,ed_.MarshallClagett (Madison, Wis., lg5g), pp.255pp. 2SE_ 89; and "Ttie Formation of Lamarclc'sEiolutionary Theory,; eriiirlri iiiin twtbnales d.'histohedes scbncec,XXXVU ((fg56). 1956), g?g+9. "
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fhe Struclureof ScienlificRevolufions ing the use of eachconceptintroduced.That can be left to the *titet of textbooks. Given a textbook, however, the creative scientistcanbegin his researchwhere it leavesoff and thus concentrateexclusivelyupon the subtlestand most esotericaspects of the natural phenomenathat concernhis group. Ald as he does this, his researchcommuniqu6swill begin to change in ways whose evolution has been too little studied but whose modernend productsare obviousto all and oppressiveto many. usuallybe embodiedin booksadNo longet*ilIhis researches . . . on Electrinity or DarExperiments like Franklin's dressed-, win's origin of species,to anyonewho might be interestedin the subjectmatter of the fteld. Insteadthey will usually appe-ar as brief articles addressedonly to professionalcolleagues,the men whoseknowledgeof a sharedparadigm can be asstrmed and who prove to bJthe only onesable to read the papersaddressedto them. or retroToday in the sciences,booksare usually e_ither.texts scientific the of or another one asPect uPon spectivereflections life. The scientistwhb writes one is more likely to find his professionalreputation impaired than enhanced.Onlf in the earlier, pre-paiadigm,stagesof the developmentof the various iia tn. book ordinarily Possessthe same relation to scien^ceg professionalachievementthat it still retains in other creative ^fields. And only in thosefields that still retain the book, with or without the article, as a vehicle for researchcommunication still solooselydrawn that the are the lines of professionalization by reading the practiprogretl follow to layman may hope and astronomy' in mathematics Both tioners' originalieports. to be intelliantiquity in already researchrelorts hid ceased research dynamics, In audience. gible to a g:enerallyeducated recapAges, and_it Middle later in the 6u""*" similarlyeioteric sevenearly the during briefly only tured generalintelligibility had one that replacedthe new wheria teenth"century Paradigm require to begln research Electrical research. guided medieval franslationfor the laymanbeforethe end of the eighteenthcentur/, and most otherfields of physicalscienceceasedto be gerraccessiblein the ninetcenth. During the sametwo cen"r"ily 20
fhe Roufeto Normol Science ttrriessimilar transitionscan be isolatedin the variousparts of the biologicalsciences. In parts of the socialsciencesthey may well be occurring today. Although it has become customary, and is surelyproper,to deplorethe widening gulf that separates the professionalscientistfrom his colleaguesin other ffelds,too Iittle attentionis paid to the essentialrelationshipbetweenthat gulf and the mechanisms intrinsicto scientiftcadvance. Ever since prehistoric antiquity one ffeld of study after another has crossedthe divide betweenwhat the historianmight call its prehistoryasa scienceand its history proper.Thesetransitionsto maturity haveseldombeenso suddenor so unequivocal as my necessarilyschematicdiscussionmay have implied. But neither have they been historicallygradual,coextensive, that is to say,with the entire developmentof the ffeldswithin which they occurred.Writers on electricity during the first four decadesof the eighteenthcenturypossessed far more information about electricalphenomenathan had their sixteenth-century predecessors. During the half-centuryafter 1740,few new sortsof electricalphenomenawere added to their lists. Nevertheless,in important respects,the electricalwritings of Cavendish, Coulomb, and Volta in the last third of the eighteenth century seemfurther removedfrom thoseof Gray, Du Fay, and even Franklin than are the writings of these early eighteenthcentury electrical discoverersfrom those of the sixteenthcentury.I2SometimebetweenL740and 1780,electricianswere for the first time enabledto take the foundationsof their field for granted.From that point they pushedon to more concreteand reconditeproblems,and increasinglythey then reported their resultsin articlesaddressedto other electriciansrather than in booksaddressedto the learnedworld at large.As a group they achievedwhat had been gained by astronomersin antiquity
2l
fhe Sfrucfure ol ScientificRevolulions
22
lll. The Noture of Normol Science What then is the nature of the more professionaland esoteric researchthat a groupt receptionof a single paradigmpermits? If the paradigm representswork that has been done once and for all, what further problemsdoesit leave the united group to resolve?Thosequestionswill seemevenmore urgent if we now note one respectin which the termsusedsofar may be misleading. In its establishedusage,a paradigm is an acceptedmodel or pattern, and that aspectof its meaninghas enabledme, Iacking a better word, to appropriate'paradigm' here. But it will shortly be clear that the senseof model'and 'pattern' that permits the appropriation is not quite the one usual in defining 'anxo, paradigm.' fn grammar, for example, artes, amat' is a paradigmbecauseit displaysthe pattern to be usedin coniugating a large number of other Latin verbs, e.9., in producing 'l,audo, lnudns,lnudat.' In this standard application, the paradigm functions by permitting the replication of examplesany one of which could in principle serveto replaceit. In a science, on the other hand,a paradigmis rarely an object for replication. Instead,Iike an acceptediudicial decisionin the commonlaw, it is an obiect for further articulation and speciffcationunder new or more stringentconditions. To seehow this can be so, we must recognizehow very limited in both scopeand precisiona paradigmcan be at the time of its first appearance. Paradigmsgain their statusbecausethey are more successfulthan their competitors in solving a few problemsthat the group of practitionershas come to recognize as acute. To be more successfulis not, however, to be either completelysuccessfulwith a singleproblem or notably successful with any largenumber.The successof a paradigm-whether Aristotle'sanalysisof motion, Ptolemy'scomputationsof planetary position, Lavoisiert application of the balance,or Maxwellis mathematization of the electromagneticfield-is at the start largely a promiseof successdiscoverablein selectedand
23
fhe Sfruclureof ScienfificRevolulions still incompleteexamples.Normal scienceconsistsin the actualization of that promise,an actualizationachievedby extending the knowledge of those facts that the paradigm displays as particularlyrevealing,by increasingthe extentof the match between those facts and the paradigm'spredictions,and by ftrrther articulation of the paradigm itself. Few people who are not actually practitioners of a mature sciencerealizehow much mop-uP work of this sort a paradigm leavesto be done or quite how fascinatingsuchwork can Prove in the execution.And thesepoints needto be understood.Mop-
to invent new theories,and they are often intolerant of thoseinventedby others.lInstead,normal-scientificresearchis directed to the aiticulation of those phenomenaand theories that the ^paradigm already supplies. Perh-apsthese'aretifects. The areasinvestigatedby PTtl scienceire, of course,minuscule;the entelprise now under dis-
restrictions that bound researchwhenever the paradigm from which they derive ceasesto function effectively. At thalP9int scientists6egin to behave differentl)', and-the nature of their researchpro6l"*r changes.In the inierim, however,during the 1 Bernard Barber, "Resistance by Scientists to Scientiffc Discovery," Scbnce,
cxxxN (196r),59G602.
24
fhe Nofure of NormqlScience the professionwill have period when the paradigmis successful, iolved problemsthat its memberscould scarcelyhave imagined and would never have undertaken without commitment to the paradigm.And at leastpart of that achievementalwaysProves to be permanent. To display more clearly what is meant by normal or p-aradigm-basedresearch,let me now attempt to classify and illustrate the problems of which normal scienceprincipally consists. For convenienceI postponetheoretical activity and begin with fact-gathering, that is, with the experimentsand observations describedin the technical journals through which scientistsinform their professionalcolleaguesof the resultsof their continuing research.On what aspectsof nature do scientistsordinarily report? What determines their choice?And, since most scientific observation consumesmuch time, equipment, and money, what motivates the scientist to Pursue that choice to a conelusion? There are, I think, only three normal foci for factual scientiftc investigation, and they are neither alwaysnor Pennanentlydistinct. First is that classof facts that the paradigm has shown to be particularly revealing of the nature of things.By employin_g them in solving problems, the paradigm has made them worth determining both with more precision and in a larger variety of situations.At one time or another,thesesigniffcantfactual determinations have included: in astronomy-stellar position and magnitude,the periods of eclipsingbinaries 1nd of planets;in ph1'sics-the specificgravities and comPressibilitiesof materials, *aue lengths and spectral intensities, electrical conductivities and contact potentials; and in chemistry-composition and combining weights, boiling points and acidity of solutions, structural formulas and optical activities. Attempts to increasethe accuracy and scope with which facts like these are known occupy a signiftcant fraction of the literature of experimental and bbservalional science.Again and again complex special apparatushas been designedfor such purPoses,and the invention, constmction, and deployment of that apparatushave demandedffrst-ratetalent, much time, and considerableffnancial
25
fhe Structureol ScienfificRevolulions backing. Synchrotronsand radiotelescopesare only the most recent examplesof the lengths to which researchworkers will go if a paradigm assuresthem that the facts they seek are important. From Tycho Brahe to E. O. Lawrence,somescientists have acquired great reputations,not from any novelty of their discoveries,but from the precision, reliability, and scope of the methods they developed for the redetermination of a previously known sort of fact. A secondusual but smallerclassof factual determinationsis
the speedof light is greaterin air than in water; or the giganticscintillation counter designedto demonstratethe existenceof
4248.
26
fhe Nolure of NormolScience the neutrino-thesepiecesof specialapparatusand many others like them illustrate the immenseefiort and ingenuity that have beenrequired to bring nature and theory into closerand eloser agreement.s That attempt to demonstrateagreementis a second type of normal experimentalwork, and it is evenmore obviously dependentthan the ffrst upon a paradigm.The existenceof the paradigm sets the problem to be solved; often the paradigm theory is implicated directly in the designof apparatusable to solvethe problem.Without the Principia,for example,measurements made with the Atwood machine would have meant nothing at all. A third class of experimentsand observationsexhausts,I think, the fact-gatheringactivitiesof normal science.It consists of empiricalwork undertakento articulatethe paradigmtheory, resolving some of its residual ambiguitiesand permitting the solution of problems to which it had previously only drawn attention.This classprovesto be the most important of all, and its descriptiondemandsits subdivision.In the more mathematical sciences,someof the experimentsaimed at articulation are directed to the determinationof physical constants.Newton's work, for example,indicated that the force between two unit massesat unit distancewould be the samefor all typesof matter at all positionsin the universe.But his own problemscould be solved without even estimatingthe size of this attraction, the universalgravitationalconstant;and no one elsedevisedapparatus able to determine it for a century after the Principia appeared. Nor was Cavendish'sfamous determination in the 1790t the last.Becauseof its centralpositionin physicaltheory, improved values of the gravitational constant have been the objectof repeatedeffortseversinceby a numberof outstanding 3 For-two o parailax relescopes, ror fwo or of tne telescopes,_see the_paralla_x see ADranam Abraham Wolf, wolt, la A n8torv Historg ol of Jctence, Science, Technology, and Piilosophy inthe-Eighteenth Centurg (2d ed.; Loidon, 1952), pp. 103-5. For the Atwood machine, machine. see N. R. Hanson, Hanson. Patterns^ Pattqns of ol Discooeru Dis_cooery ( Cambridge, 1958 ), pp. 100-102, 207-8. For the last two pieces of special appal ratus, see see-M. M. L, FoGault, Foiri:ault, "M6thode g6n6rale pour me-surer me's,,ter Ia vitess" du e I t" a lumidre dans I'air et les milieux transparints. Viteslsesrelatives de Ia lumidre dans l'air et dans l'eau . . . ," Comptes rendus . . . de I'Acad,6mie des sciences, XXX (1850),551-60; and C. L. Cowan, Ir., et al.,"Detection of the Free Neutrino: A Conffrmation," Scdence,CXXIV (f956), f03-4.
27
fhe Struclure of ScientificRevolufions experimentalists.4Other examples of the sarnc solt of corttinuing work would include determinations of the astronomical unit, Avogadro's number, Joule's coefficient, the electronic charge, and so on. Few of these elabolate efforts would have been conceived and none would have been carried out without a paradigm theory to define the problem and to guarantee the existenceof a stable solution. Efforts to articulate a paradigm are not, horvever, restricted to the determination of universal constants. They may, for example, also aim at quantitative laws: Boyle's Law relating gas pressureto volume, Coulomb's Law of electrical attraction, and foule's formula relating heat generated to electrical resistance and current are all in this category. Perhaps it is not apparent that a paradigm is prerequisite to the discovery of laws like these.We often hear that they are found by examining measurements undertaken for their own sake and without theoretical commitment. But history offers no support for so excessively Baconian a method. Boyle's experiments were not conceivable (and if conceived would have received another interpretation or none at all ) until air was recognized as an elastic fluid to which all the elaborate concepts of hydrostatics could be applied.s Coulomb's successdepended upon his constructing special apparatus to measure the force between point charges. (Those who had previously measured electrical forces using ordinary pan balances,etc., had found no consisteut or simple regularity at all. ) But that design, in turn, depended upon the previous recognition that every particle of electric fluid acts upon every other at a distance. It was for the force between such particles-the only force which might safely be asstrmed 4 H. P[oyntingJ reviews some two dozen measurementsof the gravitational I. consiant between t7+t and l90t in "Gravitation Constant and Mean Density XII, of the Earth," Encyclopaedia Britannrca (llth ed.; Cambridge, l9l0-ll), 385-89. 5 For the full transplantation -Pascal, of hydrostatic concepts into pneumatics, see The trans. L H. B. Spicrs and A. G. H. Spiers, with an Phqsical Treatises of intioduction and notes by F. Barry (New York, 1937). Torricelli's original introduction of the paralleiism ( "We live submerged at the bottom of an ocean of the element air'r) occt,rs on p. 164. Its rapid development is displayed by the two main treatises.
28
fhe Notu re oI Normol Science
a simple function of distance-that Coulomb was looking.o Joule'sexperimentscould alsobe used to illustratehow quantitative laws emergethrough paradigm articulation.In fact, so general and close is the relation between qualitative paradigm and quantitative law that, since Galileo, such Iaws have often been correctly guessedwith the aid of a paradigmyears before apparatus cotrld be designed for their experimental determination.T Finally, there is a third sort of experimentwhich aims to articulate a paradigm. More than the others this one can resemble exploration, and it is particularly prevalent in those periodsand sciencesthat deal more with the qualitative than with the quantitative aspectsof nature's regularity. Often a paradigmdevelopedfor one set of phenomenais ambiguousin its applicationto other closelyrelated ones.Then experiments are necessaryto chooseamongthe alternativeways of applying the paradigm to the new area of interest. For example,the paradigmapplicationsof the caloric theory were to heating and cooling by mixtures and by changeof state.But heat could be releasedor absorbedin many other ways-e.g., by chemical combination,by friction, and by compressionor absorptionof a gas-and to each of theseother phenomenathe theory could be applied in severalways. If the vacuum had a heat capacity, for example,heatingby compressioncould be explainedas the resultof mixing gaswith void. Or it might be due to a change in the specificheat of gaseswith changingpressure.And there were several other explanationsbesides.Many experiments were undertakento elaboratethesevariouspossibilitiesand to distinguishbetwecn them; all theseexperimentsarosefrom the caloric theory as paradigm,and all exploitedit in the designof experimentsand in the interpretationof results.sOncethe phe6 Duane Roller and Duane II. D. Roller, The Deoclopment of the Concept of Electric Charge: Electricity from the Grecks to Coulomb ( "Harvard Case Histories in Experimental Scicnce," Case 8; Cambridge, I{irss., 1954), pp. 66-80. 7 For examples, see T. S. Kuhn, "Thc I.'trnction of Measuremcnt in Modern Physical Science,"Isis, LII (f96f ), 161-93. 8 T. S. Kuhn, "Thc Caloric Thcory of Adiabatic Compression," lsi.r, XLIX ( 1958), 139-40.
fhe Struclureof ScienfificRevolufions Iromenonof heatingby compression had been established,all further experimentsin the area were paradigm-dependentin this way. Giventhe phenomenon, how elsecould an experiment to elucidateit have been chosen? Turn now to the theoretical problems of normal science, which fall into very nearly the sameclassesas the experimental and observational.A part of normal theoretical work, though only a small part, consistssimply in the use of existing theory to predict factual information of intrinsic value. The manufacture of astronomicalephemerides,the computation of lens characteristics,and the production of radio propagationcuryes are examplesof problemsof this sort. Scientists,however,generally regard them as hack work to be relegatedto engineers or technicians.At no time do very many of them appearin significant scientificiournals.But thlse iournalsdo confaitta gt""t many theoretical discussionsof problems that, to the nonscientist,must seemalmost identical. Theseare the manipulations of theory undertaken, not becausethe predictions in which they result are intrinsically valuable, but becausethey can be confronted directly with experiment.Their pulpose is to display a new applicationof the paradigmor to increasethe preeisionof an application that has already been made. The need for work of this sort arisesfrom the immensedifficulties often encounteredin developingpoints of contact between a theory and nature. These difficulties can be briefly illustrated by an examinationof the history of dynamicsafter Newton. By the early eighteenthcentury those scientistswho found a paradigm in the Principin took the generality of its conclusionsfor granted, and they had every reasonto do so. No other work known to the history of sciencehas simultaneouslypermitted solarge an increasein both the scopeand precision-ofresearch.For the heavensNewton had derivedKepler's Laws of planetary motion and also explained certain of the observedrespectsin which the moon failed to obey them. For the earth he had derived the resultsof somescatteredobservations on pendulums and the tides. With the aid of additional but he had alsobeen able to derive Boyle'sLaw adlnc assumptions,
30
fhe Nofure of NormolScience and an important formula for the speedof sound in air. Given the stateof scienceat the time, the successof the demonstrations was extremely impressive.Yet given the presumptive generality of Newton's Laws, the number of these applications was not great, and Newton developed almost no others. Furthermore, compared with what any graduate student of physics can achievewith those samelaws today, Newton's few applications were not even developed with precision. Finally, the Principia had been designedfor application chiefly to problems of celestial mechanics. How to adapt it for terrestrial applications, particularly for those of motion under constraint, was by no means clear. Terrestrial problems were, in any case, already being attackedwith great successby a quite difierent set of tech-
saw quite how.e
point_in order- to provide a unique deffnition of pendulum length.-Mo_stof his theorems,the few e*ceptionsbeing hypothetical and preliminary,alsoignoredthe effectof air resistance. These were sound physical approximations.Nevertheless,as approximationsthey restricted the agreementto be expected
3l
fhe Sfructureof ScienfificRevolulions
To derive those laws, Newton had been forced to neglect all gravitational attraction except that between individual planets and the sun. Since the planets also attract each other, only approximate agreementbetween the applied theory and telescopic observationcould be expected.l0 The ageement obtained was,of course,more than satisfactory to those who obtained it. Excepting for some terrestrial problems,no other theory could do nearly sowell. None of thosewho questionedthe validity of Newton's work did so becauseof its limitd agreementwith experiment and obsenration.Nevertheless,these limitations of agreementleft many fascinating theo Theoretical techniques retical problemsfor Newton's successors. were, for example, required for treating the motions of more than two simultaneouslyattracting bodies and for investigating the stability of perhrrbed orbits. Problemslike these occupied many of Europe's best mathematiciansduring the eighteenth and early nineteenth cenfury. Euler, Lagrange, Laplace, and Gauss all did some of their most brilliant work on problems aimed to improve the match between Newton's paradigm and observationof the heavens.Many of theseffguresworked simultaneouslyto develop the mathematicsrequired for applications that neither Newton nor the contemPoraryContinental schoolof mechanicshad even attempted. fr"y produced, for example, an immerxe fiterature and somevery Powerful mathematical techniques for hydrodynamics and for the problem of vibrating
10wolf, op. cit., pp. 75-81, 9Gl0l; and william whewell, Ilistory of the (i6v. ed.;London,1847),II,213-71. IntluctioeSciences
32
TheNqfure of Normol Science o{y, or any other branch of sciencewhosefundamental laws are fully quantitative. At least in the more mathematical sciences, most theoretical work is of this sort. But it is not all of this sort.Even in the mathematicalsciences there are also theoretical problems of paradigm articulation; and during periods when scientific development is predominantly qualitative,theseproblemsdominate.some of []re problems, in both the more quantitative and more qualitativl sciences,aim simply at clarification by reformulation. The principia-,f-orexample,did not alwaysprove an easywork to apply, partly becauseit retained some of the clumsinessinevitable in a ftrst venture and partly becauseso much of its meaning was only rmplicit in its applications. For many terrestrial applications, in any case, an apparently unrelated set of Conti-nental techniques seemedvastly more pcwerful. Therefore, from Euler an{ in the eighteenth century to Hamilton, !3srange Jacobi, and Hertz in the nineteenth, many of Europe's most briliant mathematical physicists repeatedly endeavoredto reformulate mechanical theory in an equivalent but logieally and.aesthetically more satisfying form. Thuy wished, that is, to exhibit the e4plicit and implicit lessonsof the principia and of Continental mechanicsin a logically more eoherentversion, one that would be at onc€ more uniform and lessequivocal in its application to the newly elaboratedproblems of mechanics.rr similar reformulations_ofa paradigm have occurredrepeatedty-- all o{ the sciences,but most ofthem have produceldmore substantialchangesin the paradigm than the reiormulations of the Principda cited above.-such result from the em"hatrg"s
1#;i1"*f:ffiit#"i1x '.*",1:
nJ;"*"",T:iffi :ffiff
eqgarywethere.Beroreh"':frTi':"9"'JtrJff li:l*l#;:T;
make measurementswith it, coulomb had to emp^loielectrical theory to determine how his equipment should^be'built. The 11Ren6 Dugas, Histoire d.e ln mdcandgue (Neuchatel, lg5O), Books IV_V.
33
fhe Sfructureof ScienlificRevolufions conseguenceof his measurementswas a refinement in that theory, Or again, the men who designedthe experimentsthat were to distinguish between the various theories of heating by compressionwere generally the same men who had made up the versions being compared. They were working both with fact and with theory, and their work produced nolsimply new information but a more preciseparadigm, obtained by the elimination of ambiguities that the original from which they worked had retained. In many sciences,most normal work is of this sort. Thesethree classesof problems-determinationof significant fact, matching of facts with theory, and articulation of theoryexhaust,I think, the literature of normal science,both empirical and theoretical.They do not, of course,quite exhaustthe entire literature of science.There are alsoextraordinaryproblems,and it may well be their resolution that makes the scientific enterprise as a whole so particularly worthwhile. But extraordinary problemsare not to be had for the asking.They emergeonly on specialoccasionspreparedby the advanceof normal research. Inevitably, therefore, the overwhelming majority of the problems undertakenby even the very best scientistsusually fall into one of the three categoriesoutlined above.Work under the paradigm can be conductedin no other w&/, and to desertthe paradigm is to ceasepracticing the scienceit deffnes.We shall shortly discover that such desertionsdo occur. They are the pivots about which scientificrevolutionsturn. But beforebeginning the study of such revolutions,we require a more Panoramic view of the normal-scientificpursuits that prepare the way.
34
lV. Normol Science os Puzzle'solving Perhaps the most striking feature of the normal research problemi we have just encounteredis how little- they aim to Sometimes, iroduce maior novelties,conceptualor phenomenal. as in a wave-lengthmeasurement,everythingbut the most esoteric detail of tlie result is known in advance,and the typical latitude of expectation is only somewhat wider. Coulomb's need not, perhaps,have fitted an inversesquare measurements law; the men who worked on heating by comPressionwere often preparedfor any one of severalresults.Yet even in cases like thesJthe range of anticipated,and thus of assimilable,results is alwayssmall comparedwith the range that imagination can conceive.And the pioject whose outcome doesnot fall in that narrowerrange is Gually iust a researchfailure, one which reflectsnot on nature but on the scientist. In the eighteenth century, for example,little attention was paid to the experimentsthat measuredeleetricalattractionwith devicesIike the pan balance.Becausethey yielded ueither consistentnor simple results,they could not be used to articulate the paradigm from which they derived. Therefore, they remained nlere facts,unrelatedand unrelatableto the continuing of progressof electricalresearch.Only in retrospect,possessed paradigm,can we seewhat characteristicsof eleci snbseqrrent trical phenomenathey display. Coulomb and his contempothis later paradigmor one that, raries,of course,alsopossessed attraction, yielded the same of the to applied problem when expectations.That is why Coulomb was able to design apParatus that gave a result assimilableby paradigm articulation. But it is alsowhy that result surprisedno one and why several had been able to predict it in of Coulomb'scontemporaries advance.Even the proiectwhosegoal is paradigmarticulation doesnot aim at the unexpectednovelty. But if the aim of normal scienceis not major substantivenovelties-if failure to come near the anticipatedresult is usually
35
fhe Slructureof ScienfiffcRevolutions failure as a scientist-then why are theseproblemsundertakerr at all? Part of the answerhasalreadybeendeveloped.To scientists, at least, the resultsgained in normal researchare significant becausethey add to the scopeand precisionwith *t i.r, the paradigm can be applied. Tliat answer,however, cannot accountfor the enthusiasmand devotion that scientistsdisplay for the p_roblems of normal research.No one devotesy"ati to, say,the developmentof a better spectrometeror the production of an improved solution to the problem of vibrating strings simply becauseof the importanceof the information that *itt be obtained.The data to be gainedby computing ephemerides or by further measurementswith an existing instrument are often i,rft as significant, but those activities are regularly spurnedby scientistsbecausethey are so largely repetitionsof proceduresthat have been carried through before.that rejection provides a clue to the fascinationof the normal research problem. Though its outcome can be anticipated,often in detail so great that what remainsto be known is itself uninteresting, the way to achieve that outcome remains very much in doubt. Bringing a normal researchproblem to a conclusionis achieving the anticipated in a new wo/, and it requires the solution of all sorts of complex instrumental,conceplual,and mathematicalpuzzles.The man who succeedsproves himself an expert puzzle-solver,and the challengeof the puzzle is an important part of what usually drives him on. The terms'puzzle' and'puzzle-solver'highlight severalof the themes that have become increasinglyprominent in the preceding pages.Puzzlesare, in the entirely standard meaning here employed,that specialcategoryof problemsthat can serve to test ingenuity or skill in solution.Dictionary illustrationsare 'jigsaw puzzle'and'crosswordpuzzle,'andit is the characteristics that thesesharewith the problemsof normal sciencethat we now need to isolate.One of them has just been mentioned. It is no criterion of goodnessin a puzzle that its outcomebe intrinsicallyinterestingor important.On the contrary,the really pressingproblems,€.8., a cure for cancer or the design of a
36
Normol Scienceos Puzzle'solving
of a solution is. We have already seen, however, that one of the things a scientific community acquireswith a paradigm is a criterion for choosing problems that, while the paradigm is taken for
trated by several facets of seventeenth-centuryBaconianism and by some of the contemporarysocial sciences.One of the reasonswhy normal scienceseemsto progressso rapidly is that its practitionersconcentrateon problems that only their own lack of ingenuity should keep them from solving. If, however, the problems of normal science are puzzles in this sense,we need no longer ask why scientistsattack them with such passion and devotion. A man may be attracted to sciencefor all sorts of reasons.Among them are the desire to be useful, the excitementof exploring new territory, the hope of finding order, and the drive to test establishedknowledge. These motives and others besidesalso help to determine the particular problems that will later engage him. Furthermore, though the result is occasionalfmstration, there is good reason
fhe Slructureof ScienlificReyolufions why motives like these should first attract him and then lead him on.l The scientiffcenterpriseas a whole doesfrom time to time prove useful, open up new territory, display order, and test long-acceptedbelief. Nevertheless,the indirsid:u,al engaged on a norrnal researchproblem is almostneDerdoing any one of these things. Once engaged,his motivation is of a rather difierent sort. What then challengeshim is the conviction that, if only he is skilful enough, he will succeedin solving a puzzle that no one before has solved or solved so well. Many of the geatest scientiffcminds have devoted all of their professional attention to demandingpuzzlesof this sort. On most occasions any particular ffeld of specializationoffers nothing else to do, a fact that makes it no less fascinating to the proper sort of addict. Turn now to another,more difficult, and more revealing aspect of the parallelismbetween puzzles and the problems of normal science.If it is to classifyas a puzzle,a problem must be characterizedby more trhan atrntid solutioir. There must also be rules that limit both the"trnature of acceptablesolutions and the steps by which they are to be obtained. To solve a iigsaw puzzle is not, for example,merely "to make a picfure." Either a child or a contemporaryartist could do that by scattering selected pieces, as abstract shapes,upon some neutral ground. The picture thus produced might be far better, and would certainly be more original, than the one from which the puzzle had been made. Nevertheless,such a picture would not be a solution.To achievethat all the piecesmust be used,their plain sidesmust be turned down, and they must be interlocked without forcing until no holes remain. Those are among the rules that govern iigsaw-puzzle solutions. Similar restrictions upon the admissiblesolutions of crossword puzzles,riddles, chessproblems,and so on, are readily discovered. If we can accept a considerablybroadeneduse of the term r The frustrations induced bv the confict between the.individual's role and the over-all pattern of scientihc development can, however, occasionally be quite serious. On this subject, see Lawrence S. Kubie, "Some Unsolved Problems of the Scientiftc Career," American Scientist, XLI (1953),596-613; and
XLII (1954),r04-r2.
38
Normol Scienceos Puzzlesofving 'established 'rule'-one that will occasionally equate it with 'preconception'-then the problems accesviewpoint' or with sible-within a given researchtradition display som,ething-much like this set oipunle characteristics.The man who builds an instrument to determine optical wave lengths must not be satisfied with a piece of equipment that merely attributes particular numbers to particulai spectral lines. He is not iust an explorer or measurer.On the contrary, he must'show,by analyzinghis apparatus in terms of the establishedbody of optical theory, that the numbers his instrument Producesare the ones that enter theory as wave lengths. If someresidual vaguenessin the theory or some unanalyzed component of his apparatus Prevents his completing that demonstration,his colleaguesmay well concludethat he hasmeasurednothing at all. For example, the electron-scatteringmaxima that were later diagnosedas indices of electron wave length had no apparent significance when first observedand recorded.Beforethey becamemeasures of anything, they had to be related to a theory that predic_ted the waveJike behavior of matter in motion. And even after that relation was pointed out, the apparatushad to be redesignedso that the experimentalresultsmight be correlatedunequivocally with theory.2Until thoseconditionshad been satisffed,no probIem had been solved. Similar sortsof restrictionsbound the admissiblesolutionsto theoreticalproblems.Throughout the eighteenthcentury those scientistswho tried to derive the observedmotion of the moon from Newton's laws of motion and gravitation consistently failed to do so. As a result, someof them suggestedreplacing the inversesquarelaw with a law that deviated from it at small distances.To do that, however,would have beento changethe paradigm, to define a new puzzle, and not to solvethe old one. In the event, scientistspreservedthe rules until, in 1750,one of them discoveredhow they could successfullybe applied.s 2 For a brief account of the evolution of these experiments,see page 4 of lecturein Les prir Nobel en 1937(Stockholm,1938). C. J. Davisson's 3 W. Whewell, Hi*ory of the luluctioe sciences(rev. ed.; London,1847),II, l0I-5, 220-i2z
39
fhe Sfructureof ScienfificRevolufions Only a changein the rulesof the gamecould have provided an alternative. The study of normal-scientiffctraditionsdisclosesmany additional rules, and these provide much information about the commitmentsthat scientistsderive from their paradigms.What can tve say are the main categoriesinto which theserules fall?. The most obviousand probably the most binding is exempliffed by the sorts of generalizationswe have iust noted. These are explicit statementsof scientiftc law and about scientiffc concepts and theories. While they continue to be honored, such statementshelp to set puzzlesand to limit acceptablesolutions. Newton's Laws, for example,performed thosefunctions during the eighteenthand nineteenth centuries.As long as they did so, quantity-of-matter was a fundamental ontological category for physical scientists,and the forces that act between bits of matter were a dominant topic for research.6In chemistry the laws of fixed and deffnite proportionshad, for a long time, an exactly similar force-setting the problem of atomic weights, bounding the admissible results of chemical analyses, and informing chemistswhat atoms and molecules,compoundsand mixtures were.8Maxwellt equations and the laws of statistical therrrodynamics have the same hold and function today. Rules like these are, however, neither the only nor even the most interesting variety displayedby historical study. At a level Iower or more concrete than that of laws and theories, there is, for example,a multitude of commitmentsto preferred types of instrumentationand to the ways in which acceptedinstruments may legitimately be employed. Changing attitudes toward the role of ffre in chemical analysesplayed a vital part in the dea I owe this qr.restio_n to W. O. Hagstrom, whose work in the sociology of sciencesometimesoverlapsmy own. 6 For theseaspectsof Newtonianism,seeI. B. Cohen, Frca/r;Ilnatd Neuston: An Inqulru lnto Speailatioe Neutonian Erperhpntal Scbrce atd Franklin's Work in nTectr*:U1i as an Erample Thercof (Philadelphia,1956), chap.vii, esp. pp.25L57, 27.-E77. 0 This exampleis discussedat length near the end of SectionX.
40
Normol Scienceos Puzzle-solving velopmentof chemistry in the seventeenthcenhrry.?Helmholtz, in the nineteenth,encounteredstrong resistancefrom physiologists to the notion that physical experimentation could illuminate their field.8 And in this century the curious history of chemical chromatography again illustrates the endurance of instrumental commitments that, as much as laws and theory, provide scientistswith rules of the game.eWhen we analyze the discovery of X-rays, we shall find reasonsfor commitments of this sort. Less local and temporary, though still not unchanging characteristics of science,are the higher level, quasi-metaphysical commitments that historical study so regularly displays. After about 1630,for example,and particularly after the appearance of Descartes'simmensely influential scientiffc writings, most physical scientistsassumedthat the universe was composedof microscopiccorpusclesand that all natural phenomenacould be explainedin terms of coqpuscularshape,size, motion, and interaction. That nest of commitrnentsproved to be both metaphysical and methodological.As metaphysical,it told scientists what sortsof entities the universedid and did not contain: there was only shaped matter in motion. As methodological, it told them what ultimate laws and fundamental explanationsmust be like: laws must specify co{puscularmotion and interaction, and explanationmust reduceany given natural phenomenonto colpuscularaction under theselaws. More important still, the corpuscular conception of the universe told scientistswhat many of their research problems should be. For example, a chemist who, Iike Boyle, embraced the new philosophy gave particular attention to reactionsthat could be viewed as transmutations. More clearly than any others these displayed the processof corpuscularrearrangementthat must underlie all z H. Metzger, Les doctrines chimiques en France du ddbut du xvlrc siccle d hfin du XVlIle siicle (Paris, 1928),.pp. 359$l; Marie Boas,Robert Boule atd. Seoenteenth-Century Chemistry ( Cam5ridge, lg58 ), pp. I l2-l5. t.L99^{gnigsberger, Hermann oon Helmholtz, trans. Francis A. Welby (Ox_ ford, 1906), pp. 6F66. 9_JamesE, Meinhard, "Chromatography: A Perspective," Science,CX ( lg4g), 387-92.
4l
fhe Struclureof ScienfificRevolufions chemical change.l' similar efiects of corpuscularismcan be observedin the study of mechanics,optics] and heat. Finally, at a still higher level, thereis anotherset of commitmentswithout which no man is a scientist.The scientistmust, for example,be concernedto understandthe world and to extend the precisionand scopewith which it has been ordered. That commitmentmust, in turn, lead him to scrutinize,either for himselfor through colleagues, someaspectof nature in great empirical detail. And, if that scrutiny displayspocketsof apparent disorder,then thesemust challengehim to a new reftniment of his observationaltechniquesor to a further articulation of his theories.Undoubtedlythere are still other ruleslike these, oneswhich have held for scientistsat all times. The existenceof this strong network of commitments-conceptual, theoretical, instrumental, and methodological-is a principal sourceof the metaphorthat relatesnormal scienceto puzzle-solving.Becauseit provides rules that tell the practitionerof a mature specialtywhat both the world and his science are like, he can concentratewith assuranceupon the esoteric problems that these rules and existing knowledge define for him. What then personallychallengeshim is how to bring the residualpuzzleto a solution.In theseand other respectsa discussionof puzzlesand of rules illuminatesthe nature of normal scientificpractice. Yet, in another waf t that illumination may be significantly misleading.Though there obviously are rules to which all the practitionersof a scientificspecialtyadhereat a given time, thoserulesmay not by themselvesspecifyall that the practiceof thosespecialistshas in common.Normal science is a highly determined activity, but it need not be entirely determinedby rules. That is why, at the start of this essay,I introducedsharedparadigmsrather than sharedrules, assumptions,and points of view as the sourceof coherencefor normal researchtraditions.Rules,I suggest,derive from paradigms,but paradigmscan guide researcheven in the absenceof rules. 10 For corpuscularism in general, see Marie Boas, "The Establishment of the Mechanical Philosophy," Osiris, X ( 1952), 412-541. For its effects on Boyle's chemistry, see T. S. Kuhn, "Robert Boyle and Structrrral Chemistry in the Seventeenth Century," I.si.s,XLIII (1952), 12-36.
