Dispositional Explanation and the Covering-Law Model

Dispositional Explanation and the Covering-Law Model: Response to Laird Addis Carl G. Hempel PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association, Vol. 1974. (1974), pp. 369-376. Stable URL: http://links.jstor.org/sici?sici=0270-8647%281974%291974%3C369%3ADEATCM%3E2.0.CO%3B2-R PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association is currently published by The University of Chicago Press.

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C A R L G . HEMPEL

DISPOSITIONAL EXPLANATION A N D T H E COVERING-LAW MODEL: RESPONSE TO LAIRD ADDIS The following considerations are offered in response to the critical observations and constructive proposals set forth by Laird Addis, in his paper 'On Defending the Covering-Law "Model"', concerning my explications of "rational" and dispositional explanation and concerning the claims associated with the covering-law model of explanation. 1. My main objection to Dray's construal of rational explanation, as characterized by the first schema in Addis's paper, was not that it is at odds with the covering-law model, but that the second explanans sentence, 'In a situation of type C, the appropriate thing to do is x', expresses a norm and therefore cannot possibly explain why A did in fact do x : to do that, we need, not a normative sentence, but a descriptive one, roughly to the effect that A was disposed to act in accordance with the normative principle. Accordingly, I proposed this modified construal for rational explanations of the kind considered by Dray : (PI) A was in a situation of type C (P2) A was a rational agent (Schema R, (p3) In a situation of type C, any rational agent will do x (E) (Therefore,) A did x. Here, the explanans sentences P2and P3jointly imply the required descriptive sentence just mentioned'. 2. Addis argues that, thus construed, a rational explanation is not a covering-law explanation, on the ground that the only sentence of general form occurring in the explanans, namely P,, is not an empirical law, but a definitional truth. For reasons to be suggested in Section 3, I do not think that a clear and defensible construal can be given to this claim. But suppose, for the sake of the argument, that the term 'rational agent' has been introduced explicitly as an abbreviation of 'agent who does x R. S . Cohen et al. (eds.), PSA 1974, 369-376. All Rights Reserved. Copyright @ 1976 by D. Reidel Publishing Company, Dordrechf-Holland.

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whenever he is in a situation of kind C'. Then, in virtue of this convention, the sentence P, attributes to A a general behavioral disposition, namely, to do x whenever faced with a situation of kind C; and the explanation represented by schema R accounts for A's action by showing that it was but a particular manifestation of A's general disposition to do x when in C. In this "dispositional explanation", the role of the covering law is played by the sentence P,. This sentence has nomic force; it is what Ryle calls a law-like sentence, ascribing a nomic behavior pattern to a particular individual

,.

3. But, as mentioned before, I must take exception to Addis's view that the sentence P,, and analogous sentences in other dispositional explanations, are definitional truths and therefore not empirical. Suppose that the term 'temperature' is originally introduced by the convention to add to the set of accepted physical sentences a new one saying that a physical body has a temperature of r degrees centigrade just in case a regular mercury thermometer that is in contact with the body will show a reading of r on its scale. Though adopted by convention, this dispositional "criterion sentence" for the term 'temperature in centigrades' is not treated in physics as devoid of empirical content and immune from challenge by empirical findings. For suppose that physical research, making use, among other things, of the thermometer criterion, leads to the establishment of the laws of heat exchange. These laws imply that when a mercury thermometer is inserted into a body of water, then - unless thermometer and water happen to have the same temperature before insertion - heat will flow from the warmer to the colder of the two bodies, and the resulting reading of the thermometer will not show the temperature that was to be measured. Thus, if those laws are accepted as (presumably) true, the criterion sentence for temperature must be rejected as (presumably) false. In fact, considerations of just this kind have led to modifications of the criterion sentence which provide for specific ways of compensating for the disturbing effect. Thus, the original criterion sentence, though introduced by terminological convention, has been abandoned in response to empirical findings, namely, those that led to the acceptance of the laws of heat exchange. In similar fashion, dispositional or operational criterion sentences for many other scientific terms have come to be abolished or revised even

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though they were originally accepted by linguistic convention. The preceding considerations apply equally, of course, to any dispositional criterion sentence, such as P, above, that might be used to characterize an agent as rational. The reply might suggest itself that the abolition of the criterion sentence for temperature does not amount to the retraction, as presumably false, of an empirical belief or assertion which has come into conflict with empirical evidence: that it is rather a matter of abandoning, as inexpedient, a terminologcal convention which could have been retained, but at the price of making it practically impossible to establish general and simple laws of heat exchange and of other thermodynamic processes. But there is no fundamental difference between the two kinds of theoretical change thus distinguished. For, as has been pointed out by Duhem. and repeatedly emphasized by Quine, a scientific hypothesis will normally conflict with empirical findings, not when taken in isolation, but only when combined with a more or less extensive body of further hypotheses; and in this case, the conflict does not unambiguously establish the falsehood of the given hypothesis and can be resolved not only by abandoning the latter, but alternatively by a variety of other changes within the theoretical system. And which of the possible adjustments is chosen will depend not just on the conflicting evidence, but also on the effect the contemplated modification would have upon such general features of the theory as scope, simplicity, and closeness of empirical fit. In sum, both the so-called retraction of an empirical hypothesis as presumably false and what is described as the withdrawal, as inexpedient, of a "truth-by-linguistic-convention" are based on considerations of empirical evidence and of certain overall features of the theoretical system within which the given hypothesis functions. Consequently, criterion sentences such as P3 above cannot properly be qualified as non-empirical, purely definitional, truths3