12
V. The Priority of Porodigms To discoverthe relation between rules, paradigms,and normal science,considerffrst how the historian isolatesthe particular loci of commitment that have iust been described as acceptedrules. Close historical investigation of a given specialty at a given time disclosesa set of recurrent and quasistandard illustrations of various theories in their conceptual, observational,and instrumental applications. These are the community'sparadigms,revealedin its textbooks,lectures,and laboratory exercises.By studying them and by practicing with them, the members of the correspondingcommtrnity learn their trade.The historian,of course,will discoverin addition a penumbral area occupiedby achievementswhosestatusis still in doubt, but the core of solved problemsand techniqueswill usuallybe clear.Despiteoccasionalambiguities,the paradigms of a mature scientificcommunity can be determinedwith relative ease. The determinationof sharedparadigmsis not, however,the determinationof sharedrules.That demandsa secondstep and one of a somewhatdifferent kind. When undertaking it, the historian must comparethe community'sparadigmswith each other and with its current researchreports.In doing so, his object is to discoverwhat isolableelements,explicit or implicit, the members of that community may have abstracted from their more global paradigmsand deployedas rules in their research.Anyone who has attempted to describeor analyzethe evolutionof a particular scientifictradition will necessarilyhave sought acceptedprinciples and rules of this sort. Almost certainly, as the precedingsectionindicates,he will have met with at leastpartial success. But, if his experience hasbeenat all like my own, he will have found the searchfor rulesboth more difficult and lesssatisfyingthan the searchfor paradigms.Someof the generalizationshe employs to describe the communityt sharedbeliefs will presentno problems.Others,however,in-
43
fhe Sfructureof ScientificRevolufions cluding someof those used as illustrationsabove,will seema shadetoo stlong.Phrasedin iust that way, or in any other way he can imagine,they would almostcertainlyhave beenrejected by somemembersof the group he studies.Nevertheless,if the coherenceof the researchtradition is to be understoodin terms of rules, some specificationof common ground in the correspondingarea is needed.As a result, the searchfor a body of rules competentto constitutea given normal researchtradition becomesa sourceof continual and deepfrustration. Recognizingthat frustration, however, makes it possibleto diagnoseits source.Scientistscan agree that a Newton, Lavoisier, Maxwell, or Einstein has produced an aPParentlypermanent solution to a group of outstandingproblems and still disagree,sometimeswithout being aware of it, about the parthat make those solutionsperticular abstractcharacteristics manent. They can, that is, agree in their identification of. a paradigmwithout agreeingon, or even attemptingto produce, i fril interpretationor rationalizationof it. Lack of a standard interpretationor of an agreedreduction to rules will not prevent a paradigmfrom guiding research.Normal sciencecan be determined in part by the direct inspection of paradigms,,a processthat is often aided by but does not depend upon theIndeed,the existenceof formulationof rules and assumptions. a paradigmneednot evenimply that any full set of rules exists.r Inevitably, the first efiect of thosestatementsis to raiseproblems. In ttre absenceof a competentbody of rules, what restricts the scientistto a particular normal-scientifictradition? 'direct inspectionof paradigms'mean? What can the phrase Partial ans*ets to questionslike thesewere developedby the the late Ludwig Wittgenstein,though in a very different context. Becausethat context is both more elementaryand more familiar, it will help to considerhis form of the argumentfirst. what need we know, wittgenstein asked, in order that we r Michael Polanyi has brilliantly developed a very similar lh"T."t arguing that much of the Jcientist's s,tccesi dependi upon "tacit knowledge," i.e., upon knowledqe that is acquired through practice and that cannot be articulated See his Perionul Knoulidgi (Chicago, 1958), particularly chaps. v "*fti"ittf and vi.
44
FhePriorityof Porodigms 'chair,' 'leaf,' 'game' apply terms like or unequivocallyand or without provoking argument?2 That question is very old and has generallybeen answered by saying that we must know, consciouslyor intuitivel/, what a chair, or leaf, or gamers. We must, that is, graspsomeset of attributes that all gamesand that only gameshave in common. Wittgenstein,however, concludedthat, given the way we use languageand the sort of world to which we apply it, there need be no suchset of characteristics. Though a discussionof someof the attributes sharedby a number of gamesor chairsor leaves often helps us learn how to employ the correspondingterm, there is no set of characteristicsthat is simultaneouslyapplicable to all membersof the classand to them alone. Instead, confrontedwith a previouslyunobservedactivity, we apply the 'game'because term what we are seeingbearsa close"family resemblance"to a number of the activities that we have previously learnedto call by that name.For Wittgenstein,in short, games,and chairs,and leavesare natural families,eachconstituted by a network of overlappingand crisscrossresemblances. The existenceof such a network sufficiently accountsfor our successin identifying the correspondingobject or activity. Only if the familieswe namedoverlappedand mergedgraduallyinto one another-only, that is, if there were no natural familieswould our successin identifying and naming provide evidence for a set of commoncharacteristicscorrespondingto eachof the classnameswe employ. Somethingof the samesort may very well hold for the various researchproblems and techniquesthat arise within a single normal-scientiffctradition. What thesehave in common is not that they satisfy some explicit or even some fully discoverable set of rules and assumptionsthat gives the tradition its character and its hold upon the scientific mind. Instead, they may relate by resemblanceand by modeling to one or anotherpart of the scientific corpus which the community in question al2 Ludwig Wittgenstein, Philosophical lnoestigafions, trans. G. B. M. Anscombe (New York, 1953), pp. 3I-96. Wittgenstein, however, says almost nothinq about the sort of world neccssary to support tlrc naming procedurc hc outlineij Part of the point that follows cainot thcrcforc bc attribui6d to him.
45
fhe Struclureof ScientificRevolulions
do so,they needno full set of rules.The coherencedisplayedby the reseaichtradition in which they participate may not imply even the existenceof an underlying body of rules and assumptions that additional historical or philosophical investigation might uncover. That scientistsdo not usually ask or debate what makesa particular problem or solutionlegitimate tempts us to supposethat, at least intuitively, they know the answer. But it may only indicate that neither the question nor the answeris felt to be relevantto their research.Paradigmsmay be prior to, more binding, and more completethan any set of rules for researchthat could be unequivocallyabstractedfrom them. So far this point has been entirely theoretical: paradigms could determinenormal sciencewithout the interventionof discoverablerules.Let me now try to increaseboth its clarity and ulgency by indicating some of the reasonsfor believing that paradigmsactually do operatein this manner.The ftrst, which hasalreadybeen discussedquite fully, is the severedifficulty of discoveringthe rules that have guided particular normal-scientific traditions.That difficulty is very nearly the sameas the one the philosopherencounterswhen he tries to say what all games have in common.The second,to which the first is really a corollary, is rooted in the nature of scientificeducation.Scientists,it shouldalreadybe clear,neverlearn concepts,laws,and theories in the abstract and by themselves.Instead, these intellectual tools are from the start encounteredin a historically and pedagogically prior unit that displaysthem with and through their applications.A new theory is always announcedtogether with applications to some concrete rarlge of natural phenomena; without them it would not be even a candidatefor acceptance. After it has been accepted,those same applicationsor others accompanythe theory into the textbooksfrom which the future practitioner will learn his trade. They are not there merely as 46
The PriorityoI Porodigms embroidery or even as documentation.On the contrary, the processof learninga theory dependsupon the study of applications,including practiceproblem-solvingboth with a pencil and paper and with instrumentsin the laboratory.If, for example, the studentof Newtoniandynamicsever discoversthe meaning 'space,' and'time,' he doesso less of terms like'force,''mass,' from the incompletethough sometimeshelpful definitionsin his text than by observingand participating in the application of theseconceptsto problem-solution. That processof learningby finger exerciseor by doing continues throughout the processof professionalinitiation. As the student proceedsfrom his freshmancourseto and through his doctoral dissertation,the problems assignedto him become more complexand lesscompletelyprecedented.But they continue to be closelymodeledon previousachievementsas are the problemsthat normally occupyhim during his subsequentindependentscientificcareer.One is at liberty to supposethat somewhere along the way the scientist has intuitively abstracted rules of the gamefor himself, but there is little reasonto believe it. Though many scientiststalk easily and well about the particular individual hypothesesthat underlie a concretepiece of current research,they are little better than laymenat characterizingthe establishedbasesof their field, its legitimateproblems and methods.If they have learnedsuchabstractionsat all, they show it mainly through their ability to do successfulresearch. That ability can, however, be understoodwithout recourseto hypotheticalrulesof the game. These consequences of scientific educationhave a converse that provides a third reasonto supposethat paradigmsguide researchby direct modelingas well asthrough abstractedrules. Normal sciencecan proceed without rules only so long as the relevant scientiffccommunity acceptswithout question the particular problem-solutionsalreadyachieved.Rulesshould therefore becomeimportant and the characteristicunconcern about them should vanish whenever paradigms or models are felt to be insecure.That is,moreover,exactlywhat doesoccur.The preparadigm period, in particular, is regularly marked by frequent
17
fhe Slruclureof ScienfiffcRevolulions and deep debates over legitimate methods, problems, and standards of solution, though t'hese serve rather to define schoolsthan to produce agreement.We have already noted a few of these debatesin optics and electricity, and they played an even larger role in the developmentof seventeenth-century chemistry and of early nineteenth-century geology.sFurthermore,debateslike thesedo not vanishonceand for all with the appearanceof a paradigm.Though almostnon-existentduring periods of normal science,they recur regularly just before and during scientific revolutions,the periods when paradigmsare first under attack and then subject to change.The transition from Newtonian to quantum mechanicsevoked many debates about both the nature and the standardsof physics,some of which still continue.aThere are people alive today who can rememberthe similar argumentsengenderedby Maxwell's electromagnetictheory and by statisticalmechanics.bAnd earlier still, the assimilationof Galileo'sand Newton'smechanicsgave rise to a particularly famousseriesof debateswith Aristotelians, Cartesians,and Leibnizians about the standardslegitimate to science.oWhen scientistsdisagreeabout whether the fundamental problems of their field have been solved,the searchfor While rules galnsa function that it doesnot ordinarily Possess. 3 For chemistry,seeH. Metzger, Lesdoctrineschimiquesen Franced.uddbut du XYII' d It fi; du XVIIIe $CZte( paris, 1923), pp. 2L27 ,14G49; and Marie (Cambrilge, 1958), Boas, Robert Boyl,eand Seoenteenth-Cmtury.-ChqnAp -'The Uniformitarian-Catastrophist chap. ii. For seolbqy, seeWalter F. Cannon, De6ate," fs,is,"Lf( ibOO), 38-55; and C. C. Gillispie, Genesisatd. Geology( Cambridge, Mass.,l95l)' chaps.iv-v.
48
The Priorilyof Porodigms paradigmsremain secure,however, they can function without agreement over rationalization or witrhout any attempted rationalizationat all.
clear how they can exist. If normal scienceis so rigid and if scientific communitiesare so close-knit as the preceding discussionhas implied, how can a changeof paradigm ever affect only a small subgroup?What has been said so far may have seemedto imply that normal scienceis a singlemonolithic and uniffed enterprisethat must stand or fall with any one of its paradigmsas well as with all of them together.But scienceis obviously seldom or never like that. Often, viewing all fields together, it seemsinstead a rather ramshacklestructure with litlle coherenceamong its various parts. Nothing said to this point should,however,conflict with that very familiar observation. On the contrary, substituting paradigmsfor rules should make the diversity of scientiffcftelds and specialtieseasierto understand.Explicit rules,when they exist,are usuallycommon to a very broad scientiftcgroup,but paradigmsneednot be. The practitionersof widely separatedfields,sayastronomyand taxonomic botany, are educated by exposureto quite different achievementsdescribedin very different books.And even men who, being in the same or in closely related fields, bcgin by studying many of the samebooks and achievcmcntsmay acquire rather difierent paradigmsin the courseof professional specialization. Consider,for a single example,the qrrite large and diversc community constitutedby all physicalscientists.Each member of that group today is taught the laws of, say, quantum mechanics,and most of them employ theselaws at somepoint in
49
Ihe Sfructureof ScienfificRevolutions their researchor teaching.But they do not all leam the same applications of these laws, and they are not therefore all affected in the sameways by changesin quantum-mechanical practice.On the road to professionalspecialization,a few physical scientistsencounteronly the basic principles of quantum mechanics.Others study in detail the paradigmapplicationsof theseprinciples to chemistry,still others to the physicsof the solid state,and so on. What quantum mechanicsmeansto each of them dependsupon what courseshe has had, what texts he hasread, and which journalshe studies.It follows that, though a changein quantum-mechanicallaw will be revolutionaryfor all of thesegroups,a changethat reflectsonly on one or another of the paradigm applicationsof quantum mechanicsneed be revolutionaryonly for the membersof a particular professional subspecialty.For the rest of the professionand for those who practiceother physicalsciences,that changeneed not be revolutionary at all. In short, though quantum mechanics(or Newtonian dynamics,or electromagnetictheory) is a paradigm for many scientificgroups,it is not the sameparadigmfor them all. Therefore,it can simultaneouslydetermineseveraltraditions of normal sciencethat overlap without being coextensive.A revolution produced within one of these traditions will not necessarily extendto the othersas well. One brief illustration of specialization'seffect may give this whole series of points additional force. An investigator who hopedto learn somethingabout what scientiststook the atomic theory to be asked a distinguishedphysicist and an eminent chemist whether a single atom of helium was or was not a molecule.Both answeredwithout hesitation,but their answers were not the same.For the chemistthe atom of helium was a moleculebecauseit behavedlike one with respectto the kinetic theory of gases.For the physicist,on the other hand, the helium atom was not a molecule becauseit displayed no molecular spectrum.?Presumablyboth men were talking of the samepar7 The investigator was James K. Senior, to whom I am indebted for a verbal report. Some related issues are treated in his paper, "The Vernacular of the Laboratory," PlfiIosoplry of Science, XXV (1958), 163-68.
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ThePriorilyof Porodigms ticle, but they were viewing it through their own researchtraining and practice.Their experiencein problem-solvingtold them what a molecule must be. Undoubtedly their experienceshad had much in common,but they did not, in this case,tell the two specialiststhe samething. As we proceedwe shall discoverhow consequentialparadigm differencesof this sort can occasionally be.
of Vl. Anomolyond the Emergence ScientificDiscoveries Normal science,the puzzle-solvingactivity we have itrst strccessexamined,is a highly cumulativeenterprise,emine_trtly of and precision scoPe ful in its aim, the steadyextensionof the great it fits with scientificknowledge.In all theserespects _pre-cisionthe most usial imageof scientlficwork. Yet one standard Normal science product of the scientificLnterprise_ismissin_g. sttccessfttl, when aud, theory fact or doesnot aim at noveltiesof rehowever, are, finds none.New and unsuspectedphenomella new radical and research, peatedly uncoveredby scientific iheotiei have again and again been inventedby scientists.History even suggeststhat the scientificenterprisehas developeda poiv:erfultechnique for producing surprisesof this ,rttq.tely 'th^is characteristicbf scienceis to be reconciledwith sort] If u,hat has already been said, then researchunder a Para{ig* must be a particularly efiective way of indtrcing paradigm change.Thai is what fundamentalnoveltiesof fact and theorydo. Pioduced inadvertentlyby a gameplayed under one set of rules, their assimilationrequiresthe elaborationof another set' After they have becomeparts of science,the enterprise,at.least of thoseipecialistsin whloseparticular field the noveltieslie, is neverquite the sameagain. We must now ask hJw changesof this sort can come about, jnconsideringfirst discoveries,oi noveltiesof fact, and then ventions,or noveltiesof theory. That distinctionbetweencliscovery and invention or betweenfact and theory will, however, immediatelyproveto be exceedi'glyartificial.Its artificialityis theses.Examirtan important-clueto severalof this essay'smain we shall this section' of rest ing sitected discoveriesin the epiextendcd btrt evertts qtiickly find that they _arenot isolated conlDiscovery strttctttre. recurretrt tlod.t with a regularly menceswith the-awar.tt.tt of anomaly,i.e., with the recognition that nature has somehowviolated the paradigm-induced 52
Anomolyond the Emergenceof ScienfificDiscoveries expectationsthat governnormal science.It then continueswith
establisha claim was the British scientist and divine, Joseph Priestley,who collectedthe gasreleasedby heatedred oiide^of
'F"", however, uno Bockluld, '4 Lost Letter from scheele to Lavoisier," _ I-ychnos. 1957-58, pp. Bg62, f i a difierent evaluatio" ol-5"t"ute's role.
53
Ihe Struclureof ScienliffcRevolulions
clusion that Priestleywas never able to accept. This pattern of discoveryraisesa question that can be asked about &uty novel phenomenonthat has ever entered the consciousnessof scientists.Was it Priestley or Lavoisier, if either, who ffrst discovered oxygen? In any case, when was oxygen discovered?In that form-the question could be asked even if only one claimant had existed. As a ruling about-priority and date, an answer doesnot at all concernus. Nevertheless,an attempt to produce one will illuminate the nature -of discovery, beciuse there is no answerof the kind that is sought.Discovery is not the sort of processabout which the question is aPPro-
claim to the discovery of oxygen is based uPon his priority in
thought he had obtained nitrous oxide, a specieshe already knew; in 1775he saw the gas as dephlogisticatedair, which is still not oxygen or even, for phlogistie chemists,a quite unexpected sort of gas. Lavoisier's claim may be stronger, bu_tit presentsthe sameproblems.If we refuse th9 plhn to Priestley, tnrecannot award i[ to Lavoisier for the work of.L775which led s I. B. Conant, The Ooetthroo of the PhlogkstonTheory: The Clwmical Reoolutiii of 1775-1789("Harvard Cise Histori-esin ExperimentalScience,"Case 2; Cambridge,Mass., 1950), p. 2s. This very usefuI pamphlet reprlntr many of the relevant documents.
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Anomolyond the Emergenceof ScienfificDiscoveries him to identify the gasas the "air itself entire."Presumablywe wait for the work of 1776 and L777which led Lavoisier to see not merely the gas but what the gas was. Yet even this award could be questioned,for in L777 and to the end uf his life Lavoisierinsistedthat oxygenwas an atomic "principle of acidity" and that oxygengaswasformed only when that "principle" united with caloric,the matter of heat.aShall we thereforesay that oxygenhad not yet beendiscoveredin L777?Somemay be temptedto do so.But the principleof acidity was not banished from chemistryuntil after 1810,and caloriclingered until the 1860's.Oxygenhad becomea standardchemicalsubstancebefore either of thosedates. Clearly we need a new vocabularyand conceptsftor analyzing eventslike the discoveryof oxygen.Though undoubtedly correct, the sentence,"Oxygen was discovered,"misleadsby suggestingthat discoveringsomething is a single simple act assimilableto our usual ( and alsoqtrestionable ) conceptof seeing. That is why we so readily assumethat discovering,Iike seeingor touching, should be unequivocallyattributable to an individual and to a momentin time. But the latter attribution is always impossible,and the former often is as well. Ignoring Scheele,we can safelysay that oxygenhad not been discovered before 1774,and we would probably also say that it had been discoveredby1777or shortly thereafter.But within thoselimits or otherslike them, any attempt to date the discoverymust inevitably be arbitrary becausediscoveringa new sort of phenomenon is necessarilya complexevent, one which involvesrecogn_izingboth that somethingis and ushatit is. Note, for example, that if oxygenwere dephlogisticatedair for us, we shouldinsist without hesitationthat Priestleyhad discoveredit, though we would still not know quite when. But if both observationand co_n_ceptualization, fact and assimilationto theory, are inseparally linked in discovery,then discoveryis a processand m^ust take time. only when all the relevantconceptualcategoriesare preparedin advance,in which casethe phenomenonwould not 4 _ H. Metzger, La philosoTtlie cle la muti)re clrcz Laooisier (paris, lg35); and Lraumas, oqt.cit., chap. vii.
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fhe Sfructureof ScienfiffcRevolutions be of a new sort, can discovering that and discovering what occur effortlessly,together, and in an instant. Grant now that discovery involves an extended,though not necessarilylong, processof conceptualassimilation.Can we also say that it involves a changein paradif? To that question,no general answer can yet be given, but in this case at least, the answermust be yes. What Lavoisier announcedin his PaPers from L777on was not so much the discovery of oxygen as t{re oxygen theory of combustion.That theory was the keystonefor a reformulation of chemistry so vast that it is usually called the chemical revolution. Indeed, if the discoveryof oxygenhad not been an intimate part of the emergenceof a new paradigm for chemistry, the question of priority from which we began would never have seemedso important. In this caseas in others, the value placed upon a new phenomenonand thus upon its discoverer varies with our estimate of the extent to which the phenomenonviolated paradigm-induced anticipations. Notice, however, since it will be important later, that the discovery of oxygen was not by itself the cause of the change in chemical theory. Long before he played any part in the discovery of the new gas, Lavoisier was convinced both that something was wrong with the phlogiston theory and that burning bodies absorbed some part of the atmosphere.That much he had recorded in a sealednote depositedwith the Secretaryof the French Academy in 1772.6What the work on oxygendid was to give much additional form and structure to Lavoisier's earlier sensethat somethingwas amiss.It told him a thing he was already prepared to discover-the nature of the substancethat combustionremovesfrom the atmosphere.That advanceawarenessof dificulties must be a significantpart of what enabled Lavoisier to seein experimentslike Priestley'sa gasthat Priestley had been unable to see there himself. Conversely,the fact that a maior paradigm revision was needed to seewhat Lavoisier saw must be the principal reasonwhy Priestleywas, to the end of his long life, unable to seeit. 6 The most authoritative account of the origin of Lavoisier's discontent is Henry Guerlac, Lanoisier-the Crucbl Iear: fhc Backgtound atd. Otigln ol n* First Erpefiments on Combustion in 1772 (lthaca, N.Y., f96l ).
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Anomolyond the Emergenceof ScienfificDiscoveries Two other and far briefer exampleswill reinforce much that has iust been said and simultaneouslycary us from an elucidation of the nature of discoveriestoward an understandingof the circumstancesunder which they emergein science.In an effort to represent the main ways in which discoveriescan come about, theseexamplesare chosento be different both from each other and from the discovery of oxygen. The ffrst, X-rays, is a classiccaseof discoverythrough accident, a type that occurs more frequently than the impersonalstandardsof scientific reporting allow us easilyto realize.Its story openson the day that the physicist Roentgen intermpted a normal investigationof cathode rays becausehe had noticed that a barium platinocyanide screen at some distance from his shielded apparatus glowed when the dischargewas in process.Further investigations-they requiredsevenhectic weeksduring which Roentgen rarely left the laboratory-indicated that the causeof the glow camein straight lines from the cathoderay tube, that the radiation cast shadows,could not be defected by ^ magnet, and much elsebesides.Before announcinghis discovery,Roentgen had convincedhimself that his efiect was not due to cathode rays but to an agentwith at Ieastsomesimilarity to light.6 Even so brief an epitomerevealsstriking resemblances to the discoveryof oxygen: before experimentingwith red oxide of mercury, Lavoisier had performed experiments that did not producethe_resultsanticipatedunder the phlogistonparadigm; Roentgen'sdiscoverycommencedwith the recognitionthat his screenglowed when it should not. In both casesthe perception of anomaly-of a phenomenon,that is, for which his paradigm had not readied the investigator-played an essentialrole in preparingthe way for perceptionof novelty. But, again in both cases,the perception that somethi"g had gone wrong was only the prelude to discovery. Neither oxygen nor X-rays emerged without a further processof experimentationand assimilation. At what point in Roentgen'sinvestigation,for example,ought we say that X-rays had actually been discovered?Not, in any u-_!. W. Taylor,,Plrysics, the Pioneer Science (Boston, lg4l ), pp. Zg0-g4; and _ T. W. Chalnrers,Historic Rescurches(London, lg4g), pp. 218-i-g
fhe Slruclureof ScienfiffcRevolufions case,at the ffrst instant, when all that had been noted was a glowing screen.At least one other investigator had seen that glow and, to his subsequentchagin, discoverednothing at all.? Nor, it is almost as clear, can the moment of discovery be pushedforward to a point during the last week of investigation, by whictr time Roentgen was exploring the properties of the new radiation he had already discovered.We can only say that X-raysemergedin Wiirzburg betweenNovember8 and December 28, 1895. In a third area,however,the existenceof signiffcantparallels between the discoveriesof oxygen and of X-rays is far less apparent. Unlike the discoveryof oxygen,that of X-rays was not, at least for a decadeafter the event, implicated in any obviousupheavalin scientiffctheory. In what sense,then, can the assimilationof that discoverybe said to have necessitatedparadig* change?The case for denying such a change is very strong. To be sure, the paradigmssubscribedto by Roentgen and his contemporariescould not have been trsed to predict
Lavoisier'sinterpretationof Priestley'sgas.On the contrary, in f895 acceptedscientifictheory and practiceadmitted a number of forms of radiation-visible, infrared, and ultraviolet. Why could not X-rayshave been acceptedas iust one more form of a well*nown classof natural phenomena?Why were they not, for example,received in the same\Mayas the discoveryof an additionil chemicalelement?New elementsto fill empty places in the periodic table were still being soughtand found in Roentgen's day. Their pursuit was a standard project for normal Jcience,and successwas an occasiononly for congratulations, not for surprise. ? E. T. Whittaker, A History of the Theories of Aether and Electricity, | (2d' ed.; London, l95t),358, n. l-. sir George Thomson has informed me of a second ,r""r miis. Alerted by unaccountably fogged photographic plates, Sir William Crookes was also on the track of the discovery.
58
Anomolyond the Emergence of ScienlificDiscoveries X-rays, however, were greeted not only with surprise but with shock.Lord Kelvin at first pronouncedthem an elaborate hoax.8Others,though they could not doubt the evidence,were clearly staggeredby it. Though X-rays were not prohibited by establishedtheory, they violated deeply entrenchedexpectations.Thoseexpectations,I suggest,were implicit in the design and interpretationof establishedlaboratoryprocedures.By the 1890's cathode ray equipment was widely deployed in numerous European laboratories.If Roentgen'sapparatus had producedX-rays,then a number of other experimentalists must for sometime have beenproducing thoserays without knowing it. Perhapsthoserays,which might well have other unacknowledged sourcestoo, were implicated in behavior previously explained without referenceto them. At the very least, several sortsof long familiar apparatuswould in the future have to be shielded with lead. Previously completed wort on normal projectswould now have to be done againbecauseearlier scientists had failed to recognize and control a relevant variable. X-rays,to be sure,openedup a new ffeld and thus added to the potential domain of normal science.But they also, and this is now the more important point, changedfields that had already existed. In the processthey denied previously paradigmatic typesof instrumentationtheir right to that title. fn short, consciouslyor not, the decisionto employ a particular pieceof apparatusand to useit in a particular way carriesan assumptionthat only certain sorts of circumstanceswill arise. There are instrumentalas well as theoreticalexpectations,and they have often played a decisiverole in scientificdevelopment. One such expectation is, for example, part of the story of oxygen'sbelateddiscovery.Using a standardtest for "the goodnessof air," both Priestleyand Lavoisiermixed two volumesof their gaswith onevolumeof nitric oxide,shookthe mixture over water, and measuredthe volume of the gaseousresidue.The previous experiencefrom which this standard procedure had evolved assuredthem that with atmosphericair the residue of sir wi,iam rhomson BaronKetoinof r,,*tjtili,::I;,iHtfii: [#r.'tr"
59
fhe Sfruclureof ScienfificRevolufions would be onevolumeand that for any other gas (or for polluted air) it would be greater.In the oxygenexperimentsboth found a residue closeto one volume and identified the gas accordittgly. Only much later and in part through an accidentdid Priestley renouncethe standardprocedureand try mixing nitric oxide with his gas in other proportions. He then found that with quadruple the'volume of nitric oxide there was almostno residue at all. His commitmentto the original test procedure-a procedure sanctioned by much previous experience-had been of gasesthat simultaneouslya commitmentto the non-existence could behaveas oxygendid.e Illustrations of this sort could be multiplied by reference,for example,to the belated identiffcation of uranium fission.One reasonwhy that nuclear reaction proved especially difficult to recognizewas that men who knew what to expect when bombarding uranium chosechemical testsaimed mainly at elements from the upper end of the periodic table.loOught we conclude from the frequency with which such instrumental commitments prove misleading that scienceshould abandon standard tests and standardinstruments?That would result in an inconceivable method of research.Paradigmproceduresand applications are as necessaryto scienceas paradigm laws and theories,and they have the sameeffects.Inevitably they restrict the phenomenological field accessiblefor scientific investigation at any o Conant,op. cit,, pp. l&20.
with close aftliations to physics,we cannot bring ourselvesto this le_apryhich would contradict all pre'iious experienceof nucliar physics.It may be that a seriesof strangeaccidentsrenderi our resultsdeceptiie" ( Otto Hahh and Fritz Strassman,"ULer den Nachweisund das Verhalten der bei der Bestrahlungdes Urans mittels Neutronen entstehendedErdalkalimetalle," Db Naturaissensc@ten, XXVU [f939], l5).
@
Anomolyond the Emergence of ScienfificDiscoyeries given time. Recognizingthat much, we may simultaneouslysee an essentialsensein which a discoverylike X-rays necessitates paradigmchange-and thereforechangein both proceduresand expectations-fora specialsegmentof the scientificcommunity. As a result,we may alsounderstandhow the discoveryof X-rays could seemto open a strangenew world to many scientistsand could thus participate so effectively in the crisis that led to twentieth-centuryphysics. Our ffnal exampleof scientiffcdiscovery,that of the Leyden iar, belongsto a classthat may be describedas theory-induced. Initially, the term may seemparadoxical.Much that has been said so far suggeststhat discoveriespredicted by theory in advance are parts of normal scienceand result in no new sort of. fact. I have, for example,previouslyreferred to the discoveries of new chemical elementsduring the secondhalf of the nineteenth century as proceedingfrom normal sciencein that way. But not all theories are paradigm theories.Both during prep-aradigmperiodsand during the crisesthat lead to large-scale changesof paradigm, scientistsusually develop many speculative and unarticulatedtheoriesthat can themselvespoint the ryay to discovery.Often, however, that discoveryis not quite the one anticipatedby the speculativeand tentativehypothesis. only as experimentand tentative theory are together articuIated to a match doesthe discovery emergeand the theory become a paradigm The discoveryof the Leyden jar displaysall thesefeaturesas well as the others we have observedbefore. when it began, there was no single pa_radigm for electricalresearch.Inqtead,o number of theories,all derived from relatively accessibii:ph"nomena,were in competition.None of them succeededin ordering the whole variety of electricalphenornenavery well. That failure is the sourceof severalof the anomaliesihat provide backgroundfor the discovery of the Leyden jar. oni of the comp_eting schoolsof electricianstook electricityto be a fluid, a-ndjhalconception led a number of men to aitempt bottling the fluid by holding a water-filledglassvial in their-handsa,rJ touching the water to a conductor suspendedfrom an activc 6l
fhe Slruclureol ScienfificRevolulions electrostaticgenerator.On removing the iar from the machine and touching the water (or a conductor connectedto it) with his free hand, each of theseinvestigatorsexperienceda severe shock.Those first experimentsdid not, however,provide electricianswith the Leyden iar. That deviceemergedmore slowly, and it is again impossibleto say iust when its discoverywas completed.The initial attemptsto store electricalfluid worked only becauseinvestigatorsheld the vial in their hands while standing upon the ground. Electricianshad still to learn that the iar required an outer as well as an inner conducting coating and that the fuid is not really storedin the jar at all. Somewhere in the courseof the investigationsthat showedthem this, and which introduced them to severalother anomalouseffects,the device that we call the Leyden iar emerged.Furthermore,the experimentsthat led to its emergence,many of them performed by Franklin, were alsothe onesthat necessitatedthe drasticrevision of the fuid theory and thus provided the ffrst full paradig- for electricity.ll To a greateror lesserextent (correspondingto the continuum from the shockingto the anticipatedresult), th" characteristics common to the three examplesabove are characteristic of all discoveriesfrom which new sortsof phenomenaemerge.Those characteristicsinclude: the previousawarenessof anomaly,the gradual and simultaneousemergenceof both observationaland conceptualrecognition,and the consequentchangeof paradigm categoriesand proceduresoften accompaniedby resistance. There is evenevidencethat thesesamecharacteristiesare built into the nature of the perceptualprocessitself. In a psychological experimentthat deservesto be far better known outsidethe trade,Bruner and Postmanaskedexperimentalsubjectsto identify on short and controlled exposurea seriesof playing cards. Many of the cards were normal, but somewere made anoma11 For various stirges in thc Leyde'n jirr's evolution, see I. B. Cohen, Franklin_ atd Newton: An lnf,uiry into Specufuttiue Neutonian. Erperimental Scbnce and Franklin's Work in Eleitrlcltg as an Example Thereol (Philadelphia, lg56), pp. 385-86,400-406, 452-67,506-7. The last stage is described by Whittaker, oP. cit., pp.50-52.
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Anomolyqnd the Emergenceof ScrentificDiscoyeries lous,e.9.,a red six of spadesand a black four of hearts.Each experimentalrun was constitutedby the displayof a singlecard to a single subject in a seriesof gradually increasedi*por,rr"r. A-fter each exposurethe subject was askedwhat he had seen, and the run wasterminatedby two successive correctidentifications.12 Even on the shortestexposuresmany subiectsidentifiedmost of the cards,and after a smallincreaseall the subiectsidentified them all. For the normal cardstheseidentiffcationswere usually correct,but the anomalouscardswere almostalwaysidentified, without apparent hesitation or puzzlement, as normal. The black four of heartsmight, for example,be identiffedas the four of either spadesor hearts.Without any awarenessof trouble, it was immediatelyfftted to one of the conceptualcategoriespre. pared by prior experience.One would not evenlike to say that the subjectshad seensomethingdifferent from what they identiffed. With a further increaseof exposureto the anomalous cards,subjectsdid begin to hesitateand to display awarenessof anomaly.Exposed,for example,to the red six of spades,some would say: That'sthe six of spades,but there'ssomethingwrong with it-the black hasa red border.Further increaseof exposure resultedin still more hesitationand confusionuntil finally,and sometimesquite suddenly, most subjectswould produce the correct identificationwithout hesitation.Moreover,after doing this with two or three of the anomalouscards,they would have Iittle ftrrther difficulty with the others.A few subjects,however, were never able to make the requisiteadjtrstmentof their categories. Even at forty times the averageexposurercqrrired to recognizenormal cards for what they were, lllore thirn l0 pcr. cent of the anomalouscardswere not correctlyidentificd.Ancl the subiectswho then failed often experiencedactrtepersonal distress.One of them exclaimed:"I cAn't make the strit out, whateverit is. It didn't evenlook like a card that tinre.I don't know what colorit is now or whetherit's tr sptrdeor a heart.I'nr t'I, S..-B_runer_andLco Postttutn, "On tlrc'Pcrception of Incrrrrgrrrity: A _ Paradignr," Iournal of Pusonality, XVIII (1949), gO0-:2g.
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fhe Sfruclureof ScienliffcRevolulions not even sure now what a spadeIookslike. My Godl"rs In the next section we shall occasionallysee scientistsbehaving this way too. Either as a metaphoror becauseit reflectsthe nature of the mind, that psychologicalexperiment provides a wonderfully simpleand cogentschemafor the processof scientificdiscovery. In science,as in the playing card experiment,novelty emerges only with difficulty, manifestedby resistance,against a background provided by expectation.Initially, only the anticipated and usual are experiencedeven under circumstanceswhere anomaly is later to be observed.Further acquaintance,however,doesresult in awarenessof somethingwrong or doesrelate the efiectto somethingthat has gonewrong before.That awarenessof anomalyopensa period in which conceptualcategories are adiusteduntil the initially anomaloushasbecomethe anticipated. At this point the discoveryhas been completed.I have already urged that that processor one very much like it is involved in the emergenceof all fundamentalscientificnovelties. Let me now point out that, recognizingthe process,we can at Iast begin to seewhy normal science,a pursuit not directed to noveltiesand tending at first to suppressthem, shouldnevertheIessbe so effectivein causingthem to arise. In the developmentof any science,the first received paradigm is usuallyfelt to accountquite successfullyfor most of the observationsand experimentseasilyaccessibleto that science's practitioners.Further development,therefore,ordinarily calls for the constructionof elaborateequipment,the development of an esotericvocabulary and skills, and a refinementof concepts that increasinglylessenstheir resemblanceto their ttsttal leads, orl common-sense prototypes.That professiortalization the one hand, to an immenserestrictionof the scientist'svision and to a considerableresistanceto paradigm change.The sciencehas becomeincreasinglyrigid. On the other hand, within thoseareasto which the paradigmdircctsthe attentionof the ls lbiil., p. 2f8. My colleague Postman tells me that, though knowing all about the aiparatus and displai in irdvance, he neverthelcssfound looking at the incongruoui cards acutely uirc6mfortable.
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Anomoly ond the Emergenceof ScientificDiscoveries group, normal scienceleads to a detail of information and to a precision of the observation-theory match that could be achieved in no other way. Furthermore, that detail and precision-of-matchhave a value that transcendstheir not alwaysvery high intrinsic interest. Without the special apparatus that is constructed mainly for anticipated functions, the results that lead ultimately to novelty could not occur. And even when the apparatus exists, novelty ordinarily emergesonly for the man who, knowing uith precision what he should expect,is able to recognize that something has gone wrong. Anomaly appears only against the background provided by the paradigm. The more preciseand far-reaching that paradig- is, the more sensitive an indicator it provides of anomaly and hence of an occasion for paradigm change. In the normal mode of discovery, even resistanceto changehas a use that will be explored more fully in the next section.By ensuringthat the paradigm will not be too easily surrendered,resistanceguaranteesthat scientists will not be lightly distracted and that the anomaliesthat lead to paradigm change will penetrate existing knowledge to the core. The very fact that a signiftcant scientific novelty so often emerges simultaneously from several laboratories is an index both to the strongly traditional nature of normal scienceand to the completenesswith which that traditional pursuit prepares the way for its own change.
Vll. Crisis ond the Emergence of Scientific Theories All the discoveriesconsideredin SectionVI were causesof or contributorsto paradigm change.Furthermore,the changesin which thesediscoverieswere implicated were all destructiveas well as constructive.After the discoveryhad been assimilated, scientistswere able to account for a wider range of natural phenomenaor to account with greater precision for some of those previously known. But that gain was achievedonly by discardingsomepreviouslystandardbeliefs or proceduresand, simultaneously,by replacingthose componentsof the previous paradigm with others. Shifts of this sott are, I have argued, associatedwith all discoveriesachievedthrough normal science, exceptingonly the unsurprisingonesthat had been anticipated in all but their details. Discoveriesare not, however, the only paradigm changes.In sourcesof thesedestructive-constructive the similar, but usually to consider this sectionwe shall begin of new theories. invention result from the shifts that far larger, fact sciences and theory, that in the argued already Having and not permanently invention, categorically and are discovery distinct, we can anticipateoverlapbetweenthis sectionand the last. (The impossiblesuggestionthat Priestleyfirst discovered oxygenand Lavoisier then invented it has its attractions.O*ygenhas alreadybeenencounteredasdiscovery;we shall shortly meet it again as invention.) In taking up the emergenceof new theorieswe shall inevitably extend our understandingof discovery as well. Still, overlap is not identity. The sorts of discoveriesconsideredin the last sectionwere not, at least singly, responsiblefor such paradigm shifts as the Copernican,Newtonian, chemical,and Einsteinian revolutions.Nor were they responsiblefor the somewhatsmaller,becausemore exclusively professional,changesin paradigmproducedby the wave theory of light, the dynamicaltheory of heat, or Maxwell'selectromagnetic theory. How can theories like these arise from normal 6
of Scienfiffcfheories Crisisond fhe Emergence science,an activity even less directed to their pursuit than to that of discoveries? If awarenessof anomalyplays a role in the emergenceof n_ew sortsof phenomena,it should surpriseno one that a similar but more piofound awareness is pierequisite to_ all accep!1bfe of theory. On this poinl historical evidence is, I think, change^s entirely unequivocal. The state of Ptolemaic astronomy was a scandalbefoie Copernicus'announcement.lGalileo'scontributions to the study bf motion depended closely uPon difficulties discoveredin Aristotle'stheory by scholasticcritics.2Newton's new theory of light and color originated in the discovery that none of the existing pre-paradigmtheorieswould account for the length of the spectt,t*, and the waye theory that replaced Newtoi's was anno-uncedin the midst of growing concernabout anomaliesin the relation of difiraction and polarization effects to Newton's theory.s Thermodynamicswas born from the collision of two existingnineteenth-centuryphysical theories,and quantum mechanicJfrom a variety of difficulties surrounding black-body radiation, specific heats, and the photo_electric efrect.aFurthermore,in ill these casesexcept that of Newton the awarenessof anomalyhad lastedso long and penetratedso deep that one can appropriatelydescribethe fields affectedby it aJ in a stateof growing crisis.Becauseit demandslarge-scale paradigm destruction and major shifts in the problems ""i iechniques of normal science,the emergenceo{ new theoriesis precededby a period of Pronouncedprofessionaling"t "tuily (London, 1954), P. 16. 1A. R. Hall, The ScientificReoolution,$0f18N in the Middle.lg?r lMadison, 2 MarslrallClagctt,Thc Scienceof_L[echanics of medievalelementsin a Wis., l9S9), parii II-III. A-.Koy16-displays ""T!gt C;iil"r'r tlrought in his EhtdesGalitieniei ( Paris,1939), particularly Vol. I. 3 For Newton, seeT. S. Kuhn, "Newton's Optical Pa-pers,"in lsaac Neu;ton's paoerco6 asfters in Natural PliIosophy, ed. i. B. Cohen (Cambridge, Ivlass, seeE. T. Whittaker,A i9's3l- o1,.27-45.For the prelude -(2d 'Aaher to'thL wave theor/, ed.; Lond_on,1951), and. Electricity,| ihe fheortes of ilriii'it (rev. ed.; London, b;-105, J"J w. Whewell, Ilistory of tlrc lnclucti;e Sciences 1847),II,39M66. 4 For thermodynamics,see SilvanusP. T}-ompso\ Life of William Thonxon nnrii Xelotnof Largs ( London,l9l0), I, 26&81. I.l th" quantumtlreory,scc i.iiJ n"i"h* , Thc Qiuntutn Tlrcory,trans.II. S. Ilatfteld and II. L. Brose( London, 1922),chaps.i-ii.
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fhe Sfructureof ScienlificRevolulions s-ecurity.As on-e _might-expect,that insecurity is generatedby the persistentfailure of thi puzzles of normal sciJnce to com'e out a_s $ey should. Failure of existing rules is the prelude to a searchfor new ones.
Ptolemy's system.Given a particular discrepanc/r astronomers w_ereinvariably able to eliminate it by making someparticular adiustment in Ptolemy's systemof compoundedcircles. But as time went on, a man looking at the net result of the normal researcheffort of many astronomerscould observethat astronomy's complexity was increasingfar more rapidly than its accuracy and that a discrepancycorrectedin one place was likely to showup in another.s Because the astronomical tradition was repeatedly interrupted from outside and because,in the absenceof printing, communicationbetween astronomerswas restricted,thesedifA History of Astronony from Tlulec to KepLer(2d ed.; --oI.__L..E, D-1eyer, New York, 1953), chaps.ri-xii.
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of ScienfificTheories Crisisond fhe Emergence ficultieswere only slowly recognized.But awarenessdid come' By the thirteenth centuiy Alfonso X could proclaim that if God had coosultedhim when creating the universe,he would have receivedgood advice.In the sixteenthcentury, Copernicus'coworker, domenico da Novara, held that no system so cumbersome and inaccurateas the Ptolemaichad becomecould possibly be true of nature. And Copernicushimself wrote in the Preiace to the De Reoolutionibis that the astronomical tradition he inherited had finally created only a monster. By_the early sixteenth century an incre,asingnumber of Europe'sbest astronomerswere recognizingthat the astronomicalparadigm was failing in application to its own traditional problems.That recognitioo *a-t prerequisite to Copernicus' reiection of the PtolJmaic paradifm attd his searchfor a new one. His famous preface stil provides one of the classicdescriptionsof a crisis state.o Breakdown of the normal technical puzzle-solvingactivity is not, of course,the only ingredientof the astronomicalcrisisthat faced Copernicus.An extended treatment would also discuss that made the for calendarreform, a Pressur_e the socialltess*" -precession a fuller addition' Inparticularly urgent. puzzle of -account the rise Aristotle, of would considermedieval criticism historical signiffcant of RenaissanceNeoplatonism,and other elementsbesides.But technical breakdown would still remain the core of the crisis. fn a mature science-andastronomyhad becomethat in antiquity-external factorslike thosecited above are principally signiffcantin determititg the _timing_ofbreakdown, thJease with which it can be recognized,and the area in which, becauseit is given particular attention, the breakdown ffrst occurs.Though immenselyimportant, issuesof that sort are out of boundsfor this essay. If that much is clear in the caseof the Copernicanrevolution, let us turn from it to a secondand rather different example,the crisisthat precededthe emergenceof Lavoisier'soxygentheory of combuslion.In the 1770'smany factors combinedto generate 6T. S. Kuhn, The CopernicanReoolutfon(Cambridge,Mass.,1957), pp. 135-43.
fhe Structureof ScienfificRevolulions a crisis in chemistry,and historiansare not altogether agreed about either their nature or their relative importance.Bui two of the-mare generallyacceptedas of first-rati significance:the rise of pnzumatic chemistry and the question Jf weight relations. The history of the ffrst beginsin the seventeentlicentury np and its deploymentin chemi-
:'';'1",,:TffiJ,:?;Ji chemical reactions. Butwithil*t:-::ff:'Jr"-[%ffijJi
that they may not be exceptionsat all-chtmists contiirued to believe that air wry lhe olly rott of gas.until r7s6, when Joseph B-1":\ showedthat ffxed air (cor) was consistentlydistinguishablefrom normal air, two samplesof gaswere thbught to be distinct only in their impurities.t After Black'swork the investigationof gasesproceededrapid^and Iy,_most n_otablyi_nthe handJ of Cavendisli, priestley, sche_ele, w-hotogethe-rdeveloped a number of new tectrniques c-apableof distinguishingone sampleof gas from another.-All thesemen, from Black thlough scheele,bJlieved in the phlogiston theory and often employedit in their designand intiqprCtution of experiments.scheeleactually ffrst producedoxygin by an elaborate chain of experimentsdesigned to dephlogi-sticatl heat. Yet the net result of their experimentswas a varief, of gas samplesand gas properties so elaborate that the phiogist-on theory proved ingeasingly little able to cope with iaboiatory elperien-ce.thgugh none of thesechemistsiuggested that thL theory_shguld_be replaced, they were unable to apply it consistently.By the time Lavoisier beganhis experimenti on airs in the early 1770's,there were almost as many versionsof the phlogiston theory as there were pneumatic chemists.sThat stnrtH*toryof chen*mry lg51),pp. Qd ed.;London, nrJi,l*tttfl'f a Though their main conc€rn is with a slightly later period, much relevant material is scattered throughout J. R. Partingtori and Douglas McKie's "Historical studies on the Phlogiston Theory," eirwls of sclerc6,II (rggz),961404; III (1988), l-58,837-7t; and tV (tggg), gg7-7t.
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of Scientificfheories Crisisond the Emergence proliferation of versionsof a the crisis.In his preface,Copernicur The increasingvaguenessanc giston theory for pneumatic chemis only sourceof the crisisthat confror .,rth concernedto explainthe gain in weight that most bodies experiencewhen bnrtted or roasted,and that againis a problem wiih a long prehistory. At least a few Islamic chemistshad known thal Jome meials gain weight when roasted. In the seventeenthcenfury sett"tal investigatorshad concluded from this same fact that a roastedmetal takes up some ingredient firomthe atmosphere.But in the seventeenthcentury that conto most chemists.If chemicalreacclusionseemedunnecessary color, and texture of the ingrevolume, the tions could alter weight as well?-Weight was not alter they dients, why should of quantity of matter. Bemeasure not alwayt t"k"t to be the an isolated phenomeremained roasting sides,*iigttt-gain on weight on roasting lose (e.g., wood) non. Mosinatrrral bodies they-should' to say later was as the phlogistontheory During the eight""nth century, however,theseinitially adequate reiponsesto the problem of weight-gainbecameincreasiirgly difficult to maintain. Partly becausethe balancewas incrEasinglyused as a standardchemicaltool and partly because the dev-eiopmentof pneumatic chemistry made it possille 1nd desirabletb retain tfie gaseousProductsof reactions,chemists discoveredmore and more casesin which weight-gain accom' the gradual assimilationof panied roasting. Simultaneou_sly_, to insist that gain in chemists i.lewton'sgravitationaltheory led weight must mean gain in quan' did not result in reiection of theory could be adiustedin maI negativeweight, or perhapsfir, ter-edthe roasted body as phlo explanationsbesides.But if the lead to rejection,it did lead to studiesin which this problem bulked large. One of them, "On
ffie Sfructureof ScienlificRevolufions
consider now, as a third and ftnal example, the late nineteenth century crisis in physics that ptepared the way for the emergenceof relativity theory. one root of that crisis can be
0 H. Guerlac, LaooisW-the Cructal Year (Ithaca, N.Y., 196l). Ttre entire book doctnents the evolution and ffrst recognitim of a crisis. For a clear statement of the situation with respectto Lavoisier, seep. 85. _.toM"TJaqmgl, Cl*qq_oJ .SWe: Tfu Hlstoty of Theottcs of Spce tn Physbs (Cambridge,Mass.,l9$t),-pp. ll+24.
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Crisisond the Emergenceof ScienfificTheories them during the early decadesof the eighteenth century to_be resurrectedbnly in the last decadesof the nineteenth when they had a very different relation to the practice of physics. The technical problems to which a relativistic philosophy of spacewas ultimately to be related began to enter normal sciencewith the acceptanceof the wave theory of light after about 1815,though they evokedno crisis until the 1890's.If light_is wave motion propagatedin a mechanicalether governedby Newton's Lawi, then both celestialobservationand terrestrial experiment become potentially capable of detecting drift thiough the ether. Of the celestial observations,only those of aberration promised sufrcient accuracy to provide relevant information, and the detection of ether-drift by aberration measurementstherefore becamea recognizedproblem for normal research.Much specialequipment was built to resolveit. That equipment, however, detected no observable drift, and the problem was therefore transferredfrom the experimentalists and observersto the theoreticians.During the central decades of the century Fresnel, Stokes,and others devised numerous articulations of the ether theory designedto explain the failure to observe drift. Each of these articulations assumedthat a moving body dragssomefraction of the ether with it. And each was sufficiently successfulto explain the negative results not only of celestialobservationbut alsoof terrestrialexperimentation, including the famous experimentof Michelsonand Morluy.tt There was still no conflict excepting that between the various articulations.In the absenceof relevant experimental techniques,that conflict never becameacute. The situation changedagain only with the gradual acceptance of Maxwell's electromagnetictheory in the last two decades of the nineteenth century. Maxwell himself was a Newin general tonian who believedthat light and electromagnetism of mechandisplacements of the a to variable particles were due electricity and ical ether. His earliestversionsof a theory for rr Joseph Larmor, Aether and Matter . . . lnclud.ing a Discussion of the lnfluence ol the Earth's Motion on Optical Phenomena (Cambridge, 190O), pp. 6-20,32V22.
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fhe Struclureof ScienliffcRevolufions m_agnetismmade direct use of hypothetical properties with which he endowedthis medium. Thise were dropped from his final version, but he still believed his electromagnetictheory compatiblewith somearticulationof the Newtonianmechanical
Maxwell'stheory, despiteits Newtonian origin, ultimately produced a crisis for the paradigm from which it had sprung.tg Furthermore,the locus at which that crisisbecamemost acute was provided by the problemswe have just been considering, thoseof motion with respectto the ether.
thereforewitnesseda long seriesof attempts,both experimental and theoretical,to detect motion with respectto the ether and to work ether drag into Maxwell'stheory.The former were uniformly unsuccessful, though someanalyststhought their results equivocal.The latter produced a number of. promising starts, particularly thoseof Lorentz and Fitzgerald,but they also disclosedstill other puzzlesand finally resultedin just that proliferation of competing theoriesthat we have previously found to be the concomitantof crisis.laIt is againstthat historicalsetting that Einstein'sspecialtheory of relativity emergedin 1905. Thesethree examplesare almostentirely typical. In eachcase a novel theory emergedonly after a pronouncedfailure in the 12 R. T. Glazebrook, Iamg1 Clerk Marwell and Modern Physics (London, 1896), chap. ix. For Maxwell's final attitude, see his own book, A Treatise on Electricity and, Magrctisrn (3d ed.; Oxford, 1892), p.470. rB For astronorrry's role in the development of mechanics, see Kuhn, op, cit., chap. vii. 1r Whittaker, op. cit.,I, 38G-410; and II (London, 1953), 27-40.
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of Scienfificfheories Crisisond the Emergence normal problem-solvingactivity. Furthermore, except for the caseof Copernicusin which factorsexternalto scienceplayed a particularly large role, that breakdownand the proliferation of theoriesthat is its sign occurredno more than a decadeor two before the new theory'senunciation.The novel theory seemsa direct responseto crisis.Note also,though this may not be quite so typical, that the problemswith respectto which breakdown occurredwere all of a type that had long been recognized.Previouspracticeof normal sciencehad given every reasonto consider them solvedor all but solved,which helps to explainwhy the senseof failure, when it came, could be so acute. Failure with a new sort of problem is often disappointingbut never surprising.Neither problemsnor puzzlesyield often to the first attack.Finally, theseexamplesshareanothercharacteristicthat may help to make the casefor the role of crisisimpressive:the solutionto eachof them had been at leastpartially anticipated during a period when there was no crisis in the corresponding science;and in the absenceof crisis those anticipationshad been ignored. The only completeanticipation is also the most famous,that of Copernicusby Aristarchusin the third century n.c. It is often said that if Greek sciencehad been less deductive and less ridden by dogma,heliocentricastronomymight have begun its developmenteighteen centuriesearlier than it did.15But that is to ignore all historical context.When Aristarchus'suggestion wasmade,the vastly more reasonablegeocentricsystemhad no needsthat a heliocentricsystemmight even conceivablyhitve fulfflled. The whole developmentof Ptolemaicastronomy,both its triumphs and its breakdown,falls in the centuriesafter Aristarchus' proposal.Besides,there were no obvious reasonsfor taking Aristarchusseriously.Even Copernicus'more elaborate proposalwas neither simpler nor more accuratethan Ptolemy's system.Availableobservationaltests,aswe shall seemore clear15 For Aristarchus' wor\ see T. L, Heath, Aristarchus of Samos: The Ancient Copernicus (Oxford, l9l8), Part II. For an extreme statement of the traditional position about the neglect of Aristarchus' achievement, see Arthur Koestler, ?he Sleepualkers: A History of Man's Clwnging Yision ol the l|nioerse (London, 1959),p.50.
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- ff:{'l;fr^nn ^9d,.'Kftr5*-r t\-^ fheTlructureof ScienfificRevolulions
J
lv below, provided no basisfor a choicebetweenthem. Under one of the factorsthat led astronomersto thosecircumstances, Copernicus(and one that could not have led them to Aristarchus) was the recognizedcrisis that had been responsiblefor innovationin the ffrst place.Ptolemaicastronomyhad failed to solve its problems;the time had come to give a competitor a chance.Our other two examplesprovide no similarly full anticipations.But surely one reasonwhy the theoriesof combustion by absorptionfrom the atmosphere-theoriesdevelopedin the seventeenthcentury by Rey, Hooke, and Mayow-failed to get a sufficienthearingwas that they madeno contactwith a recognized trouble spot in normal scientiftcpractice.loAnd the long neglect by eighteenth- and nineteenth-centuryscientists of Newton's relativistic critics must largely have been due to a similar failure in confrontation. Philosophersof sciencehave repeatedly demonstratedthat more than one theoretical constructioncan always be placed upon a given collection of data. History of scienceindicates that, particularly in the early developmentalstagesof a new paradigm,it is not evenvery difficult to invent such alternates. But that invention of alternatesis just what scientistsseldom undertake except during the pre-paradigmstage of their science'sdevelopmentand at very special occasionsduring its subsequentevolution.So long as the tools a paradigm supplies continue to prove capable of solving the problems it defines, sciencemovesfastestand penetratesmost deeply through conffdent employment of those tools. The reason is clear. As in manufactureso in science-retoolingis an extravaganceto be reservedfor the occasionthat demandsit. The signiffcanceof crisesis the indication they provide that an occasionfor retooling ha.sarrived. 1oPartington, op. cit., pp. 78-85.
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Vlll. The Responseto Crisis Let rrs then assumethat crisesare a necessaryprecondition for the emergenceof novel theoriesand ask next how scientists respondto their existence.Part of the answer,as obviousas it is important, can be discoveredby noting ffrst what scientists never do when confronted by even severeand prolonged anomalies.Though they may begin to losefaith and then to consider alternatives,they do not renouncethe paradigm that has led them into crisis.They do not, that is, treat anomaliesascounterinstances,though in the vocabulary of philosophy of science that is rryhatthey are. In part this generalizationis simply a statementfrom historic fact, based upon exampleslike those given above and, more extensively,below. Thesehint what our later examinationof paradigmrejectionwill disclosemore fully: once it has achievedthe statusof paradigm,a scientiffctheory is declaredinvalid only if an alternatecandidateis availableto take its place. No processyet disclosedby the historical study of scientiffcdevelopmentat all resemblesthe methodological stereotypeof falsification by direct comparisonwith nature. That remark doesnot mean that scientistsdo not reject scientiftc theories,or that experienceand experimentare not essential to the processin which they do so. But it doesmean-what will ultimately be a central point-that the act of iudgment that leadsscientiststo reiect a previouslyacceptedtheory is always comDarisonof that theory theorv with the based upon uDon more than a comparison world. The decisionto reject one paradigm is always simultaneouslythe decisionto acceptanother,and the judgment leading to that decisioninvolvesthe comparisonof both paradigms with nature an^dwith each other. There is, in addition, a secondreasonfor doubting that scientists reject paradigms becauseconfronted with anomaliesor In developingit my argumentwill itself forecounterinstances. shadow another of this essay'smain theses.The reasonsfor doubt sketchedabove were purely factual; they were, that is,
77
fhe Sfructureof ScientificRevolufions themselvescounterinstancesto a prevalent epistemological theory. As such,if my presentpoint ii correct,th-eycan at-best help to create a crisis or, more accurately,to reinforce one that is already ve1y much in existence.By themselvesthey cannot and will not falsify that philosophical theory, for its defenders will do what we h-avealready seenscientisfsdoing when con-articulations fronted by anomaly. They will devise numerous and ad hoc modiffcationsof their theory in order to eliminate any apparent conflict. Many of the relevant modificationsand qualificationsare, in fact, already in the literature. If, therefore, these epistemologicalcounterinstancesare to constitute more than a minor irritant, that will be becausethey help to permit the emergenceof a new and different analysisof sciinceiithin which_theyare no longer a sourceof trouble. Furthermore,if a typical pattern, which we shall later observein scientiffc revolutions, is applicable here, these anomalieswill then no longer seemto,be simply facts.From within a new theory of scientific knowledg.,$ry may insteadseemvery much hkl tautologies, statementsof situations that could not conceivably have i'."r, otherwise. It has often been observed,for exampre,that Newton's second law of motion, though it took centririesof difficult factual and theoretical research-to achieve, behaves for those committed to Newton'stheory very much rike a purely logical statement that no amount of observationcould tif,rt".t hisection x we shall see that the chemicallaw of ftxed proportion, which before Dalton was an occasionalexp_erimenfal finding'of very dubious generality, became after oiltont work a' iigredient of a definition of chemical compound that no expeiimental could by itself have upset.sbmethingmuch lik'e that will lork also happen to the generalilation that scLntists fail to reject paradigms when faced with anomalies or counterinstances. They could not do so and still remain scientists. _ Though history is unlikely to record their names,somemen have undoubtedly been diiven to desert science because of I see particularlv the discussion in N. R. Hanson, pattcrns of ( Cambridge, lg58 i, pp. 9S-t05.
78
Discooerg
fhe Response lo Crisis their inability to tolerate crisis.Like artists, creative scientists must occasionallybe able to live in a world out of joint-elsewhere I have describedthat necessityas "the essentialtension" implicit in scientificresearch.2But that reiection of sciencein favor of anotheroccupationis, I think, the only sort of paradigm rejection to which eounterinstancesby themselvescan lead, Once a first paradigm through which to view nature has been found, there is no such thing as researchin the absenceof any paradigm.To reject one paradigm without simultaneouslysubstituting anotheris to reiect scienceitself. That act reflectsnot on the paradigmbut on the man. Inevitably he will be seenby his colleaguesas "the carpenterwho blameshis tools." The samepoint can be made at least equally effectively in reverse: there is no such thing as researchwithout counterinstances.For what is it that differentiatesnormal sciencefrom sciencein a crisis state?Not, surely,that the former confronts no counterinstances. On the contrary,what we previouslycalled the puzzlesthat constitutenormal scienceexistonly becauseno paradigmthat providesa basisfor scientificresearchever completely resolvesall its problems.The very few that have ever seemedto do so (e.g.,geometricoptics) haveshortlyceasedto yield researchproblems at all and have instead becometools for engineering.Excepting those that are exclusivelyinstrumental, everyproblem that normal scienceseesas a puzzle can be seen,from anotherviewpoint, as a counterinstanceand thus as a sourceof crisis.Coperrricussaw as counterinstances what most of Ptolemy'sother successors had seen as puzzlesin the match between observationand theory. Lavoisier saw as a counterinstance what Priestleyhad seenasa successfully solved puzzlein the articulationof the phlogistontheory.And Einstein saw as counterinstanceswhat Lorentz, Fitzgerald, and others had seenas puzzlesin the articulation of Newton's and Max2 T. S. Kuhn, "The Essential Tension: Tradition and fnnovation in Scientiftc Research," in The Third (1959) unhsersity of utah Research conference on thc ldentification of crcatioe scientfic Taleht, c"luit w. Taylor 1s"lt L"k" City,_ 1959), pp !62:77.-F9r the-comparable"d. phenomenon among artists, see Frank Barron, "The Psychology of Imagination," Scicntifw Amuican, CXCIX ( Scptenrber, 1958), 15l-66, esp. 160.
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fhe Sfruclureof ScienlificRevolufions well's theories.Furthermore, even the existenceof crisis does not by itself transform apuzzle into a counterinstance. There is no suchsharpdividing line. Instead,by proliferatingversionsof the paradigm,crisis loosensthe rules of normal puzzle-solving in ways that ultimately permit a new paradigm to emerge. There are, I think, only two alternatives:either no scientiffc theory ever confronts a counterinstance,or all such theories confront counterinstances at all times. How can the situationhave seemedotherwise?That question necessarilyleads to the historical and critical elucidation of philosoph/, and those topics are here barred. But we can at leastnote two reasonswhy sciencehasseemedto provide so apt an illustration of the generalizationthat truth and falsity are uniquely and unequivocally determined by the confrontation of statementwith fact. Normal sciencedoesand must continually strive to bring theory and fact into closeragreement,and that activity can easilybe seenas testingor as a searchfor conffrmation or falsiffcation.Instead, its object is to solve a puzzle for whose very existencethe validity of the paradigm must be assumed.Failure to achievea solutiondiscreditsonly the scientist and not the theory.Here,evenmorethan above,the proverb applies: "It is a poor carpenterwho blameshis tools." In addition, the manner in which sciencepedagogyentanglesdiscussion of a theory with remarkson its exemplary applieationshas helped to reinforcea confirmation-theorydrawn predominantly from other sources.Given the slightestreasonfor doing so, the man who readsa sciencetext can easilytake the applicationsto be the evidencefor the theory, the reasonswhy it ought to be believed.But sciencestudentsaccepttheorieson the authority of teacherand text, not becauseof evidence.What alternatives have they, or what competence?The applicationsgiven in texts are not there as evidencebut becauselearning them is part of learning the paradigmat the baseof current practice.If applicationswere set forth as evidence,then the very failure of texts to suggestalternativeinterpretationsor to discussproblemsfor which scientistshave failed to produce paradigm solutions 80
lo Crisis fhe Response would convict their authors of extreme bias. There is not the slightest reasonfor such an indictment. How, then, to refurn to the initial question, do scientistsrespond to the awarenessof an anomaly in the fft between theory and nature? What has iust been said indicates that even a discrepancy unaccountably larger than that experiencedin other applications of the theory need not draw any very profound response.There are always somediscrepancies.Even the most stubborn ones usually respond at last to normal practice. V"ry often scientistsare willing to wait, particularly if thereare many problems available in other parts of the fteld. We have already noted, for example,that during the sixty years after Newton's original corhputation, the predicted motion of the moon's perigee remained only half of that observed.As Europe's be_st hathematical physicists continued to wrestle unsuccessfully with the well-known discrepancy, there were occasionalpro' posalsfor a modiffcation of Newton's inversesquarelaw. But no one took these proposals very seriously, and in practice this -ilot aoo*ily proved iustifted. Clairaut in patience with 1750was able to"-show that only the mathematicsof the applica' tion had been wrong and that Newtonian theory could stand as before.sEven in caseswhere no mere mistake seemsquite possible (perhaps becausethe mathematicsinvolved is simpler or of a familiar and elsewheresuccessfulsort), persistent and recognized anomaly does not always induce crisis. No one seriously questioned Newtonian theory because of the longrecognizeddiscrepanciesbetweenpredictionsfrom that theory and both the speedof sound and the motion of Mercury. Thc first discrepancywas ultimately and quite unexpectedlyresolved by experimentson heat undertaken for a very different purpose;the secondvanishedwith the general theory of relativity after a crisis that it had had no role in creating.aApparentsW. Whewell, History of the Inductioe Sclences (rev. ed.; London, 1847), II,22U2L. a For the speed of sound, see T. S. Kuhn, "The Caloric Theory of Adiabatie Compression,'fsds, XLIV (1958), lg&37. For the secular shift in Mercury's perilielion, see E. T. Whittakcr, A Flistonl of thc Thcories of Aahu and El,octri<:ity, ll ( London, 1953), l5l, 179.
8I
fhe Sfruclureof ScientificRevolutions ly neither had seemedsufficiently fundamental to evoke the malaise that goes with crisis. They could be recognized as counterinstances and still be set asidefor later work. It follows that if an anomalyis to evokecrisis,it must usually be more than just an anomaly. There are always difficulties somewherein the paradigm-naturefit; most of them are set right sooneror later, often by processesthat could not have been foreseen.The scientist who pauses to examine every anomaly he notes will seldom get significant work done. We thereforehave to ask what it is that makesan anomaly seem worth concertedscrutiny,and to that questionthere is probably no fully generalanswer.The caseswe have already examined are characteristicbut scarcelyprescriptive.Sometimesan anomaly will clearly call into questionexplicit and fundamentalgeneralizationsof the paradigm,as the problem of ether drag did for thosewho acceptedMaxwell'stheory. Or, as in the Copernican revolution, an anomalywithout apparentfundamentalimport may evokecrisis if the applicationsthat it inhibits have a particular practical importance,in this casefor calendardesign and astrology.Or, as in eighteenth-centurychemistr/, the development of normal sciencemay transform an anomaly that had previouslybeenonly a vexationinto a sourceof crisis:the problem of weight relationshad a very different statusafter the evolution of pneumatic-chemicaltechniques.Presumablythere are still other circumstances that can makean anomalyparticularly pressing,and ordinarily severalof thesewill combine.We have already noted, for example,that one sourceof the crisis that confrontedCopernicuswas the mere length of time during which astronomershad wrestledunsuccessfullywith the reduction of the residualdiscrepanciesin Ptolemy'ssystem. When, for these reasonsor others like them, an anomaly comesto seemmore than iust another puzzleof normal science, the transition to crisis and to extraordinarysciencehas begun. The anomalyitself now comesto be more generallyrecognized as such by the profession.More and more attention is devoted to it by more and more of the field'smost eminentmen. If it still continuesto resist, as it usually does not, many of them may 82
fhe Response fo Crisis
upon it will have involved someminor cr not so minor articula-
increasingly blurred. Though there still is a paradigm, few practitionersprove to be entirely agreedabout what itls. Even -solved formerly standard solutionsof problems are called in question.
monsterrather than man."sEinstein,restrictedby current usage to lessflorid language,wrote only, "rt was as if ihe ground hi'd beenpulled out from under one, with no firm founJation to be seenanywhere,upon which one could have built."6 And wolfgang Pauli, in the months before Heisenberg'spaper on matrix _^r Quoted in T. s. Kuhn, Tlrc copernican Reoolntion (cambricrge, Mass., 1957), p. f38. 0 Albert Einstein, "Autobiographical philosopherNote," in Albert Einstein: '-Scientist,ed. P. A. Schilpp (Evanston, Ill., ig4g), p. 4S.-
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fhe Sfructureof ScientiffcRevolulions mechanicspointed the way to a new quantum theory, wrote _to a friend, "Alt the moment physicsis again terribly confused.In any case,it is too difficult for me, and I wish I had been a movie comedian or something of the sort and had never heard of if contrasted physics."That testimon! is particularly imp-ressive "Heisenberg's later: five months *i[ft Pauli's words lesi than type of mechanicshas again given me hope and-ioy in life.'To bJ srrreit doesnot supply the solution to the riddle, but I believe it is againpossibleto march forward."T of breakdownare extremelyrare, Such "*fli.it-tecognitions but the efiectsof crisisdo not entirely dependuPonits conscious recognition.What can we say these efiects are?_O4y two of thern'seemto be universal. Ali crisesbegin with the blurring of a paradigm and the consequentlooseningof the rules for nor-il ,"r.irch. In this respectresearchduring crisis very much resemblesresearchduring the pre-paradigmperiod, exceptthat in the former the locus of difiet"nce is both smaller and more clearly defined. And all crisescloseiir one of three ways. Sometimes normal scienceultimatelY provoking problem desPitethe it as the end of an existing Pa problem resistseven aPParentlY: lcientists may conclude that no solution will be forthcoming in the pr"r.rrt state of their fteld. fhe problem -rslabelled and set asidiefor a future generationwith more developedtools' Or, ffnally, the casethat rititl most concernus here, a crisis m1y eng with of a new candidatefor_paradiqo with'tir" ":{ "rnurgence balde over its acceptance.This last mode of closure the ensuing will be coniidered at length in later sections,but we must anticipate a bit of what will be said there in order to complete these iemarks about the evolution and anatomy of the crisis state' new one from The transition from a paradigm -normal in crisis to a is far from emerge can science of which a new tradition or extenan articulation by achieved one a cumulativeprocess,
z Ralph Kronig, "The Turning lgfj," in Theoretical Physicsin the Tutentieth Cenhnu:AMemorilioitu*"tiWollgongPauli,ed. M. Fierzand V. F. Weissv-t, r.q6ol, pp. 22,2*i6.lfruch of this article describesthe crisis ;#'ift;; me"ha'icsin'the yearsimmediatelybefore 1925' i;'qlr;;i;;
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fhe Response fo Crisis sion of the old paradigm. Rather it is a reconstructionof the field from new fundamentals,a reconstructionthat changes someof the field's most elementarytheoreticalgeneralizations aswell asmany of its paradigmmethodsand applications.During the transition period there will be a large but never compfete overlap between the problems that can be solved by the old and by the new paradigm.But there will also be a decisive differencein the modesof solution.When the transitionis complete, the professionwill have changedits view of the field, its methods, and its goals. One perceptive historian, viewing a classiccaseof a science'sreorientation by paradigm change, recently describedit as "picking up the other end of the stick," a processthat involves "handling the same bundle of data as before,but placing them in a new systemof relationswith one another by giving them a different framework."s Others who
The precedinganticipationmay help us recognizecrisisas an appropriateprelude to the emergenceof new theories,particuIarly since we have already examineda small-scaleveision of the s-ameprocessin discussingthe emergenceof discoveries. Just becausethe emergenceof a new theory breakswith one tradition of scientiffcpractice and introducesa new one conducted under different rules and within a different universeof 8 Herbert Butterffeld,The origins of Moden science, rs0Glg00 (London, 1949),pp. f-7. o llanson,oyt,cit.,chap.i.
fhe Sfruclureof ScienlificRevolufions discourse,it is likely to occur only when the first tradition is felt to have gone badly astray.That remark is, however, no more and,unforthan a preludeto the investigationof the crisis-state, tunately, the questionsto which it leads demand_the competence of the prychologist even more than that of the historian. What is extraoidinaryiesearchlike? How is anomalymade lawlike? How do scieniistsproceed when aware only that_to,*ething has gonefundamenlallywrong_ut-l l:-velwith which their train-inghis not equipped them to deal?lhose questionsneed far moie investigalio", and it ought not all be historical.What follows will necissarily be more tentative and less complete than what has gone before. Often tt"* p"radigm emerges,at least in embryo,tefore a crisis has" developed far or been explicitly recognized. Lavoisier's work ptoltid"t a case in point. His sealednote was deposited with the French Academy less than ? -y.ar after the theory hrst thorough study of weight relations in the_phlogislo1and before Priestley'spublications had revealed the full extent of the crisis in pneumatic chemistry. Or again, ThomasYoung's first accounts of the wave theory of light appearedat a-Yg{y early stage of a developingcrisii in optics, one that would be ahbst unnoticeableexclplthat, with no assistancefrom Youn$, it had grown to an international scientific scandalwithin a decade oflhe time he ffrst wrote. In caseslike theseone can say only that a minor breakdown of the paradigm and the veryfrst bluiring of its rules for normal sciencewere sufficient to induce in someonea new way of looking at the ffeld. What intervened between the ffrst t"tttt of trouble and the recognition of an available alternate must have been largely unconscious. In other cases,however-those of Copernicus,Einstein, and contemporary nuclear theor/, for example-considerable time of breakdown and the elapsesietween the first consciousness of a new paradig-. When that occurs,the historian "*Lrg"rr"e may Iapture at least a few hints of what extraordinaryscience is iike.^Faced vrith an admittedly fundamental anomaly in theory, the scientist'sfirst efiort will often be to isolate it more precisely and to give it structure. Though now aware that th"y 86
Ihe Response lo Crisis cannot be quite right, he will push the rules of normal science harder than ever to see,in the area of difrculty, iust where and how far they can be made to work. Simultaneouslyhe will seek for ways of magnifying the breakdown,of making it more striking and perhaps also more suggestivethan it had been when displayed in experimentsthe outcome of which was thought to be known in advance.And in the latter effort, more than in any other part of the post-paradigmdevelopmentof science,he will Iook almostlike our most prevalentimage of the scientist.He will, in the first place, often seemA man searchingat random, trying experimentsjust to seewhat will happen,looking for an effect whose nature he cannot quite guess. Simultaneously, since no experiment can be conceivedwithout some sort of theory, the scientist in crisis will constantly try to generate may disclosethe road to speculativetheoriesthat, if strccessful, a new paradigm and, if unsuccessful,can be surrenderedwith relative ease. Kepler's account of his prolonged struggle with the motion of Mars and Priestleyt descriptionof his responseto the proliferation of new gasesprovide classicexamplesof the more random sort of researchproducedby the awarenessof anomaly.to But probably the best illustrationsof all comefrom contemporary researchin field theory and on fundamentalparticles.In the absenceof a crisisthat madeit necessaryto seeiust how far the rules of normal sciencecould stretch, would the immense effort requiredto detectthe neutrinohave seemedjustifted?Or, if the ruleshad not obviouslybroken down at someundisclosed point, would the radical hypothesisof parity non-conservation havebeen either suggestedor tested?Like much other research in physics during the past decade,these experimentswere in part attemptsto localizeand definethe sourceof a still difftrse set of anomalies. This sort of extraordinaryresearchis often, though by no r0 For an account of Kepler's work on Mars, see I. L. E. Dreyer, A Historu of Astronamy from Thales'to Kepler (2d ed.; New'York, 1953i, pp. 38G93'. Occasional inaccuracies do not prevent Dreyer's pr6cis from providing the material needed here. For Priestlelr see his own work, esp. Erperiments anil Obseroationson Difierent Kinds of Air (Lon
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TheStruclureof ScientificRevolutions by another.It is, I think, particumeansgenerally,accompanied Iarly in periods of acknowledgedcrisis that scientistshave turned to philosophicalanalysisas a device for unlocking the riddles of their field. Scientistshave not generally needed or wantedto be philosophers.Indeed,normal scienceusuallyholds creative philosophy at arm's length, and probably for good reasons.To the extent that normal researchwork can be conductedby usingthe paradigmasa model,rulesand a-ssumptignl need nof be made eiplicit, In SectionV we noted that the full set of rulessoughtby philosophicalanalysisneednot evenexist. But that is not to say that the searchfor assumptions(even for non-existentones) cannot be an effective way to weaken the grip of a tradition upon the mind and to suggestthe basisfor a new one. It is no accident that the emergenceof Newtonian physicsin the seventeenthcentury and of relativity and quantum mechanicsin the twentieth should have been both precededand accompaniedby fundamentalphilosophicalanalyses of the contemporaryresearchtradition.ll Nor is it an accident that in both these periods the so-calledthought experiment shouldhave played so critical a role in the progressof research. As I haveshownelsewhere,the analyticalthought experimentation that bulks so large in the writings of Galileo, Einstein, Bohr, and othersis perfectly calculatedto exposethe old Paradigm to existing knowledge in ways that isolate the root of crisiswith a clarity unattainablein the laboratory.l2 With the deployment,singly or together,of theseextraordinary procedures,one other thing may occur. By concentrating
11 For the philosophical counterpoint that accompanied seventeenth-century mechanics, se! Ren6-Dugas, La micanique au XVlle siicle (Neuchatel, 1954), Oarticularly chap. xi. Foi the similar nineteenth-century episode, see the same iuthor's earlier Book, Histoire de Ia mdcanique (Neuchatel, lg50), pp. 4f9--43. 12T. S. Kuhn, "A Function for Thought Experiments," in Mdlanges Alemndre Koyr6, ed. R. Tirt0n uncl I. B. Colrcn, t0 be published by Ilermann (Paris) in 1963.
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fhe Responsefo Crisis Lavoisier'swork on oxygenfrom Priestley's;and oxygenwasnot the only new gasthat the chemistsawareof anomalywere able to discoverin Priestley'swork. Or again,new optical discoveries accumulatedrapidly iust before and during the emergenceof the wave theory of light. Some,like polarization by reflection, were a result of the accidentsthat concentratedwork in an area of trouble makeslikely. (Malus, who made the discovery,was just starting work for the Academy'sprize essayon double refraction, a strbject widely known to be in an rursatisfactory state.) Others,Iike the Iight spot at the centerof the shadowof a circular disk, were predictionsfrom the new hypothesis,ones whose successhelped to transform it to a paradigm for later work. And still others,like the colors of scratchesand of thick plates,were effectsthat had often been seenand occasionally remarked before, but that, like Priestley'soxygen,had been assimilatedto well-known effectsin ways that preventedtheir being seenfor what they were.l3A similar account could be given of the multiple discoveriesthat, from about 1895,were a constantconcomitantof the emergenceof quanfum mechanics. Extraordinary researchmust have still other manifestations and effects,but in this areawe have scarcelybegun to discover the questionsthat needto be asked.Perhaps,however,no more are neededat this point. The precedingremarksshould suffice to show how crisis simultaneouslyloosensthe stereotypesand provides the incremental data necessaryfor a fundamental paradigm shift. Sometimesthe shape of the new paradigm is foreshadowedin the structure that extraordinaryresearchhas given to the anomaly.Einstein wrote that before he had any substitutefor classicalmechanics,he could seethe interrelation between the known anomaliesof black-body radiation, the photoelectric effect, and specific heats.laMore often no such structureis consciouslyseenin advance.Instead,the new paradigm, or a sufficienthint to permit later articulation, emerges 13 For the new optical discoveries in general, see V. Ronchi, Histoire de la lurniire (Paris, 1956), ch-ap. vii. For the earlier explanation of one of tlrese see_J.,Priestley,-The H_istory ard, Present Staie of Discooeries Relating "ffggF: to Vision, Light and Colours ( London, 1772), pp. 49&-520. l { E i r r s t e i n ,l o c , t : i t ,
fhe Sfructureof ScientiffcRevolufions all at once,sometimesin the middle of the night, in the mind of a man deeply immersedin crisis.What the nature of that ffnal stageis-how an individual invents ( or findshe has invented) a new way of giving order to data now all assembled-musthere remaininscrutableand may be perrnanentlyso.Let us here note only one thing about it. Almost always the men who achieve these fundamental inventions of a new paradigm have been either very young or very new to the ffeld whoseparadigmthey change.lbAnd perhapsthat point need not have been made explicit, for obviously these are the men who, being little cornmitted by prior practice to the traditional rules of normal science,are particularly likely to seethat thoserulesno longer deftne a playable game and to conceiveanother set that can replace them. The resulting transition to a new paradigm is scientificrevolution; a subjectthat we are at long last preparedto approach directly. Note first, however, one last and apparently elusive respectin which the material of the last three sectionshas prepared the way. Until SectionVI, where the conceptof anomaly 'extraordinary 'revolution' and was ftrst introduced, the terms science'mayhave seemedequivalent.More important, neither term may have seemedto meanmore than'non-normalsciencer' a circularity that will have bothered at least a few readers.In practice,it neednot havedone so.We are about to discoverthat a similar circularity is characteristic of scientific theories. Bothersomeor not, however, that circularity is no longer unqualiffed.This sectionof the essayand the two precedinghave educed numerouscriteria of a breakdown in normal scientific activity, criteria that do not at all dependupon whether breakdown is succeededby revolution. Confrontedwith anomaly or rc This generalization about the role of youth in fundamental scientiffc research is s6 common as to be a clich6. Furthermore, a glance at almost any list of fundamental contributions to scientific theory will provide impressionistic conffrrnation. Nevertheless, the generalization badly needs systematic investigaprovides tion. Harvey C. Lehman (Age and Achieoement [Princeton, f9$]) many usefui d"t"; but his stu?ies make no attempt to single out contr:ibutions that-involve fundamental reconceptualization. Nor do they inquire about the special circumstances, if any, that may accompany relatively late productivity in the sciences.
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fo Crisis Ihe Response with crisis, seientiststake a different attitude toward existing paradigms, and the nature of their research changesaccordingly. The proliferation of competing articulations, the willingnessto try anything, the expressionof explicit discontent, the recourse to philosophy and to debate over fundamentals, all theseare symptomsof a transition from normal to extraordinary research.It is upon their existencemore than upon that of revolutions that the notion of normal sciencedepends.
9l
lX. The Noiureond Necessity of ScientificRevolutions Theseremarkspermit us at last to considerthe problemsthat provide this essaywith its title. What are scientiffcrevolutions, and what is their function in scientiffc development?Much of the answer to these questionshas been anticipated in earlier sections.In particular, the preceding discussionhas indicated that scientiffcrevolutionsare here taken to be thosenon-cumuIative developmentalepisodesin which an older paradigm is replacedin whole or in part by an incompatiblenew one.There is more to be said,however,and an essentialpart of it can be introduced by asking one further question. Why should a changeof paradigm be called a revolution?In the face of the vast and essentialdifferencesbetween political and scientiffc development, what parallelism can iustify the metaphor that finds revolutionsin both? One aspectof the parallelismmust alreadybe apparent.Political revolutionsare inauguratedby ^ growing sense,often restricted to a segmentof the political community, that existing institutionshave ceasedadequatelyto meet the problemsposed by an environmentthat they have in part created.In much the same way, scientiftcrevolutionsare inauguratedby a growing sense,again often restricted to a narrow subdivision of the scientific community, that an existing paradigm has ceasedto function adequatelyin the explorationof an aspectof nature to which that paradigmitself had previouslyled the way. In both political and scientific developmentthe senseof malfunction that can lead to crisisis prerequisiteto revolution.Furthermore, though it admittedly strains the metaphor, that parallelism holds not only for the maior paradigm changes,like those attributable to Copernicusand Lavoisier,but also for the far smaller onesassociatedwith the assimilationof a new sort of phenomenon,like oxygenor X-rays.Scientificrevolutions,aswe noted at the end of SectionV, need seemrevolutionaryonly to 92
fhe Nofure and Necessilyof ScienfificRevolulions thosewhose paradigmsare affected by them. To outsidersthey may, Iike the Balkan revolutions of the early twentieth century, seemnormal parts of the developmentalprocess.Astronomers, for example,could acceptX-rays as a mere addition to knowledge, for their paradigms were unaffected by the existenceof the new radiation.But for men like Kelvin, Crookes,and Roentgen,whoseresearchdealt with radiationtheory or with cathode ray tubes, the emergenceof X-rays necessarilyviolated one paradigm as it created another.That is why theserays could be discovered only through something's ffrst going wrong with normal research. This genetic aspect of the parallel betwssnr political and scientificdevelopmentshould no Iongerbe open to doubt. The parallel has,however,a secondand more profound aspectupon which the signiffcanceof the ffrst depends.Political revolutions aim to changepolitical institutions in ways that those institutions themselvesprohibit. Their successtherefore necessitates the partial relinquishment of one set of institutions in favor of another, and in the interim, society is not fully governedby institutions at all. Initially it is crisisalone that attenuatesthe role of political institutionsas we have alreadyseenit attenuatethe role of paradigms. In increasing numbers individuals become increasingly estrangedfrom political life and behave more and more eccentricallywithin it. Then, as the crisis deepens,many of these individuals commit themselvesto some concreteproposal for the reconstruction of society in a new institutional framework. At that point the society is divided into competing campsor parties,one seekingto defendthe old institutionalcond, cause they differ about the institutional matrix within *hi"eli
ievolutions have had a vital role evolutionof political institutions,that role dependsupon
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fhe Sfructureol ScienfificRevolufions their being partially extrapolitical or extrainstitutional events. The remainder of this essay aims to demonstrate that the historicalstudy of paradigmchangerevealsvery similar characteristics in the evolution of the sciences.Like the choice between competingpolitical institutions,that betweencompeting paradigms proves to be a choice between incompatible modes of community life. Becauseit has that character,the choice is not and cannot be determined merely by the evaluative procedurescharacteristicof normal science,for these depend in
b'"k
Each group uses its own paradigmto arguein that paradigm'sdefense. The resulting circularity doesnot, of course,make the argumentswrong or evenineffectual.The man who premisesa paradigm when arguing in its defensecan nonethelessprovide a clear exhibit of what scientiffcpractice will be like for those who adopt the new view of nature. That exhibit can be immensely persuasive,often compellingly so. Yet, whatever its of the circular force, tis
DremNes partiesto a paradigmsare not sufficientlyextensivefor that. As in political revolutions,so in paradigm choice-there is no standardhigher than the assentof the relevant community. To discoverhow scientiffc revolutions are effected,we shall therefore have to examinenot only the impact of natureand of logic, but alsothe techniques of persuasiveargumentation effective within the quite specialgroupsthat constitutethe communityof scientists. To discoverwhy this issueof paradigm choice can never be unequivocallysettled by logic and experimentalone,we must shortly examinethe nature of the differencesthat separatethe proponentsof a traditional paradigm frorn their revolutionary That examinationis the principal object of this secsuccessors. the next. We have, however,alreadynoted numerous tion and examplesof suchdifferenccs,and no one will doubt that history I
91
fhe Nofure ond Necessifyof ScientificRevolutions can supply many others.What is more likely to be doubted than their existence-andwhat must thereforebe consideredffrst-is that such examplesprovide essentialinformation about the nature of science.Granting that paradigm reiection has been a historic fact, doesit illuminate more than human credulity and confusion?Are there intrinsic reasonswhy the assimilationof or a new scientifictheory must either a new sort of phenomen-on demandthe rejectionof an older paradigm? First notice that if there are such reasons,they do not derive from the logical structure of scientificknowledge.In principle, a new phenomenonmight emerge without reflecting destructively upon any part of past scientiffcpractice.Though discov' ering life on the moon would today be destructiveof existing paradigms (these tell us things about the moon that seemincompatiblewith life's existencethere), discoveringlife in some less well-known part of the galaxy would not. By the same token, a new theory does not have to conflict with any of its It might deal exclusivelywith phenomenanot predecessors. previously known, as the quantum theory deals (but, significantly, not exclusively) with subatomicphenomenaunknown before the twentieth century. Or again, the new theory might be simply a higher level theory than those known before, one that linked together a whole group of lower level theorieswithout substantially changing any. Today, the theory of energy conservationprovidesjust such links betweendynamics,chemistry, electricity, optics, thermal theory, and so on. Still other compatiblerelationshipsbetweenold and new theoriescan be conceived.Ary and all of them might be exemplifiedby the historicalprocessthrough which sciencehas developed.If they were, scientific developmentwould be genuinely cumulative. New sorts of phenomenawould simply discloseorder in an aspectof naturewhere nonehad beenseenbefore.In the evolution of sciencenew knowledgewould replaceignorancerather than replaceknowledgeof anotherand incompatiblesort. Of course,science (or some other enterprise,perhaps less effective) might have developedin that fully cumulativemanner. Many people have believed that it did so, and most still
95
fhe Sfruclureof ScienfificRevolutions seemto supposethat cumulationis at leastthe ideal that historical developmentwould display if only it had not so often been distorted by human idiosyncrasy.There are important reasons for that belief. In section x we shall discover[ow closelythe view of science-as-cumulation is entangled with a dominant e_pistemology that takesknowledgeto be a constructionplaced directly upon raw sensedata by the mind. And in sectionXl we shall examinethe strongsupportprovided to the samehistoriographic schemaby the techniquesof efiectivesciencepedagogy. Nevertheless,despite the immense plausibility of ihat ideal image,there is increasingreasonto wonder whether it can possibly be an imageof science.After the pre-paradigmperiod the assimilationof all new theoriesand of almost all new sorts of phenomenahas in fact demanded the destruction of a prior and a consequentconfict betweencompetingschools . .-.,paradigm of scientific thought. Cumulative acquisitionof unanticipated \ z novelties to be an almost non-existentexceptionto the proves ,_,y'1 " of scientiffc development. The man who takeshistoric fact \ule seriouslymust suspectthat sciencedoes not tend toward the ideal that our image of its cumulativenesshas suggested.Perhapsit is anothersort of enterprise. If, however,resistantfacts can carry us that far, then a second look at the ground we have already coveredmay suggestthat cumulativeacquisitionof novelty is not only rare in fact but improbable in principle. Normal research,which is cumulative, owes its successto the ability of scientistsregularly to select problemsthat can be solvedwith conceptualand instrumental techniquescloseto thosealready in existence.(That is why an excessive concel'nwith useful problems,regardlessof their relation to existingknowledgeand technique,can so easilyinhibit \ fcientific development.) The man who is striving to solve a }/ problem defined by existing knowledge and technique is not, ,/ \ however,just Iooking around. He knows what he wants to .rf achieve,and he designshis instrumentsand directshis thoughts I accordingly.Unanticipatednovelty, the new discovery,can emergeonly to the extent that his anticipationsabout nature and his instrumentsprove wrong. Often the importanceof the
ffie Nolure ond Necessilyof ScientificRevolutions resulting discovery will itself be proportional to the extent and stubbornnessof the anomaly that foreshadowedit. Obviously, then, there must be a confict between the paradigm that dis- "r closesanomaly and the one that later renders the anomaly law- $ like. The examplesof discovery through paradigm destructio ") ', examinedin SectionVI did not confrontus with mere historical accident. There is no other effective way in which discoveries might be generated. \J The sameargument applies even more clearly to the invention of new theories.There are,in principle, only three types of phenomenaabout which a new theory might be developed.The ffrst consistsof phenomenaalready well explainedby existing paradigms,and these seldom provide either motive or point of departure for theory construction. When they do, as with the three famousanticipationsdiscussedat the end of SectionVII, the theoriesthat result areseldomaccepted,becausenature provides no ground for diserimination.A secondclassof phenomena consistsof thosewhosenatureis indicated by existingpara. 4ig*r but whosedetails can be understoodonly through fuither theory articulation. These are the phenomenato which scientists direct their researchmuch of the time, but that research aims at the articulation of existing paradigmsrather than at the invention of new ones.only when theseattemptsat articulation fail do scientists encounter the third type of phenomena,the recognizedanomalieswhosecharacteristicfeature is their stubborn refusal to be assimilatedto existingparadigms.This type alone gives rise to new theories.ParadigmJprovide all phenorirena excep! anomalieswith a theory-determinedplace in the scientist'sffeld of vision. But if new theoriesare called forth to resolveanomaliesin the nature,then the successfulnew t predictions that are difterent ,decessor.That difference could lly compatible.In the processof
theory rikeenersy co_nse*",i";1;i*ntff:rji:"*:t; ftffi
superstructurethat relatesto nature only through independlnt-
97
fhe Sfructureof ScienfificRevolulions ly establishedtheories, did not develop historically without paradigmdestruction.Instead,it emergedfrom a crisisin which an essentialingredient was the incompatibility between Newtonian dynamicsand somerecentlyformulatedconsequences of the caloric theory of heat. Only after the caloric theory had been rejected could energy conservationbecomepart of science.lAnd only after it had been part of sciencefor sometime could it come to seema theory of a logically higher type, one not in conflict with its predecessors. It is hard to seehow new theoriescould arisewithout thesedestructivechangesin beliefs about nature. Though logical inclusivenessremains a permissible view of the relation betweensuccessive scientifictheories, it is a historical implausibility. A century ago it would, I think, have been possibleto let the casefor the necessityof revolutionsrest at this point. But today, unfortunately, that cannot be done becausethe view of the subjectdevelopedabovecannotbe maintainedif the most prevalent contemporaryinterpretation of the nature and function of scientifictheory is accepted.That interpretation,closelyassociated with early logical positivism and not categoricallyrejected by its successors, would restrict the range and meaning of an acceptedtheory so that it could not possiblyconflict with any later theory that made predictionsabout someof the same natural phenomena.The best-knownand the strongestcasefor this restricted conceptionof a scientifictheory emergesin discussionsof the relation between contemporaryEinsteiniandynamics and the older dynamical equationsthat descendfrom Newton's Principia. From the viewpoint of this essaythesetwo theoriesare fundamentallyincompatiblein the senseillustrated by the relation of Copernicanto Ptolemaic astronomy: Einstein'stheory can be acceptedonly with the recognition that Newton'swas wrong. Today this remainsa minority view.2We must thereforeexaminethe most prevalent obiectionsto it. 1 Silvanus P. Thompson, Life of William Thomson Baron Kehsin of Largs ( London, l9l0 ), I, 266-81. 2 See, for example, the remarks by P. P. Wiener in PhilosophV of Scicnce,
xxv (1958) , 298.
98
fhe Notu re ond Necessityof ScienfificRevolulions
99
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erties. In addition, the phlogiston theory accountedfor a number of reactionsin which acids were formed by the combustion of substanceslike carbon and sulphur. Also, it explained the decreaseof volume when combustionoccurs in a c6nffnedvolume of air-the phlogiston releasedby combustion "spoils" the elasticity of the air that absorbed it, just as fire "sioils" the elasticity of a steel spring.sIf these w"t" the only pfrutro*"n" that the phlogiston theoristshad claimed for theii fheory, that th.g-orycould never have been challenged.A similar argument will suffice for any $e9ry that has ever been successfiilly "pplied to any range of phenomenaat all. But to save theories in this wa/, their range of application must be restrictgd t-otloT phenomenaand to-that pr?iision of observationwith which the experimentalevidence-inhand already clnied just step further (and the step can _de-als.a _a scarcelybe avoided once the ffrst is taken), such a limiiation p-rohibits th_escientist from claiming to speak "scientiffcally" about any-phenomenonnot already Jbr"ru6d. Even in its pturent form the restrictionforbids the scientistto rely upon ttr"ory in his own researchwhenever that researchenters an" area or seeksa degreeof precisionfor which past practice with the theory offe1 no prec_edent. Theseprohibitions-arelogically unexc_eptionable. But the result of acceptingthem *o.tld bL the end of the researchthrough which sciencemay developfurther. By now that point too is virtually a tautology.withbut commitment to a paradigm there could be no normal science.Furthermore,that commitmentmust extendto areasand to degees oj precision for which there is no full precedent. If it did"not, the paradigm could provide no_puzzles that had not already been solved.Besides,it is not only normal sciencethat depends upon commitment to a paradigm. If existingtheory binds the 8 James B. Qo_ngt,_ooerthrou !f_1he phlogiston Theorg (cambridge, lgsO), Fp. 13-16; 33d J: R Partington, A-Short Htiorq _of Chemi*trg ( 2d ed]; London, 1.951)t PP:8H8. The ftrllest and trrost syrnpathc'tic ,r"co,,ni of the phloeiston
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o.9oTpTe.the conclusionsreachedthrough a very difierent sort of analysis , by R. B. Braithwaite, Scientifc Explanation (Cambridge, lgsg), pp. SObZ, esp.p. 76.
r00
fhe Noture qnd Necessifyof ScientificRevolutions
will inevitabty return to somethingmuch like its pre-paradigm state,a condition in which all memberspractice sciencebut in which their grossproduct scarcelyresemblesscienceat all. Is it really any-wondir that the price of significantscientificadvance is a commitmentthat runs the risk of being wrong?
\l
l4
sucha derivationlook like? Imagine a set of statements,E4 Ez, E", which together embody the laws of relativity theory. Thesestatementscontainvariablesand parametersrepresenting spatial position, time, rest mass,etc. From them, together yilh the appiratus of logic and mathematics,is deducible a whole set of further statementsincluding some that can be checked by obseruation.To prove the adequacyof Newtonian dynamics as a specialcase,we must add to the Eis additional statements, 1, restricting the range of the parametersand lfue (o/c)'<< variables.This enlargedset of statementsis then manipulated to yield a new set, Nr, Nr, . . . , N., which is identical in form with Newton's laws of motion, the law of gravity, and so on. Apparently Newtonian dynamicshas been derived from Einsteinian,subjectto a few limiting conditions. Yet the derivation is spurious,at least to this point. Though the Nis are a special caseof the laws of relativistic mechanics, they are not Newton's Laws. Or at least they are not unless thoselaws are reinterpretedin a way that would havebeen impossibleuntil after Einstein's work. The variables and Parametets that in the Einsteinian Eis representedspatial position,
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ffie Sfructureof ScientiffcRevolufions I
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sent Einsteinianspace,time, and mass.But the physical referents of these Einsteinian conceptsare by no meansidentical with thoseof the Newtoniatt.onc"pts thal bear the samename. (Newtonian massis conserved;Einsteinianis convertiblewith energy.only at low relative velocitiesmay the two be measured in the same way, and even then they -,rrt not be conceivedto be the same.) unlesswe changethedefinitions of the variables in the Nis, the statementswe have derived are not Newtonian. If w9 do changethem, we cannot properly be said to have d,erioed Newton'sLaws, at leastnot in atty i.t r" of "derive" now generally recognized.our argument has, of course,explained why^Newton'sLaws ever seemedto work. In doing 16 it h"t justified, say,an automobiledriver in acting as thoufh he lived in a Newtonianuniverse.An argumentof t[e sametype is used to justify teaching earth-center-ed astronomyto survJyors.But the argumenthas still not done what it purported to do. It has not, that is, shown Newton'sLaws to bJa limiting caseof Einstein's.For in the passageto the limit it is not onlf the forms of the laws that have changed.simultaneously*" h"u" had to alter the fundamental structural elementsof which the universe to which they apply is composed. This need to changethe meaningof establishedand familiar conceptsjs central to the revolutionary impact of Einstein's theory. Though subtler than the changis from geocentrismto heliocentrism,froy phlogiston to oxygen,or frim colpuscles to waves,the resulting conceptualtraniformation is no liss decisively destructiveof a previously establishedparadigm. we may even cometo seeit asa prototype for revolutionary reorientationsin the sciences.|ust becaussit did not involveihe itrtroduction of additional objects or concepts,the transition from Newtonian to Einsteinianmechanicsillustrateswith particular clarity the scientiffcrevolutionas a displacementof th-econceptual network through which scientistsview the world Theseremarksshouldsufficeto show what might, in another philosophicalclimate,have been taken for grant"a. et least for scientists,most of the apparentdifferencesbetweena discarded scientiffctheory and its successorare real. Though an out-of-
r02
rhe Noture ond Necessifyof scientiffcRevolulions date theory can alwaysbe viewed as a specialcaseof its up-todate succ"itot, it muit be transformedfor the purPose.And the transformation is one that can be undertaken only with the advantagesof hindsight, the explicit guidance of the more recent theor/. Furthermoie, even il that transformation wele a legitimate device to employ in intelpreting the older theory, the result of its applicafion would be a theory so restricted that it could only restatewhat was already known. Becauseof its economy, thai restatementwould have utility, but it could not suffice for the guidanceof research. Let us, therefore,now take it for between successiveparadigms are I cilable. Can we then saymore explic these are? The most apparent tyPe paradigmstr repeatedly.Successive the population of the universe and about that population's be- '.^ havior. Thuy difier, that is, about suehquestions^aJttt"existence ;ht of subatomicpa*icles, the materiality.^ottigttt, .ld th.econse* tt*#5 "" vation of heatbr of energy.Theseare the substantivedifferences \ p"t"tig-s, and they-requir-eno further illus- rXl between successiv_e tration. But paradigmsdifier in more than substance,for they are directed not only to nature but also back uPon the science that produced them. They are the sourceof the methods,problem-field, and standardsof solution acceptedby any mature scientiffc community at any given time. As a result, the reception of a new paradigm often necessitatesa redefinition of the correspondingicience.Someold problemsmay be relegatedto anothir scienceor declared entirely "unscientific." Others that were previously non-existent or trivial may, with a new PaTdig-,- become the very archetypes of signfficant scientiftc achievement.And as the problems change,so, often, does the standardthat distinguishesa real scientiffcsolution from a mere metaphysical speculation, word game, or mathematical Play. The normal-scientific tradition trhat emergesfrom a scientiftc revolution is not only incompatible but often actually incommensurablewith that which has gone before. The impact of Newton's work upon the normal seventeenth-
r03
fhe Sfructureof ScientiffcRevolufions cgntu-rytradition of scientiffcpractice provides a striking example of these subtler efiects of paradigm shift. Before Newton was born the "new science"of the c"trtury had at last succeeded in reiecting Aristotelian and scholasticexplanationsexpressedin terms of the essencesof material bodies.to say that a-stonefell it toward the center of the universe mere tautological word-play, some'been. Henceforth the entire fux of ,-appearances, including color, taste, and even weight, was to be explainedin terms or tne size, shape,position, ind motion of the elementary corpusclesof base mattLr. The attribution of other qualities io the elementaryatomswas a resort to the occult and therefore out of boundi for science. Molidre :isely when he ridiculed the doctor :acyas a soporiffcby attributing to it ng the last half of the seventeenth fgned to say that the round shapeof d them to sooth the nerves about which thuy moved.b fn an earlier period explanationsin terms of occult qualities had been an integral p".t of productive scientiftc' work. Nevertheless,the seventeenth century's new commitment to mechanico-co{puscularexplanation proved immensely fruitful for a number of sciences,ridding them of problemsthat had degeletlly accept-edsolution and suggeititrgothersto replace {ed them. In dynamics,for example,Newtont three laws of motion are less a pt{ygt of novel experimentsthan of the attempt to reinteqpretwell-known observationsin terms of the motionjand interactioT-rof primary neutral corpuscles.Consider iust one concreteillustration. Sinceneutral coqpusclescould act on each other only by_ contact, the mechanico-co{puscular.view of nature directed scientiftc attention to a brand-new subiect of study, the alteration of particulate motions by collisioni. Descartes announced the problem and provided its ffrst putative 5 For corpuscularism in general, see Marie Boas, "The Establishment of the Mechanical Philoso.ghy," olrlo x (1952), 4t2-541. For the effect of farticleshape on taste, see tbti., p. 485.
104
rhe Notu re ond Necessifyof scienfific Revolufions
tury, the corpuscularparadigm bred both a new problem and a large part of that problem's solution.o Yet, though much of Newton'swork was directed to problems and embodiedstandardsderived from the mechanico-corpuscular world view, the effect of the paradigm that resultedfrom his work was a further and partially destructivechangein the problems and standardslegitimate for science.Gravity, inteqpreted as an innate attraction between every pair of particles of matter, was an occult quality in the same senseas the scholastics' "tendency to fall" had been. Therefore, while the standardsof co{puscularismremained in effect, the searchfor a mechanical explanationof gravity was nne of the most challengingproblems for those who accepted the Prircipia as paradigm. Newton devoted much attention to it and so did many of his eighteenthThe only apparentoption wasto reject Newcentury successors. ton's theory for its failure to explain gravity, and that alternative, too, was widely adopted. Yet neither of these views ultimately triumphed. Unable either to practice sciencewithout tlte Principia or to make that work conform to the corpuscular standardsof the seventeenthcentury, scientistsgradually accepted the view that gravity was indeed innate. By the mideighteenth century that interpretation had been almost nniversally accepted, and the result was a genuine reversion (which is not the sameas a retrogression)to a scholasticstandard. Innate attractionsand repulsionsioined size,shape,posi0R. Dugas, La micanique au XVII' 284-98,84il56.
sidcle (Neuchatel, 1954), pp. 177-85,
r05
ffie Sfruclureof ScienfificRevolufions tion, and motion asphysicallyirreducibleprimary propertiesof matter.? The resulting change in the standardsand problem-ffeldof again consequential.By the 1740's, culd speakof the attractive "virtue" rt thereby inviting the ridicule that :or a century before. As they did so, :asinglydisplayedan order difierent ,wn when viewed as the effects of a :ould act only by contact.In particu-at-a-distancebecame a subject for rhenomenon we now call chargingby ized as one of its effects.Previously, renattributed to the direct action of r to the leakagesinevitable in any rew view of inductive effectswas, in rnalysisof the Leyden iar and thus to rd Newtonian paradigm for electricd electricity the only scientiffcffelds on of the searchfor forcesinnate to lf eighteenth-centuryliterature on acementseriesalsoderivesfrom this Newtonianism.Chemistswho believed in these differential attractions between the various chemicalspeciesset up previouslyunimaginedexperimentsand searchedfor new sortsof reactions.Without the data and the chemicalconceptsdevelopedin that process,the later work of Lavoisierand, moreparticularly,of Dalton would be incomprehensible.schanges in the standards governing permissible problems,concepts,and explanationscan transform a science. In the next sectionI shall even suggesta sensein which they transformthe world. 7I. B. Cohen, Franklin and Neuton: An Inquiry into Speculatioe Neutonian Erperinte-ntal Science and Franklin's Work in Eleciricity oi an Erample Thereof (Philadelphia, 1956), chaps. vi-vii. t _ {ot electricity, see ibkl, chaps. viii-ix. For chemistry, see Metzger, op. cit,, Part I.
r06
TheNofure ond Necest Other examplesof thesenonsubstr successiveparadigms can be retrievr sciencein almostany period of its del let us be content with iust two other r Before the chemical revolution, one of chemistry was to account for the stancesand for the changesthese q chemical reactions. With the aid c mentary' principles"-of which phlop was to explain why somesubstancesi combustible,and so forth. Somesuccessin this direction hadqi "'. been achieved. We have already : plained why the metalswere so muc developed a similar argument for th however, ultimately did away with thus ended by depriving chemistry potential explanatory power. To ct change in standardswas required. . teenth century failure to explain the t no indictment of a chemicaltheory.e Or again, Clerk Maxwell shared r tury proponentsof the wave theory of light the convictionthat $-S ' hg[t wavesmust be propa_gated thro.tgi material ether. De{ _;" such waves was a signing a mechanical medium to support fi:' J $' standard problem for many of his ablest contemporaries.His $ $; own theory, however,the electromal no accountat all of a medium able to clearly made such an account harc seemedbefore. Initially, Maxwell'sI for those reasons.But, Iike Newton'r difficult to dispensewith, and as it ac digm, the community'sattitude towe decadesof the twentieth century Mr existenceof a mechanicalether lool service,which it emphaticallyhad nc designsuchan etherealmedium wer e E. Meyerson,Identity atd Reality (New
TheSlructureof ScienfiffcRevolufions
q-5r " cumulationof theories.The attempt to explain gravity, though b { fruitfully abandonedby most eighteenth-centuryscientists,was
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sal may evenbe undenvayin electromagnetictheory. Space,in contemporaryphysics,is not the inert and homogenoussubstraturn-employed in both Newton's and Maxwell's theories; someof its new propertiesare not unlike thoseonce attributed to the ether; we may somedaycome to know what an electric displacementis. By shifting emphasisfrom the cognitive to the normative functionsof paradigms,the precedingexamplesenlargeour understanding of the ways in which paradigms give form to the scientiffclife. Previously,we had principally examinedthe paradigm's role as a vehicle for scientific theory. In that role it functions by telling the scientistabout the entitiesthat nature does and doesnot containand about the ways in which thoseentities behave. That information provides a map whose details are elucidatedby mature scientiffcresearch.And sincenature is too complex and varied to be explored at random, that map is as essentialas observationand experimentto science'scontinuing development.Through the theories they embody, paradigms prove to be constitutive of the researchactivity. They are also, however, constitutive of sciencein other respects,and that is now the point. In particular, our most recent examplesshow that paradigmsprovide scientistsnot only with a map but alsowith some of the directions essentialfor map-making.In learning a paradigmthe scientistacquirestheory, methods,and standards together, usually in an inextricable mixture. Therefore,when paradigms change, there are usually signiffcant shifts in the criteria determining the legitimacy both of problems and of proposedsolutions. That observationreturns us to the point from which this section began,for it providesour ffrst explicit indicationof why the\ / choice between compe_ting paradigmsregularly_raises questio-ns,K''" that cannotbe resolvedby the criteria of normal science.To the/ extent, as signiffcant as it is incomplete, that two scientiffc l schoolsdisggreeabo3rtwhat is a problem and what a solution, i they wilfinevitably_.talk through each other when debating the j' relative merits of their respectiveparadigms.In the partially i, circular argumentsthat regularly result, eachparadigmwill be " t ; r *L / 1'* '{tr,vti\tl 1 { f,iFgl' fi,q e Jl t.oO.tur-- /*-^^
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have so far argued only that paradigms are constitutive of science.Now I wish to display a sensein which they are constitutive of nature as well.
il0
X. Revolutions os Chongesof World View Examining the record of past researchfrom the vantage of contemporary historiography, the historian of sciencemay be tempted to exclaim that when paradigms change, the world itseU changeswith them. Led by a new paradigm, scientists adopt new instmments and look in new places. Even more important, during revolutionsscientistssee new and different things when looking with familiar instruments in places they have looked before. It is rather as if the professionalcommunity had been suddenly transportedto anotherplanet where familiar objects are seenin a different light and are joined by unfamiliar onesas well. Of course,nothing of quite that sort does occur: there is no geographical transplantation; outside the laboratory everyday affairs usually continue as before. Nevertheless,paradigm changesdo causescientiststo seethe world of their research-engagement differently. In so far as their only recourseto that world is through what they seeand do, we may want to say that after a revolution scientistsare respondingto a different world. It is aselementaryprototypesfor thesetransformationsof the scientist'sworld that the familiar demonstrationsof a switch in visualgestaltprove sosuggestive.What were ducksin the scientistk world before the revolution are rabbits afterwards. The man who first saw the exterior of the box from above later sees its interior from below. Transformations like these, though usually more gradual and almost always irreversible,are common concomitantsof scientifictraining. Looking at a contour map, the student seeslines on paper, the cartographera picture of a terrain. Looking at a bubble-chamberphotograph,the student seesconfusedand broken lines, the physicist a record of familiar subnuclearevents.only after a number of such transformationsof vision does the student becomean inhabitant of the scientist'sworld, seeingwhat the scientistseesand responding as the scientistdoes.The world that the studentthen enters lil
fhe Sfruclureof ScienfiffcRevolufions is not, however,fixed once and for all by the nature of the environment, on the one hand, and of science,on the other. Rather,it is determinedjointly by th_eenvironmentand the particular normal-scientifictradition that the student has 6een trained to pursue.Therefore,at times of revolution, when the normal-scientifictradition chan_ges, the scientist'sperceptionof his environmentmust be re-educated-in somefimilia'r situationshe must learn to seea new gestalt.After he hasdone sothe world of his researchwill seemlhere and there, incommensurable with the one he had inhabited before. That is another t:":ol why schoolsguided by difierent paradigmsare always slightly at cross-purposes.
the absenceof the goggles,and the result is extremedisorienta-
The subiectsof the anomalousplaying-cardexperimentdiscussedin SectionVI experienceda quite ii*ilat trinsformation. Until taught by prolongedexpos.trethat the universecontained t original experiments were by George M. stratton, "vision without _ r!" Inversion of the Retinal rmage," psychological,Reoicto, Iv (lgg7), 94l-60, 'An 4,63-81. A more up-to-date re-view ii proviied by l-Iarvey A. carr, lntroyork, duction to Space Perception (New tggS), pp. lg-57.
112
os ChongesoI World View Revolufions anomalouscards, they saw only the types of cards for which previous experiencehad equipped them. Yet once experience had provided the requisite addiUonalcategories,theywere able to sei all anomalouscardson the ffrst inspectionlong enoughto permit any identiffcation at all. Still other experimentsdemonitrate that the perceivedsize,color, and so on, of experlmentally displayedobiectsalsovaries with the subiect'sprevioustraining and experience.2Surveyingthe rich experimentalliterature from which these examplesare drawn makesone suspectthat something like a paradigm is prerequisiteto perception itself. What a man seesdepends both upon what he looks at and also upon what his previous visual-conceptualexperiencehas taught him to see.In the absenceof such training there can only be, in William James'sphrase,"a bloomin' buzzin' confusion." fn recent yearsseveralof thoseconcernedwith the history of sciencehave found the sorts of experimentsdescribed above immensely suggestive.N. R. Hanson, in particular, has used gestalt demonstrationsto elaborate some of the same consequencesof scientiffc belief that concern me here.8Other colleagues have repeatedly noted that history of science would make better and more coherentsenseif one could supposethat scientists occasionally experienced shifts of perception like those described above. Yet, though psychological experiments are suggestive,th"y cannot, in the nature of the case,be more than that. They do display characteristicsof perception that could, be central to scientiffc development, but they do not demonstratethat the eareful and controlled observation exercisedby the researchscientistat all partakesof thosecharacteristics.Furthermore,the very nature of theseexperimentsmakes any direct demonstrationof that point impossible.If historical example is to make these psychologicalexperimentsseemrele2 For examples, see Albert H. Hastorf, "Tlre Influence of Suggestion on the Relationship between Stimulus Size and Perceived Distan@," journal of Psachology, XXIX (1950), l95-l2l7; and Jerome S. Bruner, Leo Postmati, atid John Rodrigues, "Expectations and the Perception of Color," American Journal of Psychology, LXIY ( f 95f ), 2lO-27. 3 N. R. Hanson, Patterns ol Discooery (Cambridge, 1958), chap. i.
fhe Sfructureof ScientiffcRevolutions vant, we must first notice the sortsof evidencethat we may and may_not gxpecthistory to provide. subiectof a gestaltdemonstrationknows that his percep. tu tion has shifted becausehe can make it shift back and firth r^e-
as in all similar psychologicalexperiments,the efiectivenessof the demonstrationdependsupon its being analyzablein this *3y._ unless there were an external standird with respect to which a switch of vision could be demonstrated,no con-clusion about alternateperceptualpossibilitiescould be drawn. with scientificobservation,however,the situation is exactly reversed.The scientistcan have no recourseabove or beyond what he seeswith his eyesand instruments.If there were some higher authority by recourseto which his visionmight be shown to have shifted, then that authority would itself become the sourceof his data, and the behaviorof his vision would become a source_olproblems (as that of the experimentalsubjectis for the psychologist).The samesortsof p'oblemswould if the "i.ir"of the scientist could switch back and forth like the subject gestaltexperiments.The period during which light was "sometimes a wave and sometimesa particle" was a period of crisisa_period rvhen somethingwas wrong-and it ended only with the developmentof wave mechanicsand the realizationthat light was a self-consistent entity difierent from both wavesand particles. In the sciences,therefore,if perceptualswitchesac-
tt4
Revolulions os Chongesof WorldYiew company paradigm changes,we may not expect scientiststo attest to thesechangesdirectly. Looking at the moon, the convert to Copernicanismdoesnot say,"I usedto seea planet,but now f see a satellite." That locution would imply a sensein which the Ptolemaicsystemhad once been correct. Instead,a convertto the new astronomysays,"I oncetook the moonto be (or sawthe moonas) a planet,but I wasmistaken."That sortof statementdoesrecur in the aftermath of scientificrevolutions.If it ordinarily disguisesa shift of scientificvision or someother mental transformationwith the sameeffect,we may not expect direct testimonyabout that shift. Rather we must look for indirect and behavioralevidencethat the scientistwith a new paradigm seesdifferently from the way he had seenbefore. Let us then return to the data and askwhat sortsof transformationsin the scientist'sworld the historianwho believesin such changes can discover. Sir William Herschel's discovery of Uranus providesa first exampleand one that closely parallels the anomalouscard experiment.On at least seventeendifierent occasionsbetween1690and 1781,a number of astronomers, including severalof Europe'smosteminentobservers, had seena star in positionsthat we now supposemust have been occupied at the time by Uranus.One of the best observersin this group had actuallyseenthe star on four successive nights in 176gwitliout noting the motion that could have suggestedanotheridenti-
4 Peter Doig, A concise llisttn'y of Astrorutmrl (London, lg50), pp. rrs-16.
I 15
fhe Slruclureof ScienlificRevolufions had been observedofi and on for almost a century was seendif' ferently after 1781because,like an anomalousplaying card, it could no longer be fftted to the perceptual categories (star or comet) provided by the paradig* that had previously Prevailed. The shift of vision that enabled astronomersto see Uranus, the planet, does not, however, seemto have affected only the
their small size,these did not display the anomalousmagniffcation that had alerted Herschel. Nevertheless,astronomersPrepared to ffnd additionalplanetswere able,with standardinstruments, to identify twenty of them in the first fifty years of the
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that comets teenth-century astronomersrePeatedly disco_vered wanderedat will through the spacepreviouslyreservedfor the sRuclolph Wolf, Geschichte der Astronomie (Munich, 187-7),.pp. -to 513-15, it to explain explain makes i[ account makes Wolf's accottnt particularly how difficult pglf's how difficult B-gB. ttotice !"tti""tarly 6$-9t-1,ilii"" these discoveries as a conseq.tenceof Bode's Law' o Joseph Needham, science and cio{lization in china, 1959), 423-29,434J6.
lr6
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Revolutions os Chongesof World View immutable planets and stars.TThe very easeand rapidity with which astronomerssaw new things when looking at old obiects with old instrumentsmay make us wish to saythat, after Copernicus, astronomerslived in a different world. In any case,their researchrespondedasthough that were the case. The precedingexamplesare selectedfrom astronomybecause reports of celestial observation are frequently delivered in a vocabularyconsistingof relatively pure observationterms.Only in suchreportscan we hope to ftnd anything like a full parallelism betweenthe observationsof scientistsand thoseof the psychologist'sexperimentalsubjects.But we need not insist on so
however,only one of many new repulsiveefiectsthat Hauksbee saw. Through his researches,rather as in a gestalt switch, repulsion suddenly became the fundamental manifestation of electrification,and it was then attractionthat neededto be ex7T. S. Kuhn, Tlte Copernican Reoolution (Cambridge, Mass., lgET), pp. 20&9.
tt7
TlreSfruclureof ScienfificRevolufions plained.sThe electrical phenomenavisible in the early eighteenth century were both subtler and more varied than those seenby observersin the seventeenthcentury.Or again,after the assimilationof Franklin'sparadigm,the electricianlooking at a Leyden jar saw somethingdifferent from what he had seenbefore. The devicehad becomea condenser,for which neither the iar shapenor glasswas required. Instead, the two conducting coatings-oneof which had beenno part of the original deviceemergedto prominence.As both written discussionsand pictorial representationsgradually attest, two metal plates with a non-conductorbetweenthem had becomethe prototypefor the class.eSimultaneously,other inductive effectsreceivednew descriptions,and still otherswere noted for the first time. Shiftsof this sort are not restrictedto astronomyand electricity. We have already remarkedsomeof the similar transformations of vision that can be drawn from the history of chemistry. Lavoisier, we said, saw oxygen where Priestley had seen dephlogisticatedair and where othershad seennothing at all. In learning to seeoxygen,however,Lavoisier also had to change his view of many other more familiar substances.He had, for example,to see .o-po,rnd ore where Priestleyand his contemporarieshad "seenan elementaryearth,and there were other such changesbesides.At the very least,as a result of discovering oxygen,Lavoisiersawnaturedifferently.And in the absence of somerecourseto that hypotheticalfixed nature that he "saw differently," the principle of economywill urge us to say that after discoveringoxygenLavoisier worked in a different world. I shall inquire in a moment about the possibility of avoiding this strangelocution,but first we require an additional example of its use,this one deriving from one of the best known parts of the work of Galileo. Sinceremote antiquity most people have seenone or anotherheavy body swinging back and forth on a string or chain until it ffnally comesto rest.To the Aristotelians, 8 Duane Roller and Duane H. D. Roller, The Deoelopmert of the Concept of Electric Charge (Cambridge, Mass., 1954), pp. 21-29. e See the discussion in Section VII and the literature to which the reference there cited in note I will lead.
ll8
Revolulions os Chongesof World View
other hand, looking at the swinging body, saw a pendulum, a
before. Why did that shift of vision occur? Through Galileo's individual genius,of course.But note that genius does not here manifest itself in more accurateor obiectiveobservationof the swingingbody. Descriptively,the AristotelianPercePtionis iust as accurate.When Galileo reported that the pendulum'speriod was independentof amplitude for amplitudesas great as 90o, his view of the pendulumled him to seefar more regularitythan we can now discoverthere.ll Rather, what seemsto have been involved was the exploitationby geniusof perceptualpossibilities made availableby medievalparadigm shift. Galileo was " an Aristotelian. On the contrary, he not raised completely as was trained to analyzemotionsin termsof the impetustheory, a late medievalparadigmwhich held that the continuingmotion of a heavy body is due to an internal power implanted in it by the projector that initiated its motion. Jean Buridan and Nicole Oresme, the fourteenth-centuryscholasticswho brought the impetustheory to its mostperfectformulations,are the first men 10 Galileo Galilei, Dialogues concerning Tuso New Sciences,trans. H. Crew and A. de Salvio (Evanston, Ill., 1946), pp. 80-81, 162-66. 1r lbid, pp. 9l-94,244.
fhe Sfructureof ScienliffcReyolufions
string back, implantingincreasingimpetusuntil the mid-point of motion is reached;after that the impetusdisplacesthe string in the oppositedirection,againagainstthe string'stension,and so on in a symmetricprocessthat may continueindeffnitely. Later in the century oresme sketcheda similar analysisof the swingingstonein what now appearsas the first discussionof a pendulum.l'Hisview is clearlyvery closeto the onewith which Galileo first approachedthe pendulum. At least in oresme's case,and almost certainly in Galileo'sas well, it was a view madepossibleby the transitionfrom the original Aristotelianto the scholasticimpetusparadigmfor motion. until that scholastic paradigmwas invented,there were no pendulums,but only swinging stones, for the scientist to see. Pendulums were brought into existenceby somethingvery like a paradigm-induced gestaltswitch. Do we, however,really need to describewhat separatesGalileo from Aristotle, or Lavoisierfrom Priestley,as a transformation of vision?Did thesemen really see different things when laoking at the same sorts of obiects?Is there any legitimate sensein which we can say that they pursuedtheir researchin different worlds?Thosequestionscan no longer be postponed, for there is obviously another and far more usual way to describe all of the historical examplesoutlined above. Many readerswill surely want to say that what changeswith a paradig- is only the scientist'sinterpretation of observationsthat themselvesare fixed once and for all by the nattrre of the environmentand of the perceptualapparatus.On this view, Priestley and Lavoisier both saw oxygen,but they interpreted their observationsdifferently; Aristotle and Galileo both saw pendu1z M. Clagett, The Science ol Meclnnics in the Miildlc Ages ( l\ladison, Wis., 1959), pp. 537-38,570.
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os Chongesof World View Revolufions Iums,but they differed in their inteqpretationsof what thuy both had seen. Let me say at once that this very usual view of what occurs when scientistschange their minds about fundamental matters ' can be neither all wrong nor a mere mistake. Rather it is an essentialpart of a philosophicalparadigm initiated by Descartes and developedat the sametime as Newtonian dynamics.That paradigmhas servedboth scienceand philosophywell. Its exploitation, like that of dynamics itself, has been fruitful of a fundamentalunderstandingthat perhapscould not have been achievedin another way. But as the exampleof Newtonian dynamics also indicates,even the most striking past successprovides no guaranteethat crisis can be indefinitely postponed.Today researchin parts of philosophy,psychology,linguistics,and even art history, all converge to suggest that the traditional paradigmis somehowaskew.That failure to fft is alsomade increasinglyapparentby the historical study of scienceto which most of our attention is necessarilydirected here. None of thesecrisis-promotingsubjectshas yet produced a viable alternateto the traditional epistemologicalparadigm,but they do begin to suggestwhat someof that paradigm'scharaceristicswill be. I am, for example,acutely awareof the difffculties createdby sayingthat when Aristotle and Galileolookedat swinging stones,the first saw constrainedfall, the seconda pendulum.The samedifficultiesare presentedin an evenmore fundamentalform by the opening sentencesof this section: though the world doesnot changewith a changeof paradigm, the scientistafterward works in a difierent world. Nevertheless, I am convincedthat we must learn to makesenseof statements that at least resemblethese. What occurs during a scientific revolution is not fully reducible to a reinterpretationof individual and stabledata. In the first place,the data are not unequivocallystable.A pendulumis not a falling stone,nor is oxygen dephlogisticated air. Consequently,the data that scientists collect from thesediverseobjectsare, as we shall shortly see, themselvesdifferent. More important, the processby which 121
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fhe Sfruclure of Scientific Revolufions either the individual or the community makes the transition from constrained fall to the pendulum or from dephlogisticated air to oxygen is not one that resembles interpretation. How could it do so in the absence of fixed data for the scientist to interpret? Rather than being an interpreter, the scientist who embraces a new paradigm is like the man wearing inverting lenses. Confronting the same constellation of objects as before and knowing that he does so, he nevertheless ffnds them transformed through and through in many of their details. None of these remarks is intended to indicate that scientists do not characteristically interpret observations and data. On the contrary, Galileo interpreted observations on the pendulum, Aristotle observations on falling stones, Musschenbroek observations on a charge-filled bottle, and Franklin observations on a condenser. But each of these interpretations presuPPosed a paradigm. They were parts of normal science, an enterprise that, as we have already seen, aims to refine, extend, and articuIate a paradigm that is already in existence. Section III provided -atry examples in which interpretation played a central role. Those examples typify the overwhelming majority of research. In each of them the scientist, by virtue of an accepted paradigm, knew what a datum was, what instruments might be used to retrieve it, and what concepts were relevant to its interpretation. Given a paradigm, inteqpretation of data is central to the entelprise that explores it. But that interpretive enteqprise-and this was the burden of the paragraph before last-can only articulate a paradigm, no_t correct ii Paradigms are not corrigible by normal science at all. Instead, as we h"'tt" already seen, normal science ultimately Ieads only to the recognition of anomalies and to crises. And these are terminated, not by deliberation and interpretation, but by a relatively sudden and unstnrctured event like the gesalt switch. Scientists then often spgak of the "scales falling Ito- the eyes" or of the "lightning fash" that "inundates" a previously obscure ptzzle, enabling its components to be seen in r"* way that for the first time permits its solutiott. OT other "
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os Chongesof World View Revolufions occasionsthe relevant illumination comesin sleep.l3No ordi'inteqpretation' nary senseof the term fits theseflashesof intuition throughwhich a new paradigmis born. Though suchintuitions depend upon the experience,both anomalousand congruent, gainedwith the old paradigm,they are not logically or piecemeallinked to particular items of that experienceas an interpretationwould be. Instead,they gather up large portions of that experienceand transform them to the rather different bundle of experiencethat will thereafterbe linked piecemealto the new paradigmbut not to the old. To learn more about what thesedifierencesin experiencecan be, return for a momentto Aristotle,Galileo,and the pendulum. What data did the interactionof their difierent paradigmsand their common environmentmake accessibleto each of them? Seeingconstrainedfall, the Aristotelian would measure(or at least discuss-the Aristotelianseldommeasured) the weight of the stone,the vertical height to which it had been raised,and the time required for it to achieverest. Together with the resistanceof the medium, these were the conceptualcategories deployed by Aristotelian sciencewhen dealing with a falling body.raNormal researchguided by them could not have produced the laws that Galileo discovered.It could only-and by another route it did-lead to the seriesof crises from which Galileo's view of the swinging stone emerged.As a rezult of those crisesand of other intellectual changesbesides,Galileo saw the swinging stonequite differently. Archimedes'work on floating bodies made the medium non-essential;the impetus theory rendered the motion symmetrical and enduring; and NeoplatonismdirectedGalileo'sattentionto the motion'scircu13 [Jacques] Hadamard, Subconscient intuition, et logique dans lc recherche scientifique (confirence laite au Pal.aisde ln Dicourtelt" le 8 Ddcembre lg4| n.d.]), pp.7-8.A much [Alengon, fuller account, thorrgh one exclusively restricted to mathematical innovations, is trre same authois The psgcholofiy of lnrsention in the Muthematical Field (Princeton, fg4g). tn __ T. s. Kuhn, "A Function for Thought Experiments," in Mhlanges Alerandre Kgr6, ed. R. Taton and I. B. cohen, to be published by Hermain (paris) in 1963.
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The Structure of ScientificRevolulions lar form.rr' He therefore measured only weight, radius, angular displacement, and time per swing, which were precisely the data that could be interpreted to yield Galileo's laws for the pendulum. In the event, interpretation proved almost unnecessary. Given Galileo's paradigms, pendulum-like regularities were very nearly accessibleto inspection. How else are we to account for Galileo's discovery that the bob's period is entirely independent of amplitude, a discovery that the normal science stemming from Galileo had to eradicate and that we are quite unable to document today. Regularities that could not have existed for an Aristotelian (and that are, in fact, nowhere precisely exemplified by nature) were consequencesof immediate experience fot the man who saw the swinging stone as Galileo did. Perhaps that example is too fanciful since the Aristotelians recorded no discussionsof swinging stones.On their paradigm it was an extraordinarily complex phenomenon. But the Aristotelians did discuss the simpler case, stones falling without uncommon constraints, and the same differences of vision are apparent there. Contemplating a falling stone, Aristotle saw a .hotrg" of state rather than a Process.For him the relevant of a motion wele therefore total distance covered and *"u*r"t total time elapsed, parameters which yield what we should now call not tp"ed but average speed.16Similarly, becausethe stone was impelled by its nature to reach its final resting point, Aristotle saw the relevant distance Parameter at any instant during the motion as the distance to the final end point rather than as thatfrom the origin of motion.r? Those conceptual-parameters underlie and give sense to most of his well-known "laws of motion." Partly through the impetus paradigm, however, and partly throtrgh-a doctrine known as the latitude of forms, scholastic criticisnichanged this way of viewing motion. A stone moved by impetus gained more and more of it while receding from its r5 A. Kovr6. Etutles Guliltiennes (Paris, 1939), I, 46-51; and "Galileo and Pfato," 1,,u|rnaiol the llistory of ltleas,IV ( f943), 400428' rG Kulrn, "A Function for Thought Expcriments," in Mtlanges Alemndre Kotp6 (see n. 14 for full citation)' r? Koyr6, Etutles. . . , II, 7-Il.
124
os Chongesof World Yiew Revofulions starting point; distancefrom rather than distanceto therefore b"ca*I ihe te.relantparameter.In addition, Aristotle'snotion of speedwas bifurcaled by the scholasticsinto conceptsthat rootr-after Galileobecameour averagespeedand instantaneous speed.But when seenthrough the paradigmof which theseconceptionswere a part, the falling stone,like the _pendulum,exhibited its governinglaws almoston inspection.Galileowas not one of the hrst *"n-to suggestthat stonesfall with a uniformly acceleratedmotion.l8Furthermore,he had developedhis theoberem on this subjecttogetherwith many of its consequences was theorem That inclined plane. fore he experimentedwith an another one of the network of new regularities accessibleto genius in the world determined iointly by nature and by the had been paradigmsupon which Galileoand his contemporaries he chose, when still, could iaised.Living in that world, Galileo the Nevertheless, he did. explainwhy Aristotlehad seeuwhat stones falling with immediatecontent of Galileo'sexpelience was not what Aristotle'shad been. It is, of course,by no meansclear that we need be so concerned with "immediate experience"-that is, with the perceptual featuresthat a paradigm so highlights that they surrender their regularitiesalmost upon inspection.Those featuresmust obviously change with the scientist'scommitments to paradigms, but they are far from what we ordinarily have in mind when we speakof the raw data or the brute experiencefrom which scientific researchis reputed to proceed. Perhapsimmediate experienceshould be set asideas fuid, and we should that discussinsteadthe concreteoperationsand measurements the scientistperformsin his laboratory.Or perhapsthe analysis should be carried further still from the immediately given. It might, for example,be conductedin terms of someneutral observation-language, perhaps one designed to conform to the retinal imprints that mediate what the scientist sees.Only in one of theseways can we hope to retrieve a realm in which experienceis again stable once and for all-in which the penduIum and constrainedfall are not differentperceptionsbut rather 18 Clagett, op. cit., clraps. iv, vi, and ix.
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fhe Sfructureof ScienfificRevolufions different interpretationsof the unequivocaldata provided by observationof a swingingstone. But is sensoryexperiencefixed and neutral? Are theories simply tnrn-*rie iriterpretationsof given data? The epistemological_viewpoint that hasmost often guided western pirilorforthree centuriesdictatesan immediateand t neq.riuocal, _ollly Yes!In the absenceof a developedalternative,I find il impossible to_relinquishentirely that viewpoint. Yet it no longerfirnctions_effectivel/,and the attemptsto make it do so through the introduction of.a neutral languageof obs-ervatiop$ now seemtq. I "rme hopel"rs.lF ttr d$( i*f fy,lif-s ",,,i,{ 'r.l- 1r. 1gTr,':nrt.| The operationsand measurements that a scientistundertakes in the laboratory are not "the given" of experiencebut rather "the collectedwith difficulty." They are not what the scientist sees-at leastnot beforehis researchis well advancedand his attention focused.Rather,they are concreteindicesto the content of more elementary perceptions,and as such they are selectedfor the closescrutiny of normal researchonly because they promiseopportunity foi the fruitful elaborationof an accepted paradigm.Far more clearly than the immediateexperience from which they in part derive, operationsand measurements are paradigm-deterrnined. Sciencedoes not deal in all possiblelaboratorymanipulations.Instead,it selectsthoserelevant to the juxtapositionof a paradigm with the immediate experiencethat that paradigmhas partially determined.As a result, scientistswith different paradigms engagein different concretelaboratory manipulations.The measurementsto be performedon a pendulum are not the onesrelevantto a caseof constrained fall. Nor are the operationsrelevantfor the elucidation of oxygen'spropertiesuniformly the sameasthoserequired when investigatingthe characteristics of dephlogisticatedair. As for a plrre observation-language, perhapsone will yet be devised.But three centuriesafter Descartesour hope for such an eventualitystill dependsexcltrsivelytrpon a theory of perception and of the miud. And modenr psychologicarl experimentation is rapidly proliferating phenomenawith which that theory can scarcelydeal. The duck-rabbit showsthat two men 126
Revolufionsos Chonges of World View with the same retinal impressionscan see different things; the inverting lenses show that two men with different retinal impressionscan see the same thing. Psychology supplies a great deal of other evidence to the same effect, and the doubts that derive from it are readily reinforced by the history of attempts to exhibit an actual language of observation.No current attempt to achieve that end has yet come close to a generally applicable language of pure percepts. And those attempts that come closest share one characteristic that strongly reinforces several of this essay'smain theses. From the start they presuppose a paradigm, taken either from a current scientific theory or from some fraction of everyday discourse,and they then try to eliminate from it all non-logical and non-perceptual terms. In a few realms of discoursethis effort has been carried very far and with fascinating results. There can be no question that efforts of this sort are worth pursuing. But their result is a language that-like those employed in the sciences-embodies a host of expectations about nature and fails to function the moment these expectations are violated. Nelson Goodman makes exactly this point in describing the aims of his Structure of Appearance: "It is fortunate that nothing more [than phenomena known to exist] is in question; for the notion of possible' cases,of casesthat do not exist but might have existed, is far from clear."ro No language thus restricted to reporting a world fully known in advance can . produce mere neutral and objective reports on "the given." Philosophical investigation has not yet provided even a hint of what a language able to do that would be like. Under these circumstances we may at least suspect that scientists are right in principle as well as in practice when they treat
127
fhe Sfruclureof ScienfificRevolufions :rhapsalsoatomsand electrons) ; of their immediateexperience. nbodiedexperienceof ihe race,
;f;'ffi;t*;:*j,:H"Jt: compared with thes e objects :?i"-*t:::trL"rT:.::':ffi1
readings and retinal imprints are constr-r:ctsto which "l"boot" experiencehas direct accessonly when the scientist,for the specjal pyrposes of his research, arranges that one or the otler should do so. This is not to suggestthat pendulums, for example, are the only things a scientisi could poisibly see when looking at a swinging stone. (we have already noted that members oT another scientific community could see constrained fall. But it ) is to suggestthat the scientist who looks at a swinging stone can have no experience that is in principle more than "I""-J"tory s.eeing_ a pendulum. The alternative is not some hypothetical "fixed" vision, but vision through another paradigm,'o^n"which mak-esthe swinging stone something else. All of this may seem more reasonableif we again remember that neither scientists nor laymen learn to see th-"eworld piecemeal or item by item. Except when all the conceptuaj and manipulative caiegories ur" pi"p"red in advance-e.|., for the discovery of an additional tranzuranic element or foi catching sight of a new house-both scientistsand Iaymen sort out whol6 al'eastogether from the flux of experience.The child who transfers the word'marnA'from all humans to all females and then to his mother is not just learning what 'nlArna'rlcAns or who his mother is. Simultaneotrslyhe is leanring somc of the clifferences between males anclfemales as well as sonrething about the ways in which all but one fernale will behavc towarcl him. FIis reactions, expectations,and beliefs-indc,ed, nruch of his perceived world--ch:rnge accordingly. By the samc tokcrr, the Copernicans who denied its traditional title 'plarret'to thc ,,r., ou"rJnot only lcarning what'planct'nreant ol what the sun was. Iusteacl,thev lvere changing the meaning of 'planet'so that it could corrtinul to makc trsefuldistinctionsin a world where all celestitl lloclies.
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os Chongesof World View Revolutions not just the sun, were seendifferently from the way they had been seenbefore.The samepoint could be made about any of our earlier examples.To r"" 6*yg"n insteadof dephlogisticated air, the condenserinsteadof the Leyden jar, or the pendulum insteadof constrainedfall, was only one part of an integrated shift in the scientist'svision of a great many related chemical, electrical,or dynamicalphenomena.Paradigmsdeterminelarge areasof experienceat the sametime. It is, however, only after experiencehas been thus determined that the searchfor an operationaldefinition or a pure can begin. The scientist or philosopher observation-language who asks what measurementsor retinal imprints make the pendulum what it is must already be able to recognize a pendulum when he seesone. If he saw constrainedfall instead, his questioncouldnot evenbe asked.And if he sawa pendulum, but sawit in the sameway he saw a tuning fork or an oscillating balance,his question could not be answered.At least it could not be answeredin the samewo/, becauseit would not be the same question. Therefore, though they are always legitimate and are occasionallyextraordinarily fruitful, questionsabout retinal imprints or about the consequences of particular laboratory manipulations presupposea world already perceptually and conceptuallysubdividedin a certain way.In a sensesuch questionsare partsof normalscience,for they dependupon the existenceof a paradigmand they receivedifferent answersas a result of paradigmchange. To conclude this section,Iet us henceforth neglect retinal impressionsand againrestrict attention to the laboratoryoperations that provide the scientistwith concretethough fragmentary indicesto what he hasalreadyseen.One way in which such Iaboratoryoperationschangewith paradigmshas alreadybeen observed repeatedly. After a scientific revolution many old measurements and manipulationsbecomeirrelevant and are replacedby others instead.One does not apply all the same teststo oxygenas to dephlogisticated air. But changesof this sort are never total. Whateverhe may then see,the scientist after a revolution is still looking at the sameworld. Further129
fhe Sfructureof ScienlificRevolufions
sionallythe old manipulationin its new role will yield difierent concreteresults.
Affinity theory, however,drew the line separatingphysical Melzgn Neuton, Stali, Boerlnate -^:t^TI. 1930), pp. 34-68.
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et I^a doctrine chimique (paris,
os Chongesof World View Revolufions mixtures from chemicalcomPoundsin a way that has become unfamiliar sincethe assimilaiionof Dalton's work. Eightee-nthWhen century chemistsdid recognizetwo sorts of processes' of else or something producedheat, tighi, effervescence ;i;d the on If' the soit, chemicalunion *", ,""r, to have to\91place' oth., hand, the particlesin the mixture could be distinguished or rr,echuiricallyseparated,there was only physicalmixUy "y" t.,re. But in the very l"tg" ttn*ber of intermediatecases-saltin and so on-these *"t"r, alloys,gl"tt, o*y[.tt in the a1m91phe5,. crudecriteria *"r" of iittte use.Guided by their paradigm,m-ost chemistsviewed this entire intermediaterange as chemical,beof which it consistedwere all governedby causethe processes forcesof the samesort. Salt in water or oxygenin nitrogen was combinationas was the ittst as much an exampJeof chemical copper.The argumells {or tombination produced^byoxidizing -were very strong. AffinitY tds the formationof a comBesides, 's observedhomogeneitY'If, for
ilffiJ"l'n,H'"il$ilH"
settle to the bottom, Dalton, who took the atmosphere to be a mixture, was never satisfactorily able to explain oxygen's failure to do so. The assimilation of his atomic theory ultimately cre' ated an anomaly where there had been none before.tt One is tempted to say that the chemists who viewed solutions as compoundi differed from their successorsonJy over a matter of definition. In one sensethat may have been the case'But that sense is not the one that makes definitions mere conventional conveniences.In the eighteenth century mixtures were not fully distinguished from .o-po.tttds by oo-erationaltests, and Pernot have been. Even if chemists had looked for haps ih"y "o.tld have sought criteria that made the soluwould they tests, ,rr"h mixtttre-compound distinction was pa_rt The a compound. tion of their pJradigm-pnrt of the way they viewed their whole 2r lbid.. pp. 124-29,13H8. For Dalton, see Leonard K. Nash, The.Atomic' |frnorrl ("Fiarvard Case Histories in Expcrimental Science," Case 4; Uoluiii Cambridge, Mass., 1950), PP. 14-21.
t3l
ffie Sfruclureof ScientiffcRevolufions ffeld of research-andas suchit wasprior to any particular laboratory test,thoughnot to the experiinceof chemis"cc.r*,rl"ted try as a whole.
first claimed that all chemicalreactionsoccurred in fixed proportion, the latter that they did not. Each collectedimpresiive experimentalevidencefor his view. Nevertheless,the two men necessarilytalked through eachother, and their debatewas entirely_inconclusive.where Berthollet saw a compound that could vary in proportion,Proustsaw only a physicalmixture.eg To that issueneither experimentnor a change of definitional convention could be relevant. The two men were as fundamentally at cross-purposes as Galileo and Aristotle had been. This wasthe situationduring the yearswhenJohnDalton undertookthe investigations that led finally to his famouschemical atomictheory.But until the very last stagesof thoseinvesuga,rJ.R:
P a r t i n g t o n ,A S / r o r t l l i s t o r y o f C l r c n i s t r y ( g c l c c l . ; L o ' d o n , l g 5 l ) ,
pp. r6r-ffi.
23 A. N. Mcldrurn, "The Development of the Atomic Theory: ( l ) Berthollet's Doctrinc of Variirblc Proportions," Llanclrcstcr Mcmoirs, LIV (lgl0), f-16.
132
Revolulionsqs Chongeso[ World Yiew tions,Dalton was neither a chemistnor interestedin chemistry.
atomic particlesin his experimentalmixtures.It was to determine these sizes and weights that Dalton finally turned to chemistry, supposingfrom the start that, in the restricted range of reactionsthat he took to be chemical,atomscould only combine one-to-oneor in someother simple whole-numberratio.2a That natural assumptiondid enablehim to determinethe sizes and weightsof elementaryparticles,but it alsomade the law of constant proportion a tautology. For Dalton, any reaction in which the ingredients did not enter in fixed proportion was ipsofacto not a purely chemicalprocess.A law that experiment could not have establishedbefore Dalton'swork, became,once that work was accepted,a constitutiveprinciple that no single set of chemicalmeasurementscould have upset.As a result of what is perhapsour fullest exampleof a scientificrevolution,the samechemicalmanipulationsassumeda relationshipto chemical generalizationvery different from the one they had had before. Needlessto say, Dalton's conclusionswere widely attacked when first announced.Berthollet,in particular, was never convinced. Consideringthe nature of the issue,he need not have been.But to most chemistsDalton'snew paradigmproved convincing where Proust'shad not been,for it had implicationsfar wider and more important than a new criterion for distinguish24 L. K. Nash, "The
Origin
xLvII (1956),10r-16.
of Dalton's Chemical Atomic Theory,"
fsis,
r33
fhe Sfructureof ScienfificRevolulions ing mixfure froS a eompound. If, for example,atoms 3 could combine chemically only in simple whole-nurib", ratios, then a re-examinationof existingchemicaldata shoulddisclose exampJes of multiple as welf as of fixed proportions. chemists writing that the two oxidesof, r"y] contained *lpp"a ,56per cent and 72 per cent of oxyge:t_Uy ""tbon #"iglrt; instead th"y wrote that one weight of carbon **n either with l.b "'o*biri, 2.6 weightloj when the results of old manipu9x1sen. Lyt,h Iationswere recordedin this wzry,a 2:l ratio leapedto the e]re; and this occurred_inthe analysisof many *"ti-tio*r reactions and of new onesbesides.tn ldditiorr, D'"lton's paradigm made it possibleto assimilateRichtert work and to r""^it, f,rligerr"Jity. AJso,it suggestednew experiments,particularly firoru o1 Gay-Lussacon combiningvoluires, and thise yielded still other regularities, onesthat chemistshad not previously dreamed of. what chemists took from Dalton was not new experimental laws but a new way-of practicing chemistry (he him^selfcalled it the.'new systemof chimical pfilosophy"i, and this prou"d so rap_idlyfruitful that only a few-of thu oti"r chemistsin France and Britain were able to resistit.2rAs a result,chemistscameto Iive in a world where reactionsbehaved quiie difierently from the way they had before. As all this went on, one other typical and very important change occurred. Here and theru t:hr very rr,r-"ii."l iata of chemistry beganto shift. when Dalton ffrst searchedthe chemical literature for data to suppo{ his physical theory, he found somerecordsof reactionsthat fftted, but he can scarcelyhave avoided ffnding others that did not. proustt own measurements on the two oxidel_of_"opperyielded, for example,an oxygen -than weight-ratio of L.47: l rather the 2: 1 derianaea uf "trru atomic theory; and Proustis iust the man who might haveieen expectedto achievethe Daltonian ratio.z.He waslthat is, a fine 25.A. N. MeJdrum, "The_Development of the Atomic Theory: (6) The Reg"_ptigl Accorded to the Theory A-dvocated by Dalton," uo"it M;;;;;, ( r 9 r r ) , LV l-10. "iii
26For Proust, see Meldrum, "Berthollet's Doctrine of variable proportions,o -trr"' ManchesterMemoirs, LIV-(lgr0), g. The aut"ir"a_r,istoryof sr",ilI chpgel in measurementsof chemical composiiion weiqhts *" has yet to be written, but partington, op. cdr.,piovider 'n"t;"fu1-l""al ""a-"i-"i"*ic tfli.
r34
os Chongesof World View Revolufions and his view of the relationbetweenmixtttres experimentalist, was very close to Dalton's. But it is hard to utl "o-potrnds make nut.,tt fit a paradigm.That is why the puzzlesof normal underscienceare so challengingand also why measurements taken without a paradigmso seldomlead to any conclusionsat all. Chemistscould not, therefore,simply acceptDalton'stheory on the evidence,for much of that was still negative.Instead, even after acceptingthe theory, they had still to beat natttre into line, a procers which, in the event, took alrnost another generation.When it was done,eventhe percentagecomposition of well-known compoundswas different. The data themselves had changed.That is the last of the sensesin which we may want to say that after a revolution scientistswork in.a different
world. - ' ,n,u el v ly),nf-,, i
Wry;
'OOlHif;
I
r35
Xl. Thelnvisibiliry of Revotutions we must still askhow scientificrevolutionsclose.Beforedoing so, however,a last attbmpt to reinforce conviction about theii existenceand nature seemscalled for. I have so far tried to display re_volutions by illustration, and the examplescould be multiplied od nauseam.But clearry,most of them^,which were deliberatelyselectedfor their famiiiarity, have customarilybeen viewed not as revolutionsbut as addiiionsto scientificftnowledge. I!"t sameview could equally well be taken of any additional illustrations,and these*o.t[d probably be inefiective.I suggestthat there are excellentreasonswhy revolutionshave prov_ed to be so nearly invisible.Both scientistsand laymentake much of their image of creativescientificactivity frbm an authoritative sourcethat sygematically disguises-partly for importantfunctionalteatorr-t-EClq.rtA;;lnasignifi canceof Only *F"ffi" nCture of tfrat authority -l5.t$g--f"LtlgJionsis recognizedand analyzedcan one hope to make historical examplefglly effective.Furthermore,though the point can be fully developedonly in my concludingsection,the inalysis now required will begin to indicate one of the aspectsof icientific work that most clearlydistinguishesit from other creative "riry pursuit exceptperhapstheology. As the sourceof authority, I h1"9 in mind principally text-*-* togetherwith both thc nopulaiizationq*nd-i[r-b$s of_scie_nce philosophicalworks modeled on them. eti ttrt"" orJEise categories-until recentlyno other significantsourcesof information about sciencehave been availableexceptthrough the practice of research-haveone thing in common.They addresi themselvesto an already articulated body of problems, data, and theory, most often to the particular set of paradigmsto which the scientificcommunityis committedat the time they arervritten. Textbooksthemselvesaim to communicatethe vocabulary and syntax of a contemporaryscientific language.Popularizations attempt to describethesesame applicationsin a language
r36
The Invisibilityof Revolulions closerto that of everydaylife. And philosophyof science,Particularly that of the Engiish-speakingrvorld,analyzesth-elogical structure of the samecompletedbody of scientiftcknowledge. Though a fuller treatment would necessarilydeal Yi,l thJvery ,""i dittittctionsbetweenthesethree genres'it is their similaritiesthat most concern us here. All three record the thusdisplaythe basesof stableoutcomeof pastrevolutions'and To fulfill their function tradition. the current normai-scientific about the way in information they need not provide authentic then embraced and which those baseswere first recognized at by the profession.In the caseof textbooks, least, there are gobareasonswhy, in thesematters,they shouldbe system"'nrn misleading. atically We noted in SJction II that an increasingreliance on textbooks or their equivalentwas an invariableconcomitantof the emergenceof a hrst paradigm in any field of science.The conwill argue that the.,dominationof cludiig sectionof this "s.! a mattire scienceby such texts significanlly differentiatesits developmentalpattern from that of other fields.For the moment L, rr, ,i*ply taie it for grantedthat, to an extentunprecedented in other fi"idr, both thJ layman'sand the practitioner'sknowledgeof scienceis basedon textbooksand " f"* other types-of htJrature derived from them. Textbooks,however,being pedagogic vehiclesfor the perpetuationof normal science,have to probbe rewritten in whole o. i" part wheneverthe languag_e, short, In change. science normal of standatds or lem-structure, thev have to be rewritten in the aftermath of each scientiffc ,"ullrrtion, and, once rewritten, they inevitably disguisenot only the role but the very existenceof the revolut-ionsthat produced them. Unlesshe has personallyexperienceda revolution in his own lifetime, the hisiorical senseeither of the working scientistor of the lay readerof textbookliterature extendsonly to the outcomeof thl mostrecentrevolutionsin the field. senseof his Textbooksthus begin by truncating the scie_ntist's for substitute a to supply th.tt history discipline's Proceed "id sciof textbooks Characteristically, eiiminated. *fr.ithey have in an introductory ence iust a bit of history, either "orrtoi.,
137
Ihe Slruclureof ScienfificRevolulions chaptero_r, moie often,in scatteredreferencesto the greatheroes earlier age.From such refereneesboth studeritsarrd pro:f "l fessionals come toJeel,like pa-rticipantsin a long-standingListorical tradition. yet the telxtboot-derivedtradition in which scientistscome to sensetheir participation is one that, in fact, never existed.For reasonsthit are both obvious and highly functional, sciencetextbooks (and too many of the old"r"histories of science) refer to that part of the work of past scientiststhat can easily .orly be viewed as iontributions to the st'atement and solution of the texts'par_adigmproblems.partly by selection- p.ttty by djstortioi, the ici"ritirts of earlier'^gJ, ".".d represe_nted are^implicitly as having worked upon the sameiet ^same of ffxed problemi and in accordancewith the set of fixed canonsthat the most recent revolution in scientifictheory and method has made seem scientific. No wonder that texttooks and the historicaltradition they imply haveto be rewritten after each scientificrevolution.And rroiutnder that, as they are rewritten, scienceonce lgain comesto seemlargely cumulative. scientistsare not, of course,the only group itr"i tends to see its discipline'spast developinglinearly io*"ia its presentvantage. The temptation to write history backward is^both omnipresent and perennial.But scientistsare more afiectedby the temptation to rewrite history, partly becausethe resultsof scientific researchshowno obviorisdependenceupon the historical contextof the inquiry, and partly b""rutr, exiept during crisis and revolution, the scientist'scontemporarypoiitior, ,""-, so secure.More historicaldetail, whether of science'spresentor of its past, or more responsibilityto the historical d^etailsthat are presented,could only give artificial statusto human idiosyn_crasy, error, and confusion.why dignify what science'sbest and most persistentefforts have made it possibleto discard? The depreciationof historicalfact is deeply,and probably functionally, ingrained in _theideology of tlle' scientific profession, the sameprofessionthat plac"t tit" highest of all 'rrilrr", upon factual detailsof other rortr. whiteheal caught the unhistorical spirit of the scientific community when hJwrote, "A science that hesitatesto forget its foundeisis lost."yet he wasnot quite
r38
TheInvisibilityof Revolulions right, for the sciences,like other professionalenterprises,do needtheir heroesand do preservetheir names.Fortunately,instead of forgetting these heroes,scientistshave been able to forget or revisetheir works. The result is a persistenttendency to make the history of sciencelook linear or cumulative,a tendencythat even affects scientistslooking back at their own research.For example,all three of Dalton's incompatibleaccountsof the developmentof his chemicalatomismmake it appearthat he was interested from an earlydate in iust thosechemicalproblemsof combining proportionsthat he was later famousfor having solved.Actually those problems seemonly to have occurred to him with their solutions,and then not until his own creative work was very nearly complete.lWhat all of Dalton's accountsomit are the revolutionaryeffectsof applying to chemistrya set of questions and conceptspreviouslyiestricted to physicsand meteorology. That is what Dalton did, and the result was a reorientation toward the ffeld, a reorientationthat taught chemiststo ask new questionsabout and to draw new conclusionsfrom old data. Or again,Newton wrote that Galileohad discoveredthat the constantforce of gravity producesa motion ProPortionalto the squareof the time. In fact, Galileo'skinematic theoremdoes take that form when embeddedin the matrix of Newton'sown
questionsthat scientistsaskedabout motion as well as in the I L. K. Naslr, "The Origins of Dalton's Chemical Atomic Tlteory," Isil XLVII ( 1 9 5 6 ) ,1 0 l - 1 6 . 2For Newton's remark, see Florian Caiori (ed.), Sir lsaac Newton's Mathematitcal Principles of Natural Philosophy_'and H-is System of the WorA (Berkepassafe should be compared with Galileo's own ley, C"lif., 1946), p.Zt.The diicussion'inhis'bklogues conTerniigTtoo Nevl Sciences,trans. H. Crew rnd A. de Salvio (Evanston, Ill., 1946), pp. 154-76'
r39
fhe Sfructureof ScienfificRevolufions they-feltable to accept.But it is just this sort of change Ts:ver: in the formulation of questio-ns and arrs*ersthat accounts,f"ar more than novel empiiical discoveries,for the transition from Aristotelian to Galilean and from Galilean to Newtonian dynamics-By disguisingsu-chchanges,the textbook tend".r"y io makethe developmentof sciencefirr"arhidesa processthat lies at the heartof the mostsigniffcantepisodes of scientificdevelopment. rlay, eachwithin the contextof rgsof a reconstructionof history postrevolutionarysciencetexts. involved than a multiplication
cons tructions ren derrevoru.ro# tli::ff::i,f il:;l:ffi ff ;
the still visible material in sciencetexts implies prJ"*r, that, if it existed,would deny a funition." B^ecause they _revolutions aim quickly-to acquaint the student with what the conte*p;rary scientiftccommunity thinks it knows, textbookstreat ihe aws,and theoriesof the current d as nearly seriatimas possible. rresentationis unexceptionable.
ence writing andwithth"o..ffi:l1r:l*'jil";1,:ilffi.
tions discussedabove,one strong impressionis overwhelmingly likely-to follow: sciencehas reac-hed-its presentstateby ,"rT"', " of individual discoveriesand inventioristhat, when gathered together,constitutethe modern body of technical knJwledge. From the beginningof the enterprise,a textbookpris_scientiffc entationimplies,scientistshave strivenfoi the particular o81""tives that are embodiedin today'sparadigmr.Oou by one, in a processoften compa"edto the addition oi bricks to a building, scientistshave added another fact, concept,law, or theory t-o the body of information supplied in the contemporaryscience text. But that is not the way a sciencedevelops.Many of the puzzlesof contemporarynormal sciencedid not existuntil after the most recent scientificrevolution.very few of them can be 140
TheInvisibilityof Revolulions traced back to the historic beginning of the sciencewithin which they now occur. Earlier generations_Pursuedtheir own problemswith their own instrumentsand their own canonsof iolution. Nor is it just the problemsthat have changed.Rather the whole networic of facl and theory that the textbook paradigm fits to nature has shifted. Is the constancyof chemical coriposition,for example,a mere fact of experiencethat chemists iould have discou6tedby experimentwithin any one of the worlds within which chemistsfr.ut practiced?Or is it rather one element-and an indubitable one, at that-in a new fabric of associatedfact and theory that Dalton fttted to the earlier chemicalexperienceasa whole,changingthat experiencein the process?Ot by the sametoken,is the constantaccelerationproirrced by a constantforce a mere fact that studentsof dynamics have ul*ayr sought,or is it rather the answerto a questionthat first aroseonly i'ithi.t Newtonian theory and that that theory could answeriro- the body of informationavailablebefore the ked about what aPPearas the textbook presentation.But obrs well for what the text Presents )urse,do "fit the facts,"but onlY by transforming previously accessibleinformation into facts tilat, for the pt""..ai"g paiadigm, had not existed at all. And that meansthat theoriJsioo do not evolvepiecemealto fit facts that were there all the time. Rather,they emergetogetherwith the facts they fft from a revolutionaryreformulationof the preceding scierriifictradition, a tradition within which the knowledge-irediatedrelationshipbetween the scientistand nature was not quite the same. One lait examplemay clarify this accottntof the impact of development' upon ottr imageof_scientiftc textbookpresenta^tion Every ele]mentarychetriittry text must discussthe conceptof a chemical element. Almost always, when that notion is introduced,its origin is attributed to ih" t"u"nteenth-ce-nturychemChymist the attentive ist, Robert B-"oyle,in whose Sceptical 'eiement'quite closeto that in of definition find a will reader l4l
fhe Struclureof ScienfificRevolufions use today. Referenceto_Boyle'scontribution helps to make the neophyte aw_arethat chemistry did not begin with the sulfa drugs; in addition, it tells him that one of tf,e scientist'straditional tasksis to invent conceptsof this sort. As a part of the ^attribution pedagogicarsenalthat makesi man a scientist,the is immenselysuccessful.Nevertheless,it illustratesonce more the pattern of historical mistakesthat misleadsboth students
impressionof sciencefosteredwhen this sort of mistakeis first compoundedand then built into the technical structure of the
I _ T. s.- Kuhn, "Rob9-rq lgyl" and structural chemistry in the seventeenth Century," Isis, XLIII ( 1952); 2U29.
142
The Invisibilityof Revolulions or even changethe verbal formula that servesas its deftnition' have to invent or even exNor, as we hive seen,did Einstein 'space'and 'time' in order to give them new redefine plicitly h"tnitrg within the contextof his work. What"then was Boyle'shistorical function in that part of his of work that includesthe famous"deftnition"?He was a leader 'eleof relation the by changing that, a scientific revolution ment' to chemicalmanipulationand chemical theory, transformed the notion into a-tool quite different from what it had been before and transformedbbth chemistryand the chemist's Other revolutions,includingthe one that world in the process.n centers^ro.,rid Lavoisier,were required to give the conceptits modern form and function. But Boyle provides a typical exand ample both of the processinvolved at each of thesestage-s is knowledge existing when of *h*t happensio that process of aspect single other any embodiedin-a textbook.Niore than of the science,that pedagogicform has determinedotrr_image in inventiou and discovery role of nature of scidnceand of the its advance. { Marie Boas, in her Robcrt Boylc and Seoenteenth-Century C.h.?n.:tty with-Eoyle's positive contributions (Cambridge, 1958), deals in many places [1""". with-Boyle's il the l|r" oufiutiot of the concept of a- chemical element' to
Xll. The Resolutionof Revolutions The textbookswe have just only in the aftermath of a sci basesfor a new tradition of nr questionof their structurewe L is the processby which a new ( rpretation of nature, whether a first in the mind of one or a few ;t learn to see scienceand the
'Til# #i""lT.,n:,j':: ;i:; sion. Invariably their attention upon the crisis-provokingprob-
ff:,T1,':"ffi?,1,T iil#Ii: rures determined bytheord n",ll,!n:'i;#*:ffi11ili::#1
must they do, to convert th; entire professiot o, lh" relevant profe_ssional subgroup their *"y of seeingscienceand the !o world? what causesthe group to abandon"one tradition of normal researchin favor ofanother? To-seethe urgencyof thosequestions,rememberthat they are.the only reconstructionsthe historian can supply for thl philosopher'sinquiry about the testing, verificatiJn,'or falsification of establishedscientiftctheoriei. In so far as he is engaged in normal science,the researchworker is a solver of puzzles,not a tester_ofparadigms.Thoughhe ma/, during the searchfor a particular puzzletssolution, try out a numb"erof alternativeapproaches,reiecting thosethat iail to yield the desired result, he is not testing the paratlign when he does so. Insteadhe is like the chessplayer who, with a problenrstatcd and the boardphysicallyor hentally beforehini, tries out various alternativemovesin the searchfor a solution.Thesetrial attempts,whether by the chessplayer or by thc scientist,are 144
fhe Resolulionof Revolutions trials only of themselves,not of the rulesof the gam9.They ar-e possibleonly so long as the paradigm itself is taken for granted. therefore, paradigm-testingoccursonly after persistentfailure to solvea toteworthy puzzlehas given rise to crisis.And even then it occursonly aftei the senseof crisis has evoked an alternate candidatefor paradigm.In the sciencesthe testing situation never consists,as puzzle-solvingdoes,simply in the comparisonof a singleparadigmwith nature.Instead,testingoccurs as part of the c-ompetitionbetweentwo rival paradigmsfor the allegianceof the scientiftccommunity. Closely examined,this formulation displaysunexPectedand probably significantparallelsto two of the most popular con' i"-portry philosophicaltheories about verification. Few phi' losophersof sciencestill seekabsolutecriteriafor the verification of scientifictheories.Noting that no theory can everbe exposed to all possiblerelevanttests,they asknot whether a theory has been verified but rather about its probability in the light of the evidencethat actually exists.And to answerthat questionone important school is driven to comparethe ability of different theories to explain the evidence at hand. That insistenceon comparingtheoriesalso characterizesthe historicalsituation in which r ttr* theory is accepted.Very probably it points one of the directionsin which future discussionsof veriftcationshould go. In their most usttalforms, however,probabilisticverification theoriesall haverecourseto one or anotherof the Pureor neutral observationlanguagesdiscussedin Section X. One probabilistic theory asksthat we comparethe given scientifictheory with all othersthat might be imaginedto fit the samecollection of observeddata. Anotherdemandsthe constntctionin imaginationof all the teststhat the given scientifictheorynright conceivablybe askedto pass.lApparentlysomesuchconstruction is necessaryfor the computationof specificprobabilities,absoIute or relative,and it is hard to seehow sucha constructioncan 1 For a brief sketch of the main routes to probabilistic verification theories, see Ernest Nagel, Principlesof thc Thcory of Probability,Yol.I, No. 6,of. lntertwtional Encyclopctliu ol Unifcd Sciencc, pp. 6f75.
145
FfreSfruclureof ScienfifcRevolufions possiblybe achieved.If, as I have already urged, there can be no scientiffcally-or empirically neutral systeri of languageor concepts,then the proposedconstruction of altemate tests and proceed from within one or another paradigm]heorjel T::t basedtradition. Thus restricted it would have ,ro ac-"us tJal ssibletheories.As a result,prob-
;':fln'i,H t'H:'T il#T:,i:
of theories and of much widespreadevidence,the theoriesand observationsat issueare alI"yr closelyrelated to onesalreadyin existence.Verificationis Iike natural selection:it picks ouf the most viable among the actual alternativesin a particular historical situation. WhEther that choiceis the best that could have been made if still other alternativeshad been availableor if the data had been of another sort is not a question that can usefully be asked.There are no tools to employ in seekinganswersto it. A-very differentapproachto this whole network of problems -existence hasbeendevelopedby Karl R. Popperwho deniesthe of any verificationproceduresat all.'Instead, he emphasizes the importanceof falsiffcation,i.e., of the test that, becauseits outcome is negative,necessitates the rejectionof an established theory. clearly, the role thus attributed to falsificationis much like the one this essayassignsto anomalousexperiences, i.e.,to experiencesthat, by evoking crisis,prepare the way for a new theory. Nevertheless,anomalousexperiencesmay not be identifted with falsifying ones.Indeed, I doubt that ihe latter exist. As has-repeatedly been emphasizedbefore, no theory ever solvesall the puzzleswith which it is confrontedat a given time; nor are the solutions already achieved often perfect. on the contrary, it is just the incompletenessand imperfection of the existing data-theory fit that, at any time, define many of the puzzlesthat characterizenormal science.If any and every failure to fft were ground for theory rejection, all theoriesought to be rejectedat all times.On the other hand, if only severefailure ,2K..R. cnaPs. r-rv.
146
Popper, The Logic of Scientifw Discooery (New Yorlr, lg5g),
"rp.
fhe Resolutionof Revolulions to fft justiftes ft9o.y rejection, then the Popperianswill require somecriterion of "improbability" or of "degree of falsiffcation." In developingone they will almostcertainly encounterthe same network network of di difficulties that has haunted the ihe advocates advoonrecof nf the rhe various probabilistic veriffcation theories. Manl of the pr-ecedingdifficulties can be avoided by recognizing_that both of theseprevalentand opposedviews th1 "6o,rttwo underlyinglogic of scientiffcinquiry havliried to compress Iargely separateprocesses into one. popper'sanomalo,isexperience is important to sciencebecauseit ivokes competitori fo, an existing paradigm_.But falsiffcation, though it suiely occurs, doesnot l"ppg". *jth, or simply becauseoiifru ,*rrgurr., oi an anomalyor falsifying instance.Instead,it is a subseqientand separateprocessthat might equally well be called velriffcation sinceit consistsin the triumph-of a new paradigmover the old one. Furthermore, it is in that joint veriffcat-ion-falsiffcation processthat the- probabilist's comparisonof theories plays a central role. such a two-stageformulation has, I think, t^he'virtue-of great -verisimilitude,and it may also enable.r, to begin explicating the role_of agreement(oi disagreement)betwJen fact and theory_in the verfficationprocess.io th" historian,at least,it makeslittle senseto suggestthat verification is establishing the agreementof fact withlheory. All historicallysigniffcant theorieshave agreedwith the facts, but only -or" oil"rr. There is no more preciseanswerto the question*h"thu, or how well an individual th_eoryffts the facts. but questionsmuch like that can be askedwhen theoriesare taken &lectively or even in pairs. It makesa great deal of senseto ask whict of two actual and competing theories neither Priestley'snor Lavoisier' precisely with existing observat tated more than a decadein cor provided the better fit of the two. This formulation, however, makes the task of choosingbetween paradigmsIook both easierand more familiar than"it is. If there were but one set of scientiffcproblems,one world within which to work on them, and onelet of standardsfor their 147
fhe Structureof ScienfiffcRevolutions solution, paradigm competition might be settled more or less routin-elyby sgmeprocesslike counting the number of problems solved by each. But, in fact, these conditions are never met completely, the proponentsof competingparadigmsare always at leastslightly at cross-pulposes. Neittrei iide wiil grant all the non-empirical assumptionsthat the other needs in order to make its case.Like Proust and Berthollet arguing about the compositionof chemical compounds,they are bound partly to talk through eachother. Though eachmay hope to convert the other to his way of seeinghis scienceand its problems,neither may hope to prove his case.The competition between paradigmsis not the sort of battle that can be resolvedby proofs. We have alreadyseenseveralreasonswhy the proponentsof competingparadigmsmust fail to make completecontact with each other's viewpoints. Collectively these reasonshave been describedas the incommensurabilityof the pre- and postrevolutionary normal-scientiftctraditions,and we needonly recapitulate them briefy here. In the ffrst place, the proponentsof competingparadigmswill often disagreeabout the [st of probIemsthat any candidatefor paradigm must resolve.Their standards or their definitions of scienceare not the same.Must a theory of motion explain the causeof the attractive forces between particlesof matter or may it simply note the existenceof such forces?Newton's dynamicswas widely rejectedbecause, unlike both Aristotle's and Descartes'stheories,it implied the latter answerto the question.When Newton'stheory had been accepted,a questionwas thereforebanishedfrom science.That question,however,was one that generalrelativity may proudly claim to have solved. Or again, as disseminatedin the nineteenth century, Lavoisier'schemicaltheory inhibited chemists from askingwhy the metalswere so much alike, a questionthat phlogisticchemistryhad both askedand answered.The transition to Lavoisier'sparadigmhad, like the transitionto Newton's, meant a loss not only of a permissiblequestion but of an achievedsolution. That loss was not, however, permanenteither. In the twentieth century questionsabout the qualities of
fhe Resolulionof Revolulions chemical substanceshave entered scienceagain, together with some answersto them. More is involved, however, than the incommensurability of standards.Since new Paradigmsare born from old ones, they ordinarily incoqporatemuch of the vocabulary and apparatus, both conceptual and manipulative, that the traditional paradigm had previously employed.But they seldomemploy these bonowed elements in quite the traditional way. Within the new paradig*, old terms, concepts,and experimentsfall into new ielationships one with the other. The inevitable result is what we must call, though the term is not quite right, a mis' understandingbetween the two competingschools.The laymen who scoffed at Einstein's general theory of relativity because spacecould not be "curyed"-it was not that sort of thing-were not simply wrong or mistaken. Nor were the mathematicians, physicists,and philosopherswho tried to develop a Euclidean version of Einstein's theory.sWhat had previously been meant by spacnwas necessarilyfat, homogeneous,isotropic, and unaffected by the presenceof matter. If it had not been, Newtonian physics would not have worked. To make the transition to Einsteint universe, the whole conceptual web whose strands are space,time, matter, force, and so on, had to be shifted and laid down again on nature whole. Only men who had together undergone or failed to undergo that transformation would be able to discoverprecisely what they agreedor disagreedabout. Communication across the revolutionary divide is inevitably partial. Consider, for another example, the men who called Copernicusmad becausehe proclaimed that the earth moved. They were not either iust wrong or quite wrong. Part of what they meant by'earth'was ffxed position.Their earth, at least,could not be moved. Correspondingly,Copernicus'innovation was not simply to move the earth. Rather,it was a whole new way of regarding the problems of physics and astronomy, 3 For lay reactions to the concept of curved space, see Philipp Frank, Efnstein, His Lile atd Thmes, trans. and ed. G. Rosen and S. Kusaka ( New Yorlc, 1947), pp. 14248. For a few of the attempts to preserve the gains of general relativity within a Euclidean spaoe, see C. Nordmann, Einsteln otd tlb Unloerse, trans. J. McCabe (New York, 1922), chap. ix.
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fhe Sfruclureof ScientificRevolulions
Theseexamplespoint to the third and most fundamentalaspect of the incommensurabilityof competing paradigms.In a sensethat I am unable to explicatefurther, the proponentsof competingparadigmspractice their trades in different worlds. One containsconstrainedbodiesthat fall slowly, the other pendulums that repeattheir motionsagain and again.In one, solutions are compounds,in the other mixtures.One is embedded in a flat, the other in a curved, matrix of space.Practicing in different worlds,the two groupsof scientistsseedifferent things when they look from the same point in the same direction. Again, that is not to saythat they can seeanything they please. Both are looking at the world, and what they look at has not changed.But in someareasthey seedifferent things, and they seethem in different relationsone to the other. That is why a law that cannotevenbe demonstratedto one grouPof scientists
Part of the answeris that they are very often not. Copernicanism made few converts for almost a century after Copernicus' death. Newton'swork was not generallyaccepted,particularly on the Continent, for more than half a century after the Prin' { T. S. Kuhn, The Copenban Reoolution(Cambridge, Mass.,1957), chaps. iii, iv, and vii. The extent to which helioccntrismwas more than a strictly astronomical issueis a major theme of the entire book. 6 Max tammer, Cotrceptsof Space(Cambridge,Mass.,l9t4), pp. 1f8-24.
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of Revolulions fhe Resolulion PriestleYnever cipia appeared.o Lord Kelvin the electromagnet: culties of conversionhave often selves.Darwin, in a particularl' of his Origin of Species,wrote: of the trulh of the viewsgivenin this volume. . . ,I by no means expect to convince experiencednaturalistswhose minds are stocked with a multit,tde of facts all viewed, during a long courseof years,from a point of view directly oppositeto mine. . . . [B]ui t look with &nfidence to the future,-to young and rising naturalists,who will be able to view both sides of the Jris q.r"iion with impartiality)'l And Max Planck, surveyin_g that remarked in his ScientificAutobiography,sadly d*r, ""r"er "a new scientiftctruth doesnot triumph by convincingits oPPonents and making them see the light, but rather becauseits opponentseventuallydie, and a new generationgrowsup that is familiar with it."8 Thesefacts and otherslike them are too commonlyknown to need further emphasis.But they do need re-evaluation.In the past they have most often beentaken to indicate that scientists, 6eing only human,cannotalwaysadmit their errors,evenwhen confiontedwith strict proof. I would argue,rather,that in these mattersneither proof nor error is at issue.The transfer of allegiance fom paradigmto paradigmis a conversionexperience ihtt ..ttttot be forced. Lifelong resistance,particularly from those whose productive careershave committed them to an older traditio; of normal science,is not a violation of scientiftc standardsbut an indexto the natureof scientificresearchitself. that the olderparadigm is the assurance The sourceof resistance will ultimately solveall its problems,that nature can be shoved 0I. B. Cohen, Franklin and Neu:ton: An lnquiry into Speculatioe Newtonian Erperimental Siience and Fronklin's Work tn Eleitriclty is an Erample Therc' ol'(Philadelphia, 1956), pp. 93-94. ? Charles Darwin, On the Origin of Species . . . (authorized edition from 6th English ed.; New York, 1889), II, 29ts96. 8 Max Planck, Scientific Autobiographg anil Othet Papers, trans. F. Gaynor (New York, 1949), pp.33-84.
l5r
fhe Slruclureof ScientiffcReyolutions into the box the paradigmprovides.Inevitabl/, at times of revoItrtion,that assurance seemsstubbornand pigheadedas indeed becomes.But it is alsosomethingmore.That same it sometimes is what makesnormal or puzzle-solvingscienceposassurance sible.And it is only throughnormalsciencethat the professional first, in exploiting-thepotencommunityof scientistssucceeds, tial scopeand precisionof the older paradigmand, then, in isolating the difficulty through the sttrdyof which a new paradigm may emerge. -say that resistanceis inevitable and legitimate, that dtill, to paradigmchangecannotbe justiftedby proof, is not to saythat aie relevantor that scientislscannotbe persuaded ,ro "tg,t*ents to changetheir minds. Though a generationis .sometimesrequired io effect the change,scientificcommunitieshlve again been converted to new Paradigms.Furthermore, "rrd "g^itt these conversionsoccur not despitethe fact that scientistsare human but becausethey are. Though some scientists,particularly the older and more experiencLdott.t, may resist indefinitely, most of them can be reached in one way-or_another. Conversionswill occur a few at a time until, after the last holdouts have died, the whole professionwill again be practicing under a single,but now a difier"ttt, parldigttt' y" must therefore askhow conversionis inducedand how resisted. What sort of answer to that question may we expect?-fust or about becauseit is askedabout techniquesof persuasion, there in which situation a in argument and counterargument of a-1oj demanding one, new l, no Proof,o.r, q,r.rtion is a have ""i shall we been undertaken. study that has not pr^eviously In addiimpressionistic.tY*ty' and very'partial for a to settle ^combines with the result of been said tion, what has "ti""ay rather that surveyto suggest'that,when askedabout P-ersuasion has argument scientific of nature the than proof, th" qiEstio' of a embrace scientists Individual no single o, ,-rrrifor,',answer. at several for usually and ,r"* pir"dig. for all sortsof reasons worship that once.some of thesereasons-for example,the sun
helpedmakeKepleraCopernican_lieoutsidetheapparent
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fhe Resolulionof Revolulions sphereof scienceentirely.oOthers must dependupon idiosyncrasiesof autobiographyand personality.Even the nationality or the prior reputation of the innovator and his teacherscan sometimesplay a significantrole.r0Ultimately, therefore,we must learn to ask this questiondifferently. Our coneernwill not then be with the argumentsthat in fact convert one or another individual, but rather with the sort of community that always sooneror later re-formsas a single group. That problem, however,I postponeto the final section,examiningmeanwhilesome of the sortsof argumentthat proveparticularly effectivein the battlesover paradigmchange. Probably the single most prevalent claim advancedby the proponentsof a new paradigmis that they can solvethe problemsthat haveled the old oneto a crisis.When it canlegitimately be made,this claim is often the most effectiveonepossible. In the areafor which it is advancedthe paradigmis known to be in trouble.That trouble hasrepeatedlybeen explored,and attemptsto removeit have againand againprovedvain. "Cruable to discriminateparticularlyshaqpcial experiments"-those ly between the two paradigms-have been recognizedand attestedbefore the new paradigmwas even invented,Copernicus thus claimedthat he had solvedthe long-vexingproblem of the length of the calendaryear,Newton that he had reconciled terrestrialand celestialmechanics,Lavoisierthat he had solvedthe problemsof gas-identityand of weight relations,and Einstein that he had made electrodynamicscompatiblewith a revisedscienceof motion. Claimsof this sort areparticularlylikely to succeedif the new paradigmdisplaysa quantitativeprecisionstrikinglybetterthan 0 For the role of sun worsbip in Kepler's thought, see E. A. Burtt, The Metaphysical Foundatioru ol Modern Physical Science (rev. ed.; New York, 1932), pp.44-49. r0 For the role of reputation, eonsider the following: Lord Rayleigh, at a time when his reputation was established, submitted to the British Association a paper on some paradoxes of electrodynamics. His name was inadvertently omitted when the paper was ffrst sent, and tlre paper itself was at first rejected as the work of some "paradoxer." Shortly afterwards, with the author's name in place, the papgr was accepted with profuse apologies ( R. J. Strutt, 4th Baron Rayleighi lohn Williltm Strutt, Thiid Baron- Rayleigh [N6w York,
1924J, p. 228).
I53
fhe Sfructureof ScienlificRevolufions its older competitor,The quantitative superiorityof Kepler's Rudolphinetables to all those computedfrom the Ptolemaic theory was a major factor in the conversionof astronomersto Copernicanism. Newton'ssuccessin predictingquantitativeastronomicalobservations wasprobablythe singlemostimportant reasonfor his theory's triumph over its more reasonablebut uniformly qualitative competitors. And in this century the strikingquantitativesuccess of both Planck'sradiationlaw and the Bohr atom quickly persuadedmany physiciststo adopt them even though, viewing physicalscienceas a whole, both these contributions created many more problems than they solved.rr The claim to have solved the crisis-provokingproblems is, however,rarely sufficientby itself. Nor can it alwayslegitimately be made.In fact, Copernicus'theory was not more accurate than Ptolemy'sand did not lead directly to any improvementin the calendar.Or again, the wave theory of light was not, for someyears after it was ffrst announced,even as successfulas its colpuscular rival in resolving the polarization effects that were a principalcauseof the opticalcrisis.Sometimes the looser practicethat characterizes extraordinaryresearchwill produce a candidatefor paradigm that initially helpsnot at all with the problemsthat have evokedcrisis.When that occurs,evidence mustbe drawn from other parts of the fteldas it often is anyway. In thoseother areasparticularlypersuasiveargumentscan be developedif the new paradigmpermitsthe predictionof phenomenathat had been entirely unsuspected while the old one prevailed. Copernicus' theory, for example, suggested that planets should be like the earth, that Venus should show phases,and that the universemustbe vastlylargerthan had previouslybeen As a result,when sixty yearsafter his death the telesupposed. suddenly displayedmountainson the moon,the phasesof scope stars, Venus,and an immensenumberof previouslyunsuspected 11 For the problems created by the quantum theory, see F. Reiche, Tha Quantum Theiry ( London, 1922i, chaps'. ii, vi-ix. Foi the other examples in tLis paragraph, see the earlier references in this section.
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of Revolufions Ihe Resolution t{roseobservationsbrought the new theory a great many corrIn the caseof the verts,particularly among non-astronomers.l2 wave theory, one main sourceof professionalconversionswas even more dramatic. French resistancecollapsedsuddenlyand relatively completelywhen Fresnelwas able to demonstratethe existenceof a white spot at the centerof the shadowof a circular disk. That was an efiect that not evenhe had anticipatedbut that Poisson,initially one of his opponents,had shownto be a necessaryif absurd consequenceof Fresnel'stheory.rsBecause of their shock value and becausethey have so obviouslynot been "built into" the new theory from the start, argumentslike these prove especiallypersuasive.And sometimesthat extra strengthcan be exploitedeventhough the phenomenonin question had beenobservedlong before the theory that accountsfor it was first introduced.Einstein,for example,seemsnot to have anticipatedthat generalrelativity would accountwith precision for the well-known anomalyin the motion of Mercury'sperihelion, and he experienceda corresPondingtriumph when it did so.11
All the argumentsfor a new paradigm discussedso far have been based ,tpot, the competitors'comparativeability to solve problems.To icientists thoie argumentsare ordinarily thernost The precedingexamPlesshouldleave iignificant and persuasive. of their immenseappeal.But, for the source about nJ doubt revert, they are neither indishortly we shall reasonsto which Fortunately, there is also compelling. vidually nor collectively scientiststo reiect an lead can that anothersort of consideration rarely the_argum-ents, are These a new. of favor in old paradigm of sense individual's the to appeal that explicit, entiiely -"d" is said to be theory new the aesthetic-the or the appropriate "neatlr-," o-or" suitable,"or "simpler" than the old. Probably r2 Kuhn, op. cit.,pp. 219-25. r$ E. T. Whittaker, A History ol the Theoriesof Aether ard. Electricity,l (2d ed.; London,I95l), 108. 14See ibid., Il (tgSS), 151-80, for the developmentof generalrelativity. For Einstein'sreactionto the preciseagreementof the-theoJywith the observed motion of Mercury'sperihelidn,see tfr-eletter quoted in Pa A. -S-chilpp(ed')' (Evanston,Ill., 1949), p. l0l. Albert Eirwtein,ehtloiopher-Scientist
I55
fhe Sfructureof ScienlificRevolutions suchargumentsare lesseffectivein the sciencesthan in mathematics.The early_versions of mostnew paracligms are crude.By the time their full aestheticappeal.an be cllveloped,most of the communityhas beenp".t,i"d"d by other -""ni. Nevertheless,the importanceof aesthetic can sometimes "onrid"r"tions attract only a few scientiststo a w that its ultimatetriumph may
;*:,# ;li.';gly ill'J'jJ;l it the allegiairce of the scientific
community as a whole. To seethe reasonfor the importance of thesemore strbjective and aesthetic co_nsiderations, remember what a paradigm debate is about. when a new candidate for paradigm is fir:rt proposed, ithas seldom solved mor.ethan a fewof the"probl"-, ihut confront it, and most of those solutions are still far^from perfect. until Kepler, the c-opernican theory scarcely improvei ,porl the predictions of planetary position made by rtoiemy. wiren Lavoisier saw oxygen as "the air itself entire," his new theory could cope-not at all with the problems presented by the proIiferation of new gases,a poinfthat prieitley made with gieat successin his counterattack. Caseslike Fresnel's white ,pol or" extremely rare. ordinarily, it is only much later, after tlre new paradigm_hasbeen developed, accepted, and exploited that apparently decisive arguments-the Foicault pendulum to demoirstrate the rotation of the earth or the Fizeau experiment to show that light moves faster in air than in water-are developed. producing them is part of normal science, and their role ls not in paradigm debate but in postrevolutionary texts. Before those texts are written, while the debate goes on, the situation is very different. usually the opponents of a new paradigm can legitimately claim that even in the area of erisis-it is little superior to its traditional rival. of course, it handles some problems better, has disclosed some new regularities. But the older paradigm can presumably be articulated to meet these challengesas it has met others before. Both Tycho Brahe's earthcentered astronomical system and the later versions of the
t56
fhe Resolutionof Revolulions phlogistontheory were responsesto challengesposedby a new candidate for paradigm, and both were quite successful.tsIn addition, the defendersof traditional theory and procedurecan almostalwayspoint to problemsthat its new rival hasnot solved but that for their view areno problemsat all. Until the discovery of the compositionof water, the combustionof hydrogenwas a strong argument for the phlogiston theory and against Lavoisier's.And after the oxygentheory had triumphed,it could still
should in the future guide researchon problemsmany of which neither competitor can yet claim to resolvecompletely.A decision between alternateways of practicing scienceis called for, and in the circumstancesthat decisionmust be based less on t5 For Brahe'ssystem,which was geometricallyentirely e.quivalen!_to.ftqu-t: nicus" seeJ. L. E. Dreyer, A History of Astrotwnryftom Tlwles to.Kepler (.2d ed.; New York, 1953), pp.359-71. For the last versionsof the.pHo$ston theorv and their iuccest, r"i -Anrwls I. R. Partington and D. McKie, "Historical Studies of Scierce,IV (19i19), 113-49. of'the PhlogistonTheory," r0 For the problem presented by hydrogen, see l. R. Partington, A Shorl
p. 184. monoxide, For carbon carbonmonoxide, tSa. For l95f ). p. Loridon, 19-51), stont ol Chemistry_((Ed ed.:; London. of Chbmistru History der Chemie,III (Braunlchweig,1845), 29't-96. seeu."fdpp, Geschichte
157
flre Sfructureof ScienfificRevolufions past achievementthan on future promise.The man who embracesa new paradigmat an early itage must often do so in defiance of the evidenceprovided by pioblem-solving.He must, that is, have faith that the new paradigm will succeid with the T1"y large_problems that confront it, knowing orrly that the older paradigmhas failed with a few. A decisionof that kind can only be madeon faith. That is oneof the reasonswhy prior crisisprovessoimportant. scientistswho have not experiencedit will seldomrenouncethe hard evidence o{ problem-solvingto follow what may easily prove and will be widely regarded as a will-o'-the-wisp.But crisisalone is not enough.There must alsobe a basis,though it need be neither rational nor ultimately correct,for faith in the particular candidate chosen.something must make at least a few scientistsfeel that the new proposalis on the right track, and sometimesit is only personahnd inarticulateaestf,eticconsiderationsthat can do that. Men have been convertedby them at times when most of the articulable technical arguments pointed the other y"y. when ftrst introduced,neither dopernicus' astronomicaltheory nor De Broglie'stheory of matt& had many other signiftcantgroundsof appeal.Even today Einstein's general men principally on aestheticgrounds,an Fuoy attract_s appealthat few peopleoutsideof mathematicshavJ been able to feel. This is not to suggestthat new paradigmstriumph ultimatelv through some _mysticalaesthetic.on the contraiy, very few men deserta tradition for thesereasonsalone.often thosewho do turn out to have been misled. But if a paradigm is ever to -who triumph it must gain someffrst supporters,men will develop_itto_the point where hardheadedargumentscan be produced and multiplied. -And even those arguments,when ihey come, are not individually decisive. Becausescientists are reasonablemen, one or another argumentwill ultimately persuade-1ny of them. But there is no single argumentthat can or shouldpersuadethem all. Ratherthan a singlegroup conversion, what occurs is an increasingshift in the distri-butionof professionalallegiances.
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fhe Resolutionof Revolutions At the start a new candidatefor paradigmmay havefew supporters,and on occasionsthe supporters'motives may be suspect. Nevertheless, if they are competent,they will improveit, explore its possibilities,and show what it would be like to belongto the communityguided by it. And as that goeson, if the paradigm is one destinedto win its fight, the number and strengthof the persuasiveargumentsin its favor will increase. N{orescientistswill then be converted,and the explorationof the new paradigmwill go on. Graduallythe numberof experiments,instruments,articles,and books basedupon the paradigm will multiply. Still more men,convincedof the new view's fruitfulness, will adopt the new mode of practicing normal science,until at last only a few elderly hold-outsremain. And eventhey, we cannotsay,are wrong. Though the historian can alwaysffnd men-Priestley,for instance-whowereunreasonable to resistfor aslong as they did, he will not find a point at which becomesillogicalor unscientific.At mosthe may wish resistance to saythat the man who continuesto resistafter his whole professionhasbeenconvertedhasfpsofacto ceasedto be a scientist.
159
Xlll. Progress throughRevolutions -The precedingpageshave carried my schematicdescription of scientiffcdevelopmentas far as it can go in this essay.N^evertheless,they_cannot quite provide .on.lurion. If thii descrip"structureof tion has at all caught the essential a science',.oirtinuing evolution,it will simultaneously have poseda special ^*ou. problem: wly should the enteqprise'sketchei above steadily aheadi1_waysthat, r"y, political theory, or philosoplry does not? why is progressa"tl, perquisite reserv"d "*clusively for the activities *" call science?The most"hn*t usual anto that q_uestion have been denied in the body of this essay. :y"* we must concludeit by ashng whethersubstitutescanbe founi. Notice immediately that part of the question is entirely semantic.To a very greatextentthe term 'seience'isresenredfor ffeldsthat do ptogreisin obviousways.Nowheredoesthis show moreclearlythan in the recurrentdebatesaboutwhetheroneor anotherof the contempora-ry socialsciencesis really a science. These debatesh-aveparallels in the pre-paradigmperiods of fieldsthat are todayunhesitatinglylabeled^science. Their ostensible issuethroughoutis a definiiion of that vexingterm. Men argue that psychology,for example,is a sciencJbecauseit possesses such and such characteristics. Others counter that thosecharacteristics are either unnecessary or not sufficientto makea field a science.ofte_ngreatenergyis invested,greatpassionaroused,and the outsideris at a losi to know *hv."con i"ru muchdependupon a definitionof 'science'? can a deftnitionteil a man whetherhe is a scientistor not? If so,why do not natural scientistsor artistsworry about the definitionof the term? Inevitably one suspectsthat the issueis more fundamental.probably ques-ti91s like the following are really being asked:why does^my_fteld-fail to move aheadin the way thaI, say,physics does?what changesin tech'ique or methodor ideologi *o"ra enableit to do so?Theseare not, however,questionstf,at co.,ld respondto an agreementon definition.Fuithermore,if prece160
Progressthrough Revolulions dent from the natural sciencesserves,they will ceaseto be a sourceof concernnot when a definition is found, but when the
it rather economicsabout which they agree? no longer simply.seThat point ^*ry has a conversethat, though help to display the inextiicable connectionsbemantic, tween our notions of scienceand of progress.For many cen-
chiaroscuro that had made possible successivelymore perfect of nafure.l But thoseare alsothe years,Particurepresentations when little cleavagewas felt belaily during the Renaissance, and the arts.Leonardowasonly one of many tween the s-ciences
bute of both ffelds. 1E. H. Gombrich, Art and lllusion: A Study in the Psycholagy of Pictoilal Representat&m(New York, 1960), pp. ll-I2. 2 lbid.. o. 97: and Giorqio de Santillana, "The Role of Art in the Scientific ii Criti"al Froblems in the History of Science, ed. Nt. Clagett n"""irr"""i,," (Madison, Wis., 1959), PP. 3&-65.
l6l
FheStructureof ScienlificRevolulions
tific activity and the communitythat practicesit. We must learn
sciencebecauseit makesprogress? Ask now why_an enterpriselike normal scienceshould progress,and begin by recallinga few of its most salientcharacter-
recognizesa categoryof work that is, on the onehand, a creative success,but is not, on the other, an addition to the collective achievementof the group. If we doubt, as many do, that nonscientificfteldsmakeprogress,that cannotbe becauseindividual schoolsmake none.Rather,it must be becausethere are always 162
ProgressthroughRevolutions
With resPectto n the problem of Prog
t
important recepthe o.nce that already iot:d ;;:-'\tii"h"',r", for example, comm"T'y tion of a common parad-igmhas freed the scientific. the iro* the need con'stantly"tot"-"xamine its first princiPles, uPon exclusively *"*U"r, of that community can concentrate
r63
fhe Slruclureof ScientificRevolulions
tu
Progresslhrough Revolulions ,ucational initiation' In music'
ItHT:llffi,ffiil}il:'"ff
rdia of orhandbools to original role' In history' philosop\' creations, have only a ,*od"ty has a gre-atersignitliterature and the social sciences,textbook collegg course elementary the icance.But even in these ftelds of them the some sources' parallel readings in original reports ;"fr#lI.f research "*plov, the fteld, ofturr thJcontemPorary student the iesult, a is that nractitioners n#t" for each other. ,ir;r disciplines is constantly made aware of the ;;'/;;;;i of his fuhrre immense variety of proile-s that the members Even more solve. to of time, attempted ;;;;o have,in the "oitr" competing of number t p;i;;t, ire has constantlybefore him a solutions and incommensurable solutions to these__problems, ttt"t ft" must ultimately evaluate for himself' Contrast this situatibn with that in at least the contemPgrary mainly on nah'al sciences.In these ffelds the shrdent relies work, he graduate l*t[oof* ,-U1 io his third or fourth year of ask even not U"gi,*r his own research.Ytty sciencecurricula do in works not written speciallyfor stuffiJ|u1u studentsto read in research dents.The few that do assignsupplementaryreading monographs ristrici such assignmentsto trhemost p"I"* ""a .o,.rrr"r""rrld to materials that take uP Pore or less advanced in *h"r" the available texts leave ofi. Until the very last-stages substi' the education of a scientist,textbooksare systematically thgm p91h,rtedfor the creative scientiffc literature that made in their paradigms,which makesthis sible.Given n" "o"naence would wish to educational technique Possible,fiw scientists
;il;;l ;;;-"t",
itt, shouldthe studentof 4rrsrc;, for *t, "fl"t read'the worksof Newton'Farad"l TIT::i i:
about these Schrtiiinger, when eyerythrlg he needs to know and more works is iecapitulated i" a fai briefer, more precise, f;n in a number of up-to-date.textbooks? tytl"*"t* to defend th^eexcessivelengths to which Without *i;ilt one cannot this type of educationhas occasionallybeen carried, effective' frap Lirt notice that in general it has been immensely
165
fhe Sfruclureof ScienfiffcRevolulions of course,it is a narrow and rigid education,probabrymore so than any othel except perhapJ in orthodo* ih"orogi. nrt ro, normal-scientificyoi\, lor puzzle-solvingwithin th"etradition that the textboolcsdefine, the scientist is almost perfectlf equipped. Furthermore,he is well equipped for anoiher tasl as.well-the generationthrough no.m"l^icience of signiffcant erises.when they arise, the sJientistis not, of course,"equally p-repaled. tlroug! prolonged crises are probabiy ,J:'ell T".": flected in less rigid educa"tioialprictice, scientiftl tr"irr#g I no,twell designedto produce thehan who will easily disc&e, a fresh approach.Buf so long as somebody with a new "pf"r* for paradigm-us_uiilya young man or one new to the :Tgi** nero-tne loss due to rigidity accruesonly to the individual. Giygn a generationin *tri"ti to effect thJ change, individual rigidity is compatiblewith a community that can switch from to paradigm when the occasiondemands.particularFttld:g"t ly, it.is compatiblewhen that very rigidity provides the community with a sensitive indicatoi tf,"t ,ori,ething has gone wrong. In its normal state, then, a scientific community is an immenselyefficientinstrumentfor solvingthe,probi"-, or puzzles define. Furthermo"r", th'" result ;t"i;; l*:^r::*t^t']q.r rnoseproblemsmust inevita "f probleri ;. There is no her.e.Seeingthat mr righlights the second main dre iciences.Let us -part of the prr therefore turn to it a r through u*t a*ainery scienee.Why sh r be the apiarently versal coneomitantof scientiffcrevolutions?onJe ""i_ again, there is much to be learnedby askingwhat elsethe result o"fa r"uol.rtion could be. RevolutionscloJewith a total victory for one of tl:.t*g opposingcamps.will that soup ever saythat the result ot its victory hasbeensomethingIeis thin progr"rs? That wourd De rarner lrke admitting that they had been wrong and their opponentsright. To them, at least, the outcom" oir"rrolution must-beprog-ress, and they are in an excellentpositionto make eertain that future members of ,_l"i-r wiil r"t p"ri "o**,rrrity history in the sameway. sectionXI deseribed in detail the tech166
ProgressthroughRevolufions niques by which this is accomplished,and we have just-reof professionalscientificlife' to a closelyrelated "tp""t ",rir"d a scientificcommunity paradigm, past repudi"i"r When it " for- professional subject nt a as renounces, simultaneourly that para{tg* which in articles and scrutiny,-ori of the books use of no makes education had be"n embodied. Scientiftc and the classics, of library the or equivalentfor the art museum percePthe scientist's in distortion reiult is a sometimesdrastic other of the practitioners than More tion of his discipline'spast. to line straight a in leading it as to see creativefields,ir" "o-"r as it see to he comes short, the discipline'spresentvantage..In in he remains while him to progrrrr.^Noalternativeis available the field. a Inevitably those remarkswill suggestthat the member of of character typical mature scientific community is, hle the Orwell's 7984,the victim of a I that be. Furthermore, that sug propriate. There are lossesas u iiottJ, and scientiststend to be On the other hand, no exPlanal
it [i" orrt"ome of thosl debatesmight still be revolution, but science of would not be scientificrevolutiott.The very existence to choosebetrveenparadigms depends ^the uPon vesting the-gowgr -"rib.r, of a s[eci"t titta of community. Justhow special in that community -ut[ be if scienceis to survive and grow may of humanity'shold on the U, itrai..ted by the very tenuousness scientiffcenterprise.Every civilizationof which we haverecords 8 Historians of science often encounter this blindness in a particularly striking the sciences is very otten fotr". T.h" group of students who come to them from the. most frushating usually it is also teach..Put th"t rewardini the most ;;;;t rtludents "know the right ansriers," it is particularly at the start. Becaur'""J"i"" terms. diffi""lt to make them analyze an older science in its own
167
TfreStruclureol ScienlificRevolulions hasposse_ssed a technologr,on art, a religion,a political system, laws, and so on. In .rn/'""res thosefaJetsof ciuiliz"tion have been as developed_as our own. But only the civilizations that deseendfrom Hellenic Greecehave possessed more than the most rudimentaryscience.The buk of scientificknowledgeis a product of Europein the last four centuries.No othe, pt""', time has supported the very special communities from which "rJ scientificproductivity comei. what are the essentialcharacteristicsof theseeommunities? obviousl/, need vastly more study. In this area onry the lhey most tentative generalizationsare possible. Nevertheless, a number of requisitesfor membershipin a professional scientiffc group must already be strikingly clear. Tie scientistmust, for example,be concernedto solvJprobremsabout the behavior of is concern with nature mav be ms on which he works mrrst be
rather the well-definedcommunityof the scientist's professional compeers.one of the strongest,if still unwritten, -iu, of scientific life is th_eprohibitio.r Jf to headsof stateor to the "pp"als populaceat large in matterssciintific. Recognitionof the existence of a uniquely,competentprofessiorral"gro,rp and acceptanceof its role as the exclusiveirbiter of prif"rJiorral achievement has further implications.The group;s-"mbers, as individt'als and by virtui of their sh"rei-training must be seenas the solepossessors of the rul"Jof"rrd the"*p"ri"rr"r, gime o, oi some basis for_ unequivocal judgments. To doubt -equivalent that they sharedsomesuch basisfor evalrr"iion, wourd be to admit the existenceof incompatible standards of scientific achievement.That admissionwould i_nevitablyraise the ques' tion whether truth in the scieneescan be orr". This small list of characteristicscommon to scientific communities has been entirely from the practice of normal {i1*n science,and it shouldhave been.That is the^activityfor which t68
Progressthrough Revolulions that despite the scientistis ordinarily trained. Note, however, communities such to set its small sizethe list is alieady sufficient
groupso:3 ail othe, *" "----- professional r--",",-'^t'::.1*t?:; the list accounts for that despite its source in- normalI"scie^nce ,'s t -_,_--^-.^t,,1i^nc revolutions
d*o'v +;fu
rrv'r
many specialfeaturesof the and particu servedthat
responseduring
that sclen
maximizingthe number and
t glllr,l'.sfl :1,1,19':.^r.
thougl
:rew Paradtg-t :-d
permit addit
all the capabili-
s besides
To say this much is not [o
that *"-,"PlYr:"-
:::: *"'gge't unique d;""i' "'"i1i'-"'jr'e t :":1YTi":::t:.,f:f::; 5ffi:iiil':'w;h;;"-J';ad11ote9.t"l-f ;;ru;;"rrt"rio'
::T^""1'.Ilt"1i:
that a comof that ,oti. B,tt it does-suggest
m u n i t y o f s c i e n t i f f c s p e c i a l i s t s w i l l d o a l l t h a t i t c a n t otreat ensure that it can the continuing gro*ih of the assembleddata
r69
fhe Strucfureof ScientificRevolulions with.pr.ecisionand detail. In the processthe community will sustainlosses.often someold problemsmust be banish.d. Ftrquently, in addition, revolutioir narrows the scopeof the communitys professionalconcerns,increasesthe exient of its specialization, and attenuates its communication with otfrer
ffixl;:rff,Tl"jL';;:H
he proliferation of scientificspey single specialtyalone.yet deindividual communities'the nature of such communiu", o,thu both thelist of p'roblemssolvil individual problenr-solutions w nature of the community provir a-nywf)r a_tall in which it can be provided.what better criterion than the decisionof the scientiftcgroup courd there be? Theselast paragraphspoint the dire-ctionsin which I believe a more refined solution oJ the problem of progressin the sciencesmust be soug-ht.Perhap_s th"y indicateihaf scientific progressis not quite what we had taken it to be. But they simultineouslyshgw that a sort of progresswill inevitably ch'aracterize the scientific enteqpriseso ionf as such an enteqprisesunrives. In the sciencestheie need noibe progressof an6ther sort. we may' to be more precise,h_aveto_rilinquish the notion, explicit or-implicit,-that changesof paradigm carry scientistsand tiose who learn from them closer and cl6ser to ihe tmth. It is now time to notice that until the last very few pagesthe 'truth'had term entered this essayonly in a quotatlioi frorn Francis Bacon. And even in those pages it entired only as a sourcefor the scientist'sconviction thit incompatibleruies for doing science_cannotcoexist except during rdvolutions when the profession'smain task is to eliirinate al-isetsbut one. The developmental process described in this essay has been a imitive beginnings-a process acterizedby an increasinglyde;of nature.But nothing that has rocessof evolution toward any170
ProgressthrovghRevofutions thing. Inevitably that lacunawill have disturbedTany readers. the one enterWe ire all deepiyaecustomedto seeingscience_as by nature in set goal to some nearer draws constantly that prise advance. such goal? can we not account for But need there be any -and its s,tc"ets in terms of evolution existence both science's knowledge at any gt-":-" t-ime? of state from the community's there is someone full, objecthat imagine Doesit really help to that the proper measure of and nat-ure of tive, true """o.rrri to which it brings us closer is extent the scientificachievement to substituteevolutionlearn can we If to that ultimate goal? evolution-toward-what-we-wish-tofor from-what-we-dolknow know, a number of vexing problemsmay vanish in the process. must lie the problem of Somewherein this maze,-fot "*"-ple, induction. of this I cannot yet specify in any detail the consequences recognize to it helps But alternate viiw of scientificadvance. that the conceptual transpositionhere recommendedis very closeto one that the Wes[ undertook iust a century ago. It is particularly helpful becausein-both casesthe main obstacleto transpositibnis the same. When Darwin first _publishedhis theoi of evolution by natural selectionin 1859, what most bothered many profesiionalswas neither the notion of sp-ecies changenor the possibledescentof man from aPgs.The evidence pointng to evoiution,including the evolutionof man, had been i.",r*,r'i"ting for decades,and the idea of evolution had been *id.ly disseminatedbefore. Though evolution, suggested "tid as"r"uch,did encounterresistance,particularly from somereligious groups,it was by no nle1nstlie greatestof the difficulties the Darwiniansfaced.That difficulty stemmedfrom an idea that wasmore nearly Danryin'sown. All the well-knownpre-Darwinian evolutionary theories-those of Lamarck, Chambers,Spencer, and the GermanNatutphilosophen-had taken evolution to be a goal-directedprocess.The "idea" of man and of the contemporary flora and fauna was thought to have_b9.l pres-ent froni the hrst creation of life, perhapsin the mind of God. That idea or plan had provided the direction and the guiding force to t7l
For many-men the abolition of that teleologicalkind of evo. Iution was the.mostsignificantand leastp"ti"bt" of Darwin,s The Origin of Spe _suggestions.s by God or nature. Instead,na given environmentand with tl hand, was responsiblefor the p more elaborate,further articule organisms.Even such marvelo and hand of man-organswhosr powerful argumentsfor the exir an advance.plan-wereproductsof a process^that ,no""J steadiIy from primitive beginningsbut touard no goal. The belief that natural selectign,-resultiigfrom merecomp"etitionbetween olganismsfor survival,could have producedmin together with the higher animalsand plantswastire mostdifficult a"rrd disturb_ ing aspectof Darwin's theory. \ ment,'and'progress' meanin th, many people, such terms sudd The analogythat relatesthe er Iution of scientiffcideascan easilybe pushed too far. But with respectto the issuesof this closingteclio' it is very nearly perfect.-The processdescribedin section XII as the resolution of revolutionsis the selectionby confict within the scientific com_ munity of the fittest way to practice future science.The net result of a sequenceof such revolutionaryselections,separated by periods of normal research,is the wonderfully adapted set of instrumentswe eall modern scientiffcknowledge.su^ccessive stagesin_thatdevelopmentalprocessare marked by an increase in articulation and specializalion.And the entire process may have occurred, as we now supposebiological evilution did, {._LnrenEiseley, Darusin'scertury: Eoolution and the Menwho Dtscooered, _ It (New York, t058), chaps.ii, iv-v1 6 For a particularlvacuteaccountof one prominentDarwinian,sstrugglewith
oup'e"'aii i' ov'ft1r rcIc i ci'iiu'ffi;'il:: l[fri:TJ:'affrt;tH$"' 172
ProgressthroughRevolulions without beneftt of a set goal, a Permanent ffxed scientiffc tnrth, of which each stage in ti'e deveiopment of scientiftc knowledge -is a better exemPlar. e"yo"e who ias followed the argument this far will nevertheleis feel the need to ask why the Ivohtionaly Processshould that work. What must nature, including man' be like in order be commtrnities scientiffc shoild Why all? at r.i"i." be possible Why able to rrr"h a ftrm consensusunattainable in other ffelds? acrol should consensusendure another?And whY should Parac an instrument more Perfect in r fore? From one Point of view are as ffrst, have already been answered.But trom anotner tney o{ynot is beg3rlit essay !h.e opu" as they *lr" when this that must be ipecial. The world of which scientiffc "o-rn*ity quite specialcharacthat community is a part must _alsop-ossess it the start to know*"tt i" io closerthan teristics,arrdwe "r, world be ing whai thesemust be. That problem-What must the created however, not, iii.? il;;a"r tt man may kn'ow it?-was "t and it itself, science as it.is as old [, this essay.On the "orrt "ry, place' in this answered be remainsunanswered.But it need not Any conception of nahrre -"oTP by proof iJcomPatible with the vLtoped here. Since this view i t"*"tiott of scientiffc life, ther ploying it in attemPts to solvr rernain.
173
Postscript-l969 It has now been almost seven years since this book was first published.l In the interim bot\tir" rurponseof critics *t own further work have increased*y rrrrdurrtandingof ";d a numbe'r of the issuesit raises. on fundamentarsmy viewpoint is very learly unchanged, but I now recognize aspectsof its initial formulation that create grafuitous iifficulties arrd misunderstandings.since someof thosemisunderstandings h"";t;;;;y own, their elimination enablesme to gain groind that should ultimately provide the basis for a new version of the book.2 Meanwhile, I welcomethe chanceto sketch needed ,urririo*,io commenton somereiteratedcriticisms,and to suggest directfons in which-ml g*T thou.ghtis presently developfgJ several of the key difficurtiesof my originaitext crusterabout ilAns with them.a lhe gonceptof a paradigrn,and -y dir"ufi; In the subsectionthat foilows at bnce,I rugg"rtih" d"ri;;;il-t of disenta"ghg that conceptfrom th" nouJriof a scientific community, indicatehow this -aybedone, and discuss somesigniftl This postscriotwas ffrst -shGil; prepared at the sugge_stion of my onetime student and longtimg f1bn4 bt. ivlf"y"-, o-T-th" u"r""irrty of Tokyo, for inclusion in his lapanesetransrauoooi ihi, b";k. i; sr"i;r"r to him ior the idea, for his patiente in awaiting il; iltu;", * ."a rrip"fr"rrlion to incrude the result in the linglish t""g""g" "a?ti"i." 2 For this edition I have attempted no systematicrewriting, restricting arterations to a few tvpog.."p$"ar eno'rs pt.o ti,o p"r."gu, ;i-i":i'";;;;"ii"rr"r"ii, enors. one of tir'ese"is^th_e e"r;;ft;;;f principiain the A;fi;3;"il*.*t of eighteentt-"""trf, -p.*eJha'i"s on pp. gGgg, above. The other *::Iry:S tlte response concerru; to criseson ae. 3 Other indicationswill | 6 of mine: ..Reflectionon My Critics," in fmre Lak
q;",trl-;i xiiitie-k
digms," in Frederick.Swb. Ill., 1970 or lgTl ). botil^c below as "Reflections" and
;ei;; tb.;;;d;;y
*i11
;"tJ''fiffffff ::i:!:ientific Thcortes (urbana,
e the ffrst of these essays
ifl'"ii,fl,-th
orr"to-t-
a For particularlv cogelt criticism of my_iniri_al presentation o_fparadigms see: Margarel Masternian,
;;i;;^ 'The N;hr"; ; ;'il;il,i:\;' and Dudlev shaoere, :.rh" stucture Jf s"iuof;ff. n"""I"L"*,,, Reoieus,Lxx[I ( fsdl L g83_gn.--*- "'
174
of Knour.ed,se; phirosophibar
PostscriPt of the resulting analytic separation.Next I cant consequences considerwhat occurswhen paradigmsare soughtby examining the behaviorof the membersof a prersiurslydeterminnd scientiftc community.That procedtrrequickly disclosesthat in much 'paradigm'is used in two different senses' of the book th! term On the one hand, it standsfoi the entire constellationof beliefs, values,techniques,and so on sharedby the membersof a given community.On the other, it denotesone sort of elementin that which, employed-as the concretepuzzle-solutions constellati,on, modelsor examPles,can t"-pI""" explicit rule1 as a basisfor the solution of the remaining puzzlesof ttor-"l science.The first senseof the term, call it"the sociological,is the subiectof Subsection2, below; Subsection3 is devotedto paradigmsasexempast achievements. plary ' pfiilorophically, at least, this second senseof _'paradigm'is the deepei of the two, and the claims I have made in its name are the main sourcesfor the controversiesand misunderstandings that the book has evoked,particularly for the chargethat I *""k" of sciencea subiective and irrational enterprise'These issuesare consideredin Subsections4 and 5. The first argues that termslike'subjective'and'intuitive' cannotappropriatelybe of knowledg. ryt I have described applied to the "o-por,"nts al^tacitly embedded in shared examplls, Though t,t"\ knowledge is not, without essentialchange,subiect to paraphrasein terirs of rules and criteria, it is neverthelesssystematic,time tested, and in somesensecorrigible. subsection5 applies that argument to the problem of choice between two incompatible thiories, urging ii brief conclusionthat men who hold incommensurable-,riJ*points be thought of as members of different and that their communicationproblems Ianguage "o-*.r^nities b"in^iy"ed asproblemsof translation.Three residualissuesare 6 and 7. The first condir",rsr"d in the concludingSubsections, sidersthe chargethat the v]ew of sciencedevelopedin this book is through-and--thtoughrelativistic. The secondbeginsby inquiring wheiher my arguirent really sufiers,ashasbeensaid,from a betweerithe descripiiveand the normativemodes;it "oifurion concludeswith brief remarkson a topic deservinga separate
175
Postscript essay_: the extent to which the book'smain thesesmay legitimately be appliedto fieldsother than science. l. Paradigmsand Community Structure The term'paradigm' enters the preceding pagesearly, and its manner of entry is intrinsically c]rcular..{p"riaigm ii what of a scientificcommunity share,^ arrd,Zon ersery, -a n | . lh::embers i]l,l', com.munity consistsof men who ,h*r" paradigm. a, ,|,::i:l,ilc circularitiesare vicious (I shail defend yt.:+wil,;,rflot,"u of 't'ail;l'47 similar strucfurelate in this nosrcr.rinr),but this"r, "r[-,r-ent one ii ro.rr"u ii |,, 'J of real difficu " ities can and should be isolatedwithou_tprior recourseto paradigms;the latter can then be discoveredby scrutinizing the behaiior of a given commu_ lity's members.If this book were being rewrit"ten,it would thereforeopen with a discussionof the community structureof science,a topic that has rpc-entlybecomea signihcant subiect of sociologic{ researchand that hirtori"n, of science are alsobeginning take seriously.Preliminaryresults,many of them still !o unpublished,suggestthat the empijcar techniq,r"'rr"q,rlr"d fo, -harrd its exploration are non-trivial, bui someare in arrd others are sure_to be developed.bMost practicing scientists respond at once to_questionsabout their community tiking "mr"uons, for grantedthat responsibilityfor the variouscurrent specialtiei is distriburtedamonggoups oi at leastroughlydeterminite membership. I shail theiefore here assu-""tt more ,yri"*"u" "t meansfor their identiftcationwill be found.Instead of presenting preliminary researchresults,let me briefy articulate^theinhritive notion of community that underlies much in the earlier chaptersof this book. It is a notion now widely shareJ by ,"iurr_ tists,sociologists, and a number of historiar* oi science. 6W. O. Hagstrom,The Scientific Coymyydty (New york, tg65), and v; D. J. Price and D. de B-Beaver, 'c"ii"-Ui.ition in an invisibie chaps.iv Ameficanpsycholngisr,XXI (1966), rbf f_ra; bi"rr" Cr"nu-;So"i"l College,,, Sa-"t,rrc in a Groupof Scientists:A Test of the 'Invisible C"if"g";Hyplah;;I SociobgiCalRersieut,xxXIV (1969)-, 5s,5_52;-N.-C."ruuffiir,-siJh i,n erican arnongBiolo.gicarscientffir,_(p\p, diss., Harvara u"i""irlty,'r-g-6it, ia*ortu ..rn" Micro-Structureof an Invisible colleg^e:Tbe phate c-"p' "rra fi"ilr"r"a i'p"pu, an annual meetingof the AmericanSociological"Association, Boston,196g)."t
176
PostscriPt A scientiftc community consists,on this view, qf th: practiin most tioners of a scientiffcspecialty.To an extent_unparalleled oih", fields, they have undergonesimilar educations1nd professional initiations; in the processthey have absorbedthe same technical literature and dta*n many of the same lessonsfrom the it. Us,rally the boundariesof that siandard Iiterature mark community each and matter, subiect Iimits of a scientific "+has a subiect -"tt", of its own. There are schoolsin the "Jfy sciences,communities,that is, which approachthe samesubject viewpoints. But they are far rarer there than l;;;"mpatible in other ftelis; they are il*"yt in competition;-andtheir comPeof a ,i,i* is us,raily qui"tty ended' As a result, the members as others by seen are and see themselves scientific ***rriity shared of set a of pursuit the men uniquely responsiblefor the jo"fr, including'th" tt"itting of theii successors.Within such groups communication is reiatively fufl and professional iudgii""i relatively unanimous. Becausethe attention of different scientific communitiesis, on the other hand, focusedon different is somematters, professionalcommunicationacrossSou-Plines may, and often results in misunderstanding, times "rirro,rr, unsuspected if pursued, evoke signiffcant and previously disagreement. Communitiesin this senseexist,of course,at numerouslevels' nitY of all naturd scientists' At he main scientific Professional /sicists, chemists, astronomers, xe maior grouPings,communitY ed excePtlt the fringes' Subiect
',#ni 1,"'ioll;';ilH1,'3ff;'ff
organic chemists,and niques will also isolate *"iot subgro_ups: solid-stateand highg them, chemists-"*ot p"ih"p, protein on. It is only at the so and astronomlrs, radio ftrrtgy pfrysicists, How, to take emerge. problems next"llier level ih"t "-pirical the phage isolated have one would a contemporary example^, this For acclaim? PurPose9"" 3Tt group Prior to its public have recourseto atiendanc€at specialconferences,to the distri-
177
Postscripf sciences.To that end it may help to point out that the transition need not (I now think should not) be associatedwith the first acquisition of a paradif. The members of all scientiffc com'pre-paradigm" period, munities, including the schoolsof the labelled share the sorts of elements which I have collectively 'a paradigm.' What changeswith the transition to maturi$ is not the presenceof a paradigpnbut rather its nature. Otly after the changeis normal puzzle-solvingresearchpossible.Many of the attributes of a developed science which I have above asse ciated with the acquisitionof a paradig- I would thereforenow diseussas consequencesof the acquisition of the sort of Paradigm that identifies challengingpuzzles, suppliesclues to their solution, and guaranteesthat the t*ly clever practitioner will succeed.Only those who have taken courage from observing that their own ffeld (or school) htt paradigms are likely to feel that somethingimportant is sacriffcedby the change. A secondissue,more important at least to historians,concerns this book's implicit one-to-oneidentification of scientfic communities with scientiftc subject matters. I have, that is, repeat'electricity,' 'physical and optics,' edly acted as though, so/, treat'must name scientiffc communitiesbecausethey do name subiect matters for research.The only alternative my text has seemedto allow is that all these subiectshave belonged to the physicscommunity. Identiftcationsof that sort will not, however, usually withstand examination,as my colleaguesin history have repeatedly pointed out. There was, for example, no physics community before the mid-nineteenth century, and it was then formed by the merger of parts of two previously separatecommunities, mathematicsand nahrral philosophy(physiquee@ri' mcntal,e).What is today the subiect matter for a single broad community has been variously distributed among diverse communities in the past. Other narrower subiects,for exampleheat and the theory of matter, have existe
179
Postscripf over time. A paradigm govenu, in the first instance, not a
attention are likely to vanish. A number of commentatorshave, for example,used the theory of matter to suggestthat I drastically overstatethe unanimity of scientistsin their allegiance to a paradigm. Until comparativelyrecently, they point out, those theories have been topics for continuing disagreement and debate.I agreewith the description but think it no counterexample.Theoriesof matter were not, at leastuntil about 1920, the special province or the subject matter for any scientific community. fnstead, they were tools for a large number of specialists' groups. Members of difierent communities sometimes chose different tools and criticized the choice made by others. Even more important, a theory of matter is not the sort of topic on which the membersof even a single community mgst necessarilyagree. The need for agreement depends on what it is the community does.Chemistry in the first tialf of the nineteenth century provides a casein point. Though severalof the community'sfundamentaltools-constantproportion,multiple proportion, and combining weights-had become common property as a result of Dalton's atomic theory, it was quite possiblefor chemists,after the event,to basetheir work on these toolsand to disagree,sornetimesvehemently,about the existence of atoms. Someother difficulties and misunderstandingswill, I believe, be dissolvedin the sameway. Partly becauseof the examplesI have chosen and partly becauseof my vaguenessabout the nafure and size of the relevant communities,a few readersof this book have concluded that my concern is primarily or exclusivelywith major revolutionssuch as those associatedwith Copernicus,Newton, Darwin, or Einstein.A clearerdelineation of community strucfure should, however, help to enforce the rather different impressionI have tried to create.A revolution 180
Posfscript is for me a special sort of change involving a _certainsort of reconstructionof group commitments.But it neednotbe a large change,nor needlt tr"m revolutionaryt9 thoseoutsidea singJe comriunity, consistingperhapsof fewer than twenty-five-people. little-recognizedor disIt is iust becausethii type of "h"tge, cussedin the literature of the philosophyof science'occursso regularly on this smaller scale that revolutionary,_as against ctrlmulative,changeso badly needsto be understood. One last alteratlott,closelyrelated to the preceding,mayhelp to facilitate that understanding. A number of critics have doubted whether crisis,the commonawarenessthat something has gone wrong, precedesrevolutionsso invariably as I have implied in my original text. Nothing imPortantto my argument deiends, howevei, on crises'being an absoluteprerequisiteto reiolutions; they need only be the usual prelude, supplpng, that is, a self-correctingmechanismwhich ensuresthat the rigidity of normal scierice will not forever go unchalleng{. R6volutionsmay also be induced in other ways' though I think In addition, I would now point out what the they seldo* "r". abserrceof an adequatediscussionof community structure has need not be generatedby the work of obscuredabove' "iir", them and that sometimesunderthe communitythat experiences tet.tlt. New instrumentslike the electron goesrevolution " "s -i"ror.ope or new laws like Maxwell's may develop in one specialtland their assimilationcreatecrisisin another. 2. Paradignxses the Corutellntion of Group Commitments Turn now to paradigmsand askwhat they can possiblybe._My original text leavest o mor" obscureor important_question._One syripathetic reader,who sharesmy conviction that_'paradigm,r"--u, the central philosophicalelementsof the book, prepared a partial analyticindex and concludedthat the term is usedin at Ieasttwenty-two difierent ways.?Most of thosedifferencesare' (",g., Newton'sLaws I now think, due to stylisticinconsistencies are sometimesa paradigm,sometimesparts of a paradigm,and ? Masterman, op. cit.
r8l
Postscripf
182
PosfscriPt bolic form : f : rna or I - V /R. Others are ordinarily exprerye{ in words: "elementscombinein constantproportionby weight," or "action equalsreaction." If it were not for the generalaccep!like these,there would be no points at which anceof e*pr-essions group *"rirb"r, could attach the powerful techniqugsof logical ind mathematicalmanipulation in their puzzle-solvingenterprise. Though the exarn-pleof taxonomy suggeststhat normal the power of a sciencecan proceedwitf, few such expressions, scienceseemsquite generallyto increasewith the number of gn"rib ati6ns its practioners have at their disPosal' symbolic ' Th"r" {eneralizationslooklike laws of nature, but their function for gioup membersis not often that alone.Sometimesit is: - RI2.When that law was for e*amlpleihe Joule-LenzLaw, H membersalreadyknew_wh al H , R, and I discovered, "o*i.rnity simply told them something stood for, and these genercEzations resistancethat they had and current, about the behavior oi heat, earlier in the as discussion often, not known before. But more se-rve simultaneously book indicates, symbolic generalizations in separated sharply a second.function, one thit is ordinarily : R, v r or trut / Like f analysesby philosophersof science. th"y fu.rction in part as laws but also in, part as deftnitions of ro-" of the symlok they deploy. Furthe_rmore,the balance betweentheir inseparableiegisLtive and definitionalforce shifts over time. In anotier c'ontexithesepoints would repay detailed analysis,for the nature of the commitment to a law is very difierent from that of commitment to a definition. Laws are often conigible piecemeal, but deftnitions, being tautologies, of Ohm's are not. For e*aiople, part of what the acceptance 'resist'current' and t"defittition of both Law demanded ** " ance'; if those terms had continued to mean what they had meantbefore,Ohm'sLaw could not havebeenright; that is why it was so strenuouslyopposedas, say, the Joule-LenzLaw was not.8Probablythat situation is typical. I currently suspectthat e For signiftcant parts of this epis,ode_see: T. M. Brown, "The Electric Current in Earlv lfineteentfi-Century Fre^nchPhysics," Historical Studies in the Physical ( t96g), 6t-103; and Morton Schagrin, "Resistanceto Ohm's Law," i"in"it,I Am,erican loumal of Physics,XK ( 1963 ), 53il47 .
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Posfscripf all revolutionsin_volve, amongother things,the abandonmentof generalizationsthe force of which had pieviously been in some pa_rtthat of tautologies.Did Einsteinshowthat simultaneitywas relative or did he alter the notion of simultaneityitself? !v"r" thosewho heardparadoxin the phrase'relativity of simultan"ity simply wrong? consider next a sggo-nd type of componentof the disciprinary matrix, one aboutwhich a_gooddeal hasbeensaidin tny *igirr"t text under suchrubricsas 'metaphysicalparadigmr'or.th" oo'ut"physical parts of paradigms.'I^ h"'n" in mindshared commitments to such beliefs as: heat is the kinetic energy of the constituent parts of bodies; all perceptiblephenomenaare due to -"to-, the interaction of qualitativily neutral in the void, or, alternativ"ll, matter and force,or to fields.Rewriting the book !o now I would describesuchcommitmentsas beliefsinlarticular l categorymodelsto include also the electric circuit may be redynamic system;the molecules billiard balls in randommotion. commitment varies, with non;pectnrmfrom heuristicto onto_ similarfunctions.Among other with preferred or permissible
jl;15J#,j;:",ffi1 n:f,j:
determination of the roster of unsolved puzzles and in the evaruation of the importance of each. Note, however, that the members of scientific communities may not have to share even heuristic_models, though they usually do so. I have already_pointed out that membersh'ip in it u munity of chemists during the first half of the nineteenth "o# century did not demand a belief in atoms. A third sort of element in the disciplinary matrix I shall here describe as values. usually they are rior" widely shared different communities than eiiher symbolic g#eralizations "*onlor models, and they do much to provide * ,"rrr""of community to natural scientists as a whole. Thotrgh they function at all times, their particular importance emerg:eswhen the members of a
t81
Postscript or, later, choosebeparticular community must identify crisis_ i*""r, incompatible ways of practicing their discipline. Probably the moit deeply held values concern predictions:_they should be accurate;quantitative predictions are preferable to qualitative ones; whatever the margin of Permissibleerror, it shouldbe consistentlysatisfiedin a given field; and soon. There are also,however,valuesto be used in judging whole theories: they must, first and foremost,Permit puzzle-formulationand solution;where possiblethey should be simple, self-consistent, and plausible,compatible,that is, with other theoriescurrently deployed.( I now think it a weaknessof my original text that so Iittle attention is given to such values as internal and external consistencyin coniidering sourcesof crisisand factorsin theory choice.) Other sortsof valuesexist aswell-for example,science should (or need not) be socially useful-but the preceding shouldindicatewhat I have in mind. one aspectof sharedvaluesdoes,however,require particular of mention. To greaterextent than other sortsof compo-nents " the disciplinary"matrix,valuesmay be sharedby men who differ in their appfication. |udgments of accuracy are rela-tively, though nof intirely, stable from one time to another and from orr" il"rnber to anothet in a particular grouP.But iudgmentsof simplicity, consistency,plauribility, and so-onoftenvary greatly fro* individual to individual. What was for Einstein an insupportable inconsistencyin the old quantum tltory, o-ne-th-at iendered the pursuit of normal scienceimpossible'was for Bohr and othert diffi".tlty that could be expectedto work itself out " by normal means. Even more important, il those sifuations where values must be applied, different values, taken alone, would often dictate different choices.One theory may be more accuratebut lessconsistentor plausiblethan another;againthe old quantumtheoryprovidesan examPle.In short,thoughvalues *id"ly sharedLy scientistsand though commitmentto them "t" is both deepand constitutiveof science,the applicationof values is sometimesconsiderablyafiectedby the featuresof individual personality and biography that difierentiate the members of the group. To many readersof the precedingchapters,this characteristic 185
Postscripf of the operation of sharedvarueshas seemeda major weakness of gI position. BecauseI insist that what scientistsshareis not sufficient to commanduniform assentabout such matters as the nries or the distinction between ;is-provokingone, I am occasioniectivity and even irrationality.o haracteristicsdisplayed by value shared values can be -imporJa_nt even though the membersof the the sameway. (If that were not tcial philosophicproblems about rn did not all paint alike during
,mentar pattern i,ttTffl#T'"lx:jil; :r tl" ;dfrl-"gi.r" hat value was abandorrid.to ;hat wourd h"p-
pen in the sciencesif consistency_ ceased-tobe a primary lr"lrru. second,individual variability ir ihe application oi rh"r"i values may serye functions essentialto scierrce.The points at which values must be applied are invariably also thosl at which risks must be taken. Most anomaliesare iesolvedby normal means; most-proposalsfor new theories_do prove to be wrong. If ali membersof a community_respondedt6 eachanomaly* f ro,rr.= of crisis or embracedeach new theory advancedby'a colleague, sciencewould cease.If, on the other hand, no one reactel to anomalies or te brand-new theories in high-risk ways, there would be few or no revolutions. In matters ike these ihe resort to sharedvaluesrather than to sharedrules governingindividual choice m1r pe the communityt way of distributiig risk and aszuringthe long-term successof its enteqprise. Turn now to a fourth sort of elementin the disciplinary matrix, not the only other hnd but the last I shall discussi"r". For it the 'paredigrn'would term be entirely appropriate, both philologio s-ee particularly:
Dudley
shapere, "Meaning
and scientiftc
change,,, in Mind and.cosmosiEssays ti conkmporalvscidce ,ia i;n-ti"iieLfij'rirT'uriil o_fPiruburghserils in the phil6so9h!of science,rlr intt u,iigh, igooj, ".gnly Israelscheffier,science 4l-85; and.sutieictiitty alG; ill:, r90z[ -"* L"a tri.i -'" essays of sir Karl PopperandImreLakatosinciutth rhr;;;ig"; " "t 10seethe discussion at the beginningof sectionxIII, above.
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Postscripf cally and autobiographically; this is the component of a group's shared commitments which first led me to the choice of that word. Because the term has assumed a life of its own, however, 'exemplars.' By it I mean, initially, the I shall here substitute cpncrete problem-solutions that students encounter from the start of their scientific education, whether in laboratories, on examinations, or at the ends of chapters in science texts. To these shared examples should, however, be added at least some of the technical problem-solutions found in the periodical literature that scientists encounter during their post-educational research careers and that also show them by example how their job is to be done. More than other sorts of components of the disciplinary matrix, differences between sets of exemplars provide the community fine-structure of science. All physicists, for example, beSn by learning the same exemplars: problems such as the inclined plane, the conical pendulum, and Keplerian orbits; instruments such as the vernier, the calorimeter, and the Wheatstone bridge. As their training develops, however, the symbolic generalizations they share are increasingly illustrated by different exemplars. Though both solid-state and field-theoretic physicists share the Schrcidinger equation, only its more elementary applications are common to both groups. 3. Parad.igms as Slnred Examples The paradigt as shared example is the central element of what I now take to be the most novel and least understood aspect of this book. Exemplars will therefore require more attention than the other sorts of components of the disciplinary matrix. Philosophers of science have not ordinarily discussed the problems encountered by a student in laboratories or in science texts, for these are thought to supply only practice in the application of what the sfudent already knows. He cannot, it is said, solve problems at all unless he has first learned the theory and some rules for applying it. Scientific knowledge is embedded in theory and rules; problems are supplied to gain facility in their application. I have tried to argue, however, that this localization of
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Posfscripl the cognitive content of scienceis wrong. After the sfudent has done many problems,he may gain only added facility by solving more. But at the start and for sometime after, doing problems is learning consequentialthings about nature. In the absenceof such exemplars,the laws and theorieshe has previously learned would havelittle empiricalcontent. To indicate what I have in mind I revert briefly to symbolic generalizations.One widely sharedexampleis Newton's Second Law of Motion, generallywritten asf : ma.Tlte sociologist,say, or the linguist who discoversthat the correspondingexpression is unproblematically uttered and received by the membersof a given community will not, without much additiond investigation, have learned a greatdeal about what either the expression or the terms in it mean,about how the scientistsof the community attach the expressionto nature. Indeed, the fact that they accept it without question and use it as a point at which to introduce logical and mathematical manipulation does not of itself imply that they agreeat all about such matters asmeaning and application. Of course they do agree to a considerable extent, or the fact would rapidly emergefrom their subsequent conversation.But one may well ask at what point and by what meansthey have come to do so. How have they learned, faced with a given experimentalsituation, to pick out the relevant forces,masses,and accelerations? In practice, though this aspect of the situation is seldom or never noted, what students have to learn is even more complex than that. It is not quite the casethat logical and mathematical manipulation are applied directly to f - ma. \\at expression proveson examinationto be a law-sketchor a law-schema.As the sfudentor the practicingscientistmovesfrom oneproblemsituation to the next, the symbolic generalizationto which such manipulations apply changes.For the case of free fall, f : ma becomesng:
*#;
to mgsing: -*tffitfor
for the simplependulumit is transformed a pair of interactingharmonicoscilla-
tors it becomestwo equations,the first of which maybe written
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Posfscript ilst,
mrffi
t f krsr : kr(s, - sr * d); and for more complexsitua-
tions, such as the gyroscope,it takesstill other forms, the family resemblanceof which to f - mn is still harder to discover.Yet, while learning to identify forces, masses,and accelerationsin a variety of physical situations not previously encountered, the student has also learned to design the appropriate version of f : *a through which to interrelate them, often a version for which he hasencounteredno literal equivalent before. How has he learned to do this? A phenomenonfamiliar to both students of scienceand historians of scienceprovides a clue. The former regularly report that they have read through a chapter of their text, understood it perfectly, but nonethelesshad difrculty solving a number of the problems at the chapter's end. Ordinarily, also, those difficulties dissolvein the sameway. The student discovers,with or without the assistanceof his instructor, away to seehis problem as llke a problem he has already encountered.Having seenthe resemblance,graspedthe analory between two or more distinct problems,he can interrelate symbolsand attach them to nafure in the ways that have proved effective before. The law-sketch, sayf - rnn, hasfunctioned as a tool, informing the sfudent what similarities to look for, signalingthe gestaltinwhich the situation is to be seen.The resultant ability to seea variety of situations aslike eachother,assubjectsfor I : rruror someother symbolic generalization,is, I think, the main thing a student acquiresby doing exemplary problems,whether with a pencil and paper or in a well-designedlaboratory. After he has completed a certain number, which may vary widely from one individual to the next, he views the situations that confront him as a scientist in the samegestalt as other membersof his specialists'group.For him they are no longer the samesituationshe had encounteredwhen his training began.He has meanwhileassimilateda time-tested and goup-licensed way of seeing. The role of acquired similarity relations also showsclearly in the history of science.Scientistssolvepuzzlesby modeling them on previous puzzle-solutions,often with only minimal recourse
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Posfscripf to symbolic generalizatiorx. Galileo found that a ball rolling down an incline acquiresiust enoughvelocity to return it to the same vertical height on a secondincline of any slope,and he learned to seethat experimentalsituation as like the pendulum with a point-massfor a bob. Huyghensthen solvedthe problem of the center of oscillation of a physical pendulu* by imagining that the extendedbody of the latter was composedof Galilean point-pendula, the bonds between which could be instantaneouslyreleasedat any point in the swing. After the bonds were released,the individual point-pendulawould swing freely, but their collective center of gravity when each attained its highest point would, like that of Galileo'spendulum, rise only to the height from which the center of graviry of the extendedpendulum had begunto fall. Finally, Daniel Bernoulli discoveredhow to make the flow of water from an orifice resembleHuyghens' pendulum. Determine the descentof the center of gravity of the water in tank and iet during an infinitesimal interval of time. Next imagine that eachparticle of water afterward movesseparately upward to the maximum height attainable with the velocity acquiredduring that interval. The ascentof the center of gravity of the individual particlesmust then equal the descent of the center of gravity of the water in tank and iet. From that view of the problem the long-soughtspeedof eflux followed at once.11
That example should begin to make clear what I mean by learning from problems to see situations as like each other, as subiects for the application of the same scientiffc law or lawsketch. Simultaneouslyit should show why I refer to the consequential knowledge of nature acquired while learning the simiIarity relationshipand thereafterembodiedin a way of viewing 11 For the example, see: Ren6 Dugas, A History of Mechanics, trans. J. R. Maddox (Neuchatel, 1955), pp. f3S-36, 18il93, and Daniel Bernoulli, Hydrodynamica, sioe d.e oiribus et motibus flaidorum, commentarii opus acadernicum (Strasbourg, 1738), Sec. iii. For the extent to which mechanics progressed during the ffrst half of the eighteenth century by modelling one problem-solution on another, see Clifford Truesdell, "Reactions of Late Baroque Mechanics to Success, Conjecture, Error, and Failure in Newton's Principia," Texas Quarteily, x (1967),23H8.
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Posfscripf physical sihrationsrather than in rules or laws. The three problems in the example,all of them exemplarsfor eighteenth-century mechanicians,deploy only one law of nature. Known as the Principleof oisoirsa,itwasusuallystatedas: "Acfual descent equals potential ascent." Bernoulli's application of the law should suggesthow consequentialit was. Yet the verbal state' ment of the law, taken by itself, is virtually impotent. hesent it to a contemporarysfudent of physics,who lcnowsthe words and can do all these problems but now employs different means. Then imagine what the words, though all well known, can have said to a man who did not lcnoweven the problems.For him the generabzationcould begin to function only when he learned to iecognize"actual descents"and "potential ascents"as i_ngedientsbf nature, and that is to learn something,prior to the law, about the situationsthat nature doesand doesnot present.That sort of learning is not acquired by exclusivelyverbal means. Rather it comesas one is given words together with concrete
doing it. 4. Tacit Krnwledge and.lntuition That reference to tacit knowledge and the concurrent reiection of rulesisolatesanotherproblem that hasbotheredmany of my critics and seemedto provide a basisfor chargesof subjectivity and irrationality. Somereadershave felt that I was trying to makesciencereston unanalyzableindividual intuitions rather than on logic and law. But that interpretation goesastray in two essentialrespects.First, if I am talking at all aboutintuitions, they are not individual. Rather they are the tested and shared of the membersof a successfulgroup,and the novice possessions acquiresthem through training as a part of his preparation for Second,they are not in principle unanalyzgroup-membership. able. On the contrary, I am currently experimentingwith a
r9r
Posfscripf computer program designedto investigatetheir properties at an elementarylevel. About that program I shall have nothing to say here,r2but evenmentionof it shouldmakemy mostessentialpoint. When I speakof knowledgeembeddedin sharedexemplars,I am not referring to a mode of knowing that is less systematicor less analyzablethan knowledge embeddedin mles, Iaws, or criteria of identiffcation.Instead I have in mind a manner of lcnowing which is miscontruedif reconstructedin terms of rules that are ffrst abstractedfrom exemplarsand thereafter function in their stead. Or, to put the same point differently, when I speak of acquiring from exemplarsthe ability to recognizea given situation as like someand unlike others that one has seenbefore, I am not suggestinga processthat is not potentially fully explicable in terms of neuro-cerebralmechanism.Instead I am claiming that the explication will not, by its nature, answer the question, "Similar with respect to what?" That question is a requestfor a rule, in this casefor the criteria by which partieular sitr,rationsare grouped into similarity sets,and I am arguing that the temptation to seekcriteria (or at least a full set) shouldbe resistedin this case.It is not, however, systembut a particular sort of systemthat I am opposing. To give that point substance,I must briefly digress. What follows seemsobvious to me now, but the constantrecoursein my ori$nal text to phraseslike "the world changes"suggests that it has not alwaysbeen so. If two peoplestand at the same place and gaze in the samedirection, we must, under pain of solipsism,conclude that they receive closely similar stimuli. ( If both could put their eyes at the same place, the stimuli would be identical. ) But people do not seestimuli; our knowlof them is highly theoretical and abstract. Instead they "dgu sensations,and we are under no compulsionto supposethat have the sensationsof our two viewersare the same.(Scepticsmight remember that color blindnesswas nowhere noticed until John Dalton's description of it in L7gL.) Otr the contrary, much r2 Some information on this subject can be found in "Second Thoughts."
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PostscriPt neural processingtakes place between the receipt of a stimulus and the awarenessof a sensation.Among the few things that we larow about it with assuranceare: that very different stimuli can that the samestimulusc-anproduce producethe samesensations; very difierent sensations;and, ftnally, that the route from stimulusio sensationis in part conditionedby education.Individuals raised in difierent societiesbehaveon someoccasionsas though they saw difierent things. If we were not tempted to identify stimuli one-to-onewith sensations,we might recognizethat they actuallydo so. Notice now that two groups,the membersof whichhave syst_ematically difierent sensationson receipt of the samestimuli, do in somesensekve in difierent worlds.We posit the existenceof stimuli to explain our perceptions of the world, and rye posit their immutJbi[ty to avoid both individual and social solipsism. About neither posit have I the slightest reservation. But our world is populaled in the first instancenot by stimuli but by the obiectso1o,-ttsensations,and theseneed not be the same,individual to individual or group to group. To the extent,of course, that individualsbelongto the samegrouPand thus shareeducation, language,exPerience,and culhrre,we have 89od reasonto supposethai th"ir sensationsare the same.How elseare we to understand the fulness of their communicationand the communality of their behavioral responsesto their environment? flr.y must seethings, Processstimuli, in much the sameways. But where the difierentiation and specializationof groups begins,we have no similar evidencefor the immutability of sensation. Mere parochialism,I suspect,makesus suPPosethat the route from stimuli to sensationis the samefor the membersof all grouPs. Relurning now to exemplarsand mles, what I have been trying to suggest,in however preliminary a fashion,is this. One of the fundamental techniques by which the members of a group,whether an entire culture or a specialists'sub-community within it, learn to seethe samethings when confrontedwith the samestimuli is by being shownexamplesof situationsthat their in the group have already Iearned to see as like predecessors
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Postscripf each other and as different from other sorts of situations.These similar sifuationsmay be successive sensorypresentationsof the sameindividual-say of mother,who is ultimately recognizedon sight-aswhat she is and as difierent from father or sister. Thry may be presentationsof the membersof natural families, t"y of swanson the one hand and of geeseon the other. or they may, for the membersof more specializedgroups,be examplesof the Newtoniansituation,of situations,that is, that are alike in being zubject to a version of the symbolic form f : ^a and that ar6 different from those sifuations to which, for example,the lawsketchesof optics apply. Grant for the moment that somethingof this sort doesoccur. ought Y" say that what has been acquired from exemplarsis rulesand the ability to apply them?That descriptionis tempting becauseour seeinga sifuationas like oneswe have before must be the result of neural processing,fully "n"o,rni"r"d governedby physicaland chemicallaws. In this sense,on"" *u h".toelearned t9 d9 it, recognitionof similarity must be as fully systematicas the beating of our hearts.But that very parallel suggeststhat recognition may also be involuntary, e processover which we have no control. If it is, then we may.tofproperly conceiveit as somethingwe manageby rules and criteria. To speak of it in thoseterms implies"pplying that we have accessto alternatives, that we might, for example,havedisobeyeda rule, or misapplied a criterion, or experimentedwith some other way of seeing.r' Those,I takeit, arejust the sortsof things we cannotdo. Or, more precisely,those are things we cannot do until after wehavehad a sensation,perceivedsomething.Then we do often seekcriteria and put them to use.Then we may engagein interpretation, a deliberative processby which we chooseamong alternativesaswe do not in perceptionitself. Perhaps,for example, somethingis odd about what we have seen (rememberthe anomalousplaying cards). Turning a corner we see mother 13 This point might never have needed making if all laws were like Newton's 'breaking and all rules like the Ten Commandments. a _In that case the phrase law' would be nonsense, and a rejection of rules would nol seem to impli a process not governed by law. Unfortunately, traffic laws and similar producti of legislation can be broken, which makes the confusion easy.
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PostscriPt entering a downtown store at a time we had thought she was home.dontemplatingwhat we have seenwe suddenlyexclaim, "That wasn'tmothet,for shehasred hair!" Entering the storewe seethe woman again and cannotunderstandhow shecould have been taken for rn'other.Or, perhapswe seethe tail feathersof a waterfowl feeding from the bottom of a shallowpool. Is it a swan or a goose?We c6ntemplatewhat we have seen,mentally 9o-prr#g the tail featherswith those of swansand geesewe have we simP-lywant i""tt 6.fore. Or, perhaps,being proto-scientists, of swans, (the whjteness to know some ginerai characieristic already can we for example) of"themembersof a natural family have prewe what recognizi with ease.Again, we contemplate given the viouily perceived,r""r"hing for what the membersof family havein common. and in them we do seek Thlse are all deliberativeprocesses, and deploy criteria and rules.We try, that is, to interpret sensations alieady at hand, to analyzewhat is for us the given. Foyever we do that, the proc"it"r involved must ultimately be neural, and they are therefore governed by the same_phyncochprnical laws dhat govern perception on the one hand and the beating of our heartson the other. But the fact that the system obeysih" t*-. laws in all three casesprovidesno reasonto suppose that our neural apparatusis Programmedto,operate the ,"-" way in interpretationasin peiceptionor in either asin the in this book is beatingof orrt hearts.What I have beenopposingnot before, but Descartes since therefJrethe attempt,traditional ia as to analyze perception as an interpretive Process' an unconsciousversionof what we do after we have perceived' What makesthe integrity of perceptionworth emphasizingis, is embodiedin the neural of course,that somuch p"ti "*p"rience An ap-propriately apparatusthat transformsstimuli to sensations. value. To say survival piogt"--ed perceptualmechanismhas percephave different ih"t ttr" -"-6"rt of difierent grouPsmay that imply to not is tionswhen confrontedwith the samestimuli environmertts In many they may havejust any perceptionsat all. a group that could not tell wolves from dogs could not endure. Nor would a group of nuclearphysiciststoday strrviveas scien-
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Posfscript tists if unable to recognizethe tracks of alpha particles and elechons.rt is-iustbecausesovery few waysof seeingwill do that the onesthat have withstood the testsof group ,rrc ate worth transmitting from generationto generation.Equally, it is because they have beenselectedfor their successover historic time that ye-1nu-stspeakof the experienceand knowledge of nature embedded in the stimulus-to-sensation route. Perhaps'knowledge'isthe wrong word, but there are reasons for employing it. what is built into the neural process that transformsstimuli to sensationshasthe following characteristics: it has been transmittedthrough education;it his, by trial, been found more effective than its historical competitorsin a group's current environment;and, ffnally, it is subiect to changeboth fugug} further educationand through the-discoveryofmisftts with the environment.Those are characteristicsof knowledge, and they -explain why I use the term. But it is strange ,rra!", for one other characteristicis missing.we have no dirJct """Jsto to what it is we know, no rules or generalizations with which expr_ess this knowledge. Rules which could supply that access would refer to stimuli not sensations,and stimuli *" can know only thro|Bh elaborate theory. In its absence,the larowledge embeddedin the stimulus-to-sensation route remainstacit. Though it is obviously preliminary and need not be correct in all details,what has iust been said about sensationis meant Iiterally. At the very leastit is a hypothesisabout vision which should be subiect_toexperimentalinvestigationthough probably not to direct check.But talk like this ofieeing and sensltion here also seruesmetaphoricalfunctions as it doeJin the body of the book. we do not see electrons,but rather their tracks or elsebubbles of vapor in a cloud chamber.we do not seeelectric currents at all, but rather the needleof an ammeteror galvanometer. Yet in the preceding pages,particularly in Section X, I have repeatedly acted as though we did perceive theoretical entitieslike currents,electrons,and fields,asthough we learned to do so from examinationof exemplars,and as though in these casestoo it would be wrong to replacetalk of seeingwith talk of criteria and interpretation.The metaphorthat transfers'seeing'
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Poslscript to contextslike theseis scarcelya sufrcient basisfor such claims. In the long run it will need to be eliminated in favor of a more literal mode of discourse. The computer program referred to above begrls to mggest nor ways in which that may be done,but neither available_space eliminlfi"g my the extent of my presentunderstandingpermits the metaphor heie.tn Instead I shall try briefy to bulwark it. Seeing*"t"r droplets or a needle against a numerical scaleis a primitive perceptual experiencefor the man unacquaintedwith lbud chamberJand ammeters.It thus requires contemplation, analysis,and interpretation (or else the intervention of external authority) before conclusionscan be reachedabout electronsor of the man who has learned about currents. But the position -and had much exemplary experience with these instnrments them is very difierent, and there are colrespondingdifferences in the *"y h" processesthe stimuli that reach him from them. Regarding the t"pot in his breath on a cold winter afternoon, hisiensation may be the sameasthat of a laymll, bgt viewing a cloud chamber'he sees (here literally) not droplets but the tracks of electrons,alpha particles, and so on. Those tracks are, if you will, criteria that h- interprets as indices_of _th9Presenoe ofihe coresponding particles,but that route is both shorter and difierent from the one taken by the man who inteqpretsdroplets. Or considerthe scientistinspecting an ammeter to determine the number against which the needle has settled. His sensation probably is thJ sameasthe layman's,particularly if the latter has De 14 For 14 "Second Thoughts" crypbc remark'i-may nemarxs may be to[owing gryPU" the following Thoughts'- the of "second readers of For readers leading. The possibility of immediate recognition oF the m-embers of natural ihe existence,.aftgr existence, after neiral processing, of empty perceptual families famiti& depen^cts_uporr "eital,n5gcessing, were a rc to-be.discriminated. tn hc discrimineted. If- for.example, for examole. S"t: there *,"f", If, space betrieen thi families "
ide'ntifvine a the6retical entity, that entity can bi eliminated from the ontology
these entities however. these o thlnri hv srrhstihrtion ln the absencl absenc-eof of such such rules, rules. however, fti-the of "f a theori by substitution. their existence. existence. denran& their then denran& the theory theory then are not eliminable;; the
197
Postscripf read other sorts of meters before. But he has seen the meter
prior experienceand training. 5. Exemplnrs,Incomrnerwurabiktg,and Reoolutions
_._r6_Thepoints that follow are dealt with in more detail in secs. v and vi of "Reflectiois." rG see the works cited in note g, above, and also the essay by Stephen Toulmin in Growth of Knowledge.
198
PostscriPf in a cannot communicatewith each other at all; as a result, good t-o recourse no be can there debate over theory-choice that are i"6o"r, instead theory must be chosen for reasons mystical sort,of some subjectile; ultimately personalani "PF":
6il;i1
for tire decisionactually-t"i:*| iesponsible
Y::: these othelrparts of the book, the passa$eson which ,#;;y r - -^^( of charges for responsiEle been misconstructions rest have
199
Poslscript metalsfrom the set of compoundsto the set of elementsplayed an essentialrole in the emergenceof a new theoryof combustion, of acidity, and of physical and chemical combination. In short order those changeshad spreadthrough all of chemistry. Not surprisingly, therefore, when such redistributions occur, two men whoie discoursehad previouslyproceededwith apparently full understandingmay suddenlyfind themselvesrespondingto the samestimulus with incompatible descriptionsand generalizations.Thosedifficultieswill not be felt in all areasof eventheir scientiffcdiscourse,but they will ariseand will then clustermost densely about the phenomenauPon which the choice of theory most centrallydepends. Suchproblems,though they first becomeevident in communication, are not merely linguistic, and they cannot be resolved simply by stipulating the definitionsof troublesometerms. Because the words about which difficulties cluster have been learnedin part from direct applicationto exemplars,the partici"I use the pants in J communicationbreakdown cannot say, 'element' (Ot 'mixt1re,' Or 'planet,' Or 'unConStrained WOrd motion') in ways determined by the following criteria." They cannot, that is, resort to a neutral languagewhich both use in the sameway and which is adequateto the statementof both their theoriei or even of both those theories'empirical consequences. Part of the difierence is prior to_the application of the languagesin which it is neverthelessreflected. The men who experience such communication breakdowns must, however,have somerecourse.The stimuli that impinge upon them are the same.So is their generalneural apparafus, however differently programmed. Furthermore, except in a small, if all-important, area of experienceeven their neural programmingmust be very nearly the same,for-they share a the immediatepast.As a result,both their _evel7hirt6ry, "*""pt day and -ort of their scientificworld and languJBeare shared. Given that much in common,they should be able to find out a are great deal about how they difier. The technique_s_required of or parts or comfortable, iot, however,either straightforward, them recognize rarely the scientist'snormal arsenal.Scientists
201
1trrr
\^/VU PL,P @'+r h ^fidr
lno g^trirvt"Vlk eo*icrip,
"Y l^@^rynsuaU
for quite what they are, and they seldom use them for longer than is required to induce conversionor convince themselies that it will not be obtained. Bri"fly put, what the participants in a communicauonbreakdown can do is recognize each other as members of difierent lyEage communitiesand then become translators.t?Taking the differencesbetween their own intra- and inter-group disl course as itself a subject for study, they can first attempt to d used unproblematically within e?ch community, ge nej_e4beless-foei_ol@le within for inter-group discussions.(Lo-cutlonsthat present no suctr difficulties may !e lomophonically translated.) Having isolated such areas of difficulty i" scientific communication, they can -efiort next resort to their shared everyday vocabulariesin an further to elucidate their troubies. Each m€r/, that is, try to discoverwhat the other would seeand say when presentedwith a stimulus to which his own verbal responr" *o.rld be difierent. rf they can sufrciently refrain from explaining anomalousbehavior as the consequenceof mere ot *Jdrr"ss, they may "ttoi in time become very good predictors of each other's behavioi. Each will have learned to translate the other's theory and its consequences into his own languageand simultaneouslyto describe in his language the world to which that theory applies. That is what the historianof scienceregularly does (or sho:uld ) when dealingwith out-of-datescientifictheories. since translation, if pursued, allows the participants in a communicationbrea\down to experiencevicariouslysomething of the merits and defects of eac[ other's points of view, it is i potent tool both for persuasionand for conversion.But even persuasionneed not succeed,and, if it does, it need not be r7 The already classic source for most of the relevant aspects of translation is w. v. o. Quine, word and obiect (cambridge, Mass., aid New york, rg60), claps, i and ii. But Quine seems to assume 6at two men receivi"g tt ,"-" stimulus must have thL same sensation and therefore has little to sai about " the extent to which a translator must be able to describe the world td *hich thu language bsrjrg translated_applies. For the latter point see, E. A. Nida, "Lirrguistics-and Ethnology.in Trinslation p.r.oblem^s,'irrDel Hymes (ed.), Language and Cultue in Societqj ( New york, lg64 ), pp. Sil-SZ.
202
Poslscript acoompaniedor followed by conversion.The two experiences rt" ttoi the same, an important distinction that I have only recently fully recognized. fo plrr,r"d" rori"ooe is, I take it, to convince him that one's o*rrlri"* is superiorand ought thereforesupplanthis own. That much is occasionallyachievedwithout recourseto anything UI" translation. In its absencemany of the explanationsand problem-statementsendorsedby themembers of one scientiffcgroup will be opaque to the other. But each languagecommunity "T usually ptodrr"" from the start a few concrete researchresults that, iho"gh describable in sentencesunderstood in the same way by .-ittr groups,cannot yet be actounted for by-the other inlts olwntermt. if th" new viewpojnt enduresfor a *--nttity time and continues to be fruitful, the research results verbalizable in this way are likely to gow in number. For somemen such results alon'ewill be decisive.Th.y can say: I don't know how the proponentsof the new view succeed,but I must learn; whatevei they are doing, it is clearly righ1. Th"! reaction comes particularly easily to m6n iust e-nteri$ F9 profession,for trhey L"t" not yit acquired the specialvocabulariesand commitments
uiiltyS-IanguAgeinto the dther's. As translation from ong-p furthermore, somemembersof each communit{ *ty pioceAA-s, alsobegin vicariously to understandhow a statementpreviously opaque-couldseeman explanationto membersof the opposing gt""p. The availability of t..hniques like these does not, of bnti", guaranteeperiuasion. For molt people translation is a threatenlng procesi, and it is entirely foreign to normal science'
203
Posfscripf counter-arguments are, in any case, always available, and no rules prescribe how the balance must be struck. Nevertheless, as argument piles on and as challenge after challenge is TryTent successfully met, only blind stubbornn"rr ."i at the end acc6unt for continued resistance. being case, a second aspect of translation, long -the " TI familiar to both historians and linguisis, becomes crucially iml portant. To translate a theory or worldview into one's o*., l"rr-
conversion. But neither good reasons nor translation constihlte conversion, and it is that process we must explicate in order to understand an essential sort of scientific chanle.
201
Posfscript 6. Reoolutiansand' Aelatioisnt' One consequenceof the position iust outlined-h* particularly bothered a rrjumberof my critics.ls fruy find my viewpoint relativistic, particularly as it is developed in jt_re.lastsection of this book. f"fy remarks about translation highlight the reasons for the chargl. The proponentsof difierent theoriesare like the members ot- difio"ttt language-culture communities. Recognizing the parallelir* r.rgguststhat in some senseboa! grouP: -"y 6" right. Applied to culture and its developmentthat Position is relativistic. But applied to scienceit may not be, and it is in any-case-far from rnere relativism in a respect that its critics have failed to see.Taken asa SouP or in SouP sciences ile, I have argued, Though the values that ,h"I d' derive from other asPectsof thei: ability to set ,tp *d to solve puzzlespresentedby nahrre is, in for most members of value c-onfict, the dominant "tit"tiott "rr" of ,"i"rrtiftc group. Like any other value, puz"Je-solvingability " equivo:calin application. Two men who share it may proves neverthelessdifier in thi iudgmentsthey draw from its use' But the behavior of a commu"ity"*tti"h makesit preeminent*tll b: r ;"i difierent from that of one which doesnot. In the sciences, the b"li"u., the high value accorded topuzzle-solvingabilityhas following consequences. Imagine an eiolutionary tree representing-the developmgnt of thelodern scientiftc specialtiesfrom their common o_ngi* in, say,primitive natural pirilosophyand the crafts. A line drawn up that tree, never doubing baZ'k,from the trunk to the ,iP -d by ,6*" branch would trace i ,,r"""rrion of theories related descent.Considering any two such theories,chosenfrom points not too near theit otlgi", it should be easyjo design llitl of :{teria that would enable an uncommitted observerto distinguish the earlier from the more recent theory time after time. Among 18Shapere, "structure of scientiffc Revolutions," and Popper n Gtouth of Y*noledge.
Posfscrr'pf the most useful would be: accur quantitative prediction; the bala day subjectmatter; and the num I.essuseful for this puryose,thor of scientiftc life, *ould be such
ays the sensein which I am a lfr}Orcss
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there';.t, i"ii"";;: ;;i.#;::l"ll"j ontorogy"iJd#J;'lil',Jl:,11,',,"1Jffi #fi'tTfr::"* in princip.le.Besider,i-"_nir.orian,
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r am
I can see in
ontological development.On the contrary, in some
206
PostscriPt important respects,though by no meansin all, Einstein'sgeneral the'oryof relaiivity is cloier to Aristotle's than either of them is to Ne*io.r's. fhough the temptation to describethat position as relativistic is understandable,the description seems to me wrong. Conversely,if the position be relativism, I cannot see that the relativisi loses ariything needed to account for the nature and developmentof the sciences' 7. The Nature of Scierwe I conclude with a brief discussionof two recurrent reactions to my original text, the first critical, the secondfavorable, and neither, I"think, quite right. Though the two r-elateneither to what has been rid to fir nor to each other, both have been sufficiently prevalent to demand at least someresP_onse. A few i"id"tr of my original text have noticed that I repeatedly passback and forih betweenthe descriptiveand the normative modes, a transition particularly marked in occasional-passagesthat open with, "B^,rtthat is not what scientistsd'o," and by claiming that scientistsought not do so. Some critics "lo's" violatlorrfrrsing descriptiJn with prescription, claim that I 'Is' "m imply cannot i"! tf," time-honored philosophiial theorem: 'ought.'10 fhat theorem has, in practice, become a ta$, and it is no longer everywherehonorJd. A number of contemporaryphilosoph"L have discoveredimportant contextsin which the normaiive ard the descriptivear^einextricablymixed.zo'Is'and'ought' are by no meansalivayssoseparateasthey -haveseemed.But no ,""o,rrr" to the subtletiesof iontemPorary linguistic philosophy is neededto unravel what has seemedconfusedabout this aspect a viewpoint or of my position. The preceding Pages-P-r:sent, philosophies other like and, of scie-nce, natuie the th"oty "Uo,tt which way_in the for consequences has of ,ci"rr"e, the theory Though succeed' to is enteryrise if their scientistsshouldbehave le For one of many examples, see P. K. Feyerabend's essay in Groath of Knowladge. zo stanley cavell, Must we Mean What we say? ( New York, 1969 ), chap' i'
207
Postscript
of the nafure of sciencethey constitute anomalousbehavior. The circularity of that argumentis not, I think, vicious.The consequences of the viewpointbeing discussedarenot exhausted by the observations upon which it reited at the start.Even before
they read its main thesesas applicableto many other fields as well. I see what they mean and would not like to discourage their attempts to extend the position, but their reaction hL neverthelessp_uzzledme. To the extent that the book portrays scientiffcdevelopmentasa succession of tradition-boundperiods puncfuated by non-cumulativebreaks,its thesesare undoubtedly of y1d" applicability. But they should be, for they are borrowedfrom other fields.Historiansof literature,of music,of
beenwidely thought to developin a difierent way. conceivably the notion of a paradigm as a concreteachievement,an exemp!"I, is a secondconhibution. I suspect,for example,that some of the notoriousdifficultiessurroundingthe notion of style in the
208
PostscriPt arts may vanish if paintings can be seento be modeled on one anotheirather than produied in conformity to someabstracted canonsof style.2l This boo[, however, was intended also to make another sort of point, one that has been less clearly visible to many-of its ,""^d"rr. llhough scientiftc developmentmay resemblethat in closely than has often been supposed,it is also other fields -* strikingly difierent. To say, for example, that the sciences,at a certain point in their development,progressin a least "TtLt way that other ffeldsho not, cannothave been all wrong, lvhatprogressitself may be. one of the obiectsof the book was "u", to examiie suchdifierencesand begin accountingfor them. Consider,for example,the reiteratedemphasis,above,on the absenceor, as I shouldnow say,on the relative scarcityof competing schools in the developed sciences.Or remember my i"-"ikr about the extent to which the membersof a given scientific community provide the only audienceand the only judgesof that communiiyt work. Or think again about the sp_ecial-nature of scientiftceducation,aboutpuzzlJ-solvingas a goal,and about the value systemwhich the scientificgroup deploysinperiods of crisisand iecision. The book isolatesother featuresof the same sort, none necessarilyuniqtre to science but in coniunction settingthe activity apart. About all thesefeaturesof sciencethere is a great deal more to be learned.Having openedthis postscriptby emphasizing-the need to study the community structure of science,I shall close by underscoringthe need for similar and, above all, for comparativestudy Jf th" corresPondingcommunitiesin other fields. -Ho* doesone elect and how is one electedto membershipin a and particular community,scientificor not? What is the_proc-ess the What does to the group? *tt"t are the stagesof socialization or individual deviations, what group collectivelytu" as its goals; imperthe control it iollective, will it tolerate; and how does missibleaberration?A fuller understandingof sciencewill de2r For this point as well as a more extended discussion of what is special abo,u,t the sciences, iee T. S. Kuhn, "Comment [on the Relations of Science and Art]," Comparatioe Studies in Philosophy and Histoty, Xl ( 1969 ), 40L12'
209
Posfscrpf pend on answersto other sorts of questionsas well, but there is no areain which more work is so badly needed.Scientiffcknowldgu, like language, is intrinsically ihe common property of a group or else nothing at all. To understand it welhalll nled to know the special characteristicsof the groups that create and useit.
2ro
lndex This indexhasbeenpreparedby PeterJ. Riggs,andboth authorandpubthisadditionandseeingit into lisherareindebtedto him for recommending print. A d h o c ,1 3 , 3 0 , 7 8 , 8 3 AlfonsoX,69 Annual stellarPuallax,26 Anomalies,6244, 67,82,87, 113 15,123 Archimedes, Aristarchus,75,76 Aristotle,2, 10, 12,15,48,6ffi9, 72, lO4,I 18-20,l2l-25, 140, 148,163 Bacon,Sir Francis,16,18,28,37, 170 Bl a ck,J . ,15, 70 Boyle,R., 28,41,14143 Brahe,Tlcho, 26,156
Darwin,C., 20, l5l, 17l-72 De Broglie,L., 158 R. (or Cartesian),41, Descartes, , 50 4 8 ,1 2 1, 1 2 6 ,1 4 8 1 "DifferentWorlds,"I 18,150 53,62,9G97 Discovery, A., G7 , 12,26,M, 66, Einstein, '14, 89,98-99,101-2,108, 83, 1 4 3 ,1 4 8 4 91, 5 3 ,155,158,165 4, 13-15,16,17-18, Electricity, 2V22, 28,35,6l-62, 106-7, I l7-18 Esotericproblems,24 Essentialtension,79 Extraordinaryscience,82-89
Clairant,8l Boxes,5, 152 Conceptual C o n s e n s u1s1,, 1 5 1, 5 3 l, 6 l , 1 7 3 Copernicus(and/orCoPernicism): 6, 8, 26,67-69,7 l, 74-76, 82, 83,I 15-16,128,149,150,1525 3 ,154- 55,157,158 Coulomb,C., 21,28-29,33,35 Crisis,67:75,80,82,84-86,181 2-3, 52,84, Cumulativeprocess, 9 5 . 9 6 ,1 6 1
Falsification, 77-'l 9, | 46-47 1 0 ,1 3,15,17,18, F ra n k l i n ,8 ., 20,62, 106,I 18,122,l5l
Dalton,J. (and/orDalton'schemistry),78, 106,130-35,139,l4l
103,I I 2, Incommensurability, 148,150,l98ff
Galilei,Galileo,3, 29,31, 48,67, 118-20,l2l-25,13940 Geology,10,22,48 GestaltSwitch,vi, 63,85, I l 1 - 1 4 ,1 5 0 Hutton,J., 15
lndex Kelvin,Lord, 59, 93,98n. Kepler,J., 30, 32,87,l5LS4, 1 5 6,lgg Lavoisier,A. (and/orLavoisier's chemistry),6, 10,4, 5L56, 57, 59:72, 79, 79,96,gg, 106-7, I lg, 120,130,14243,ln4g, 153,156-57,163 Leibniz,G. W., 48,72 LeydenJar,17,6142, 106,l l8, 129 Lyell, Sir Charles,l0 Lunarmotion,30, 39, 8l Maturescience,10,24,69 Ma xwell,J .C, 7, 28, 4 0 ,U ,4 8 , 58,66, 73-74,90,92, 107,lW Meaningchange,128,2014 Mercury(planet),81, 155 Neutrino(particle),27, 87 Newton,Sir Isaac(and/orNewtonianism), 6, 10,12-13,lS, 2G27,3G-33,3940,4, 4749, 67,7 l, 72-74, 76, 79, 79, gg-gg, l0l-5, 106,107, l2l,13940, 149,150,153, 154,157,163,165 Normalscience,5-6,10,2L34, 80 Nuclearfission,60 Observationlanguage,125-26, 129 Optics,ll-14, 16,39,42, 48,67, 7 9 , 89, 15L55 Paradigm, 10, 15,l8-19, 23,434,lg2-lgl Paradigmchoice,94, 109-10,
212
l M ,1 4 7 -5 9 Pauli,W., 83-84 Perception , ll2-13 Phlogiston, 53-56,57-59, 7G\72, 79,g5,gg-100,102,106,107, l2l-22,126, l2g,157 Planck,M., 12,l5l, 154 Planet(s),25,128 Popper, Sir Karl, 14H7,186n., 205n. Priestley, 1., 53-56,58, 59-60,66, 69,79,96,gg,I l g, 120,147,159 Progress,20,37, Chap.XIII esp. 160,162,166 Ptolemy,10,23,6749, 7 S-76, g 2 , g gl,l 5 , 1 5 4 , 1 5 6 Puzzlesolving,36-39 Quantumtheory,48, 49-50, 8 3 -8 4 ,9 9 ,9 5l ,0g, 154 Quine,W. V. O., vi,202n. Resistance, 62, 65,83, I 5 I Revolutions in science,G8, 92-99, t0t-2 Roentgen, W., 57-58,93 Scheele, C.,53,55,70 Scientific community,167-79, l7Gg0, 195-97 Tacitknowledge, 4,l9l Textbookscience,I 3G38 Uranus(planet),I 15-16 Venus(planet),154 Verisimilitude, v Wittgenstein,L.,45 "Worldchanges," I I l, I 18,l2l, 150 X-rays,7, 41, 57-59,61,92-93
9 " ( 6vzzo