4. For brevity of exposition, the preceding argument treats dispositional terms as standing for specific, "single-trackM4, dispositions. But this is a considerable oversimplification : such expressions as 'has a temperature of 7 degrees centigrade', 'electrically charged', 'greedy', 'rational agent', must be regarded as dispositional in a broad sense, i.e., as each standing for a large bundle of dispositions, corresponding to the diverse ways in

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which electric charge, greed, a temperature of 7 degrees centigrade, etc., can manifest themselves under different conditions. In particular, any of the various "operational" criteria of application for a term may be regarded as specifying one of the dispositions the term stands for. But this conception requires further refinement. What determines the set of dispositions that a given term stands for? Surely, it is not specified by some sort of explicit enumeration; rather, it is determined by a system of hypotheses or a theory in the context of which the term is used. To put the point schematically: a theory T i n which a term, say 'D', occurs may imply sentences of the following kind:

(H)

(x)[Dx3 ( t )(SxtXJRxt)].

This is then one of the dispositional criteria provided by T for the presence of D ; it states that if an object has the property D, then it will respond in manner R whenever it is in a stimulus-, or test-, situation S. In many cases, especially when D is a quantitative feature, Twill imply infinitely many such such dispositional criteria for D. And when the theory changes - e.g., by the addition of new laws linking the temperature of a body to other quantitative characteristics of the body or its environment - then the set of dispositions that T links to D changes accordingly. But the term 'D'cannot be said to stand just for the conjunction, as it were, of the various dispositions the theory associates with it; for T will normally include further, nondispositional, hypotheses about D. For example, T will typically contain theoretical principles that link D to other attributes each of which has a set of dispositional aspects: thus, thermodynamic theory links the temperature of a gas to its pressure and volume; and a theory of rational action may link an agent's decisions to his objectives, values, and beliefs, all of which have dispositional asp e c t ~ In . ~ short, what I called a broadly dispositional term is more adequately conceived in analogy to the "theoretical terms" of physics, as standing for a theoretically characterized property, which is linked to other such properties by the basic principles of a theory T, and which can manifest itself in a vast variety of ways, as indicated by the corresponding dispositional criteria that T implies for it. And if such a property, say D,is invoked for explanatory purposes, it is the theoretical principles concerning D that do the explaining. Dispositional explanation is basically of a kind with theoretical explanation.

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There is a certain affinity, I think, between this conception and Addis's claim that every dispositional explanation is uniquely related to a potential covering-law explanation. The law in question is made evident, according to Addis, if the dispositional property, say D, of reacting in manner R under circumstances of kind S is analyzed as follows: (Schema A ) 'x has the property D' is tantamount to 'there is a nondispositional property f such that x has f and anything that has f reacts in manner R when in situation S'. Proposals to analyse or define dispositional properties by expressions of just this form have, in fact, been made by some earlier writers, especially Eino Kaila and Arthur Pap.6 They were prompted by Carnap's observation7 that the apparently obvious definition of 'Dx' by 'whenever Sx then Rx' would attribute the disposition D to, among other things, all those objects which are never exposed to situation S : for them the antecedent of the defining conditional is false, and the conditional thus trivially true. Thus, any rock that is never immersed in water would trivially satisfy the definiens for ' x is soluble'. Now, definitions conforming to schema A yield the same consequence unless the range of the permissible properties f is suitably restricted: a rock never placed in water would still qualify as soluble because it has the property of dissolving-whenever-placed-in-water; and, as required in schema A , anything that has this property does dissolve when placed in water. Addis's restriction off to non-dispositional properties is intended to rule out such undesired consequences. In the case of dispositions like solubility, elasticity, or electric conductivity, the relevant non-dispositional properties might be identified with certain microstructural characteristics which account for a substance having the dispositions in question. But the general requirement that f be "non-dispositional" might be questioned on the ground that any property may be amenable to dispositional characterizati~n.~ Pap, in his definition, ,requiresf to be a "kind-property", i.e., a property "with respect to [the possession of] which a thing cannot meaningfully be said to change" '; but the notion of meaningful assertability faces obvious difficulties of its own. However, as I have tried to show, an analysis of rational and other dispositional explanations need presuppose no "definitions" of disposi-

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tional terms. Theoretical principles of the character of H above are sufficient for the purpose. D is a property dealt with by a theory, and the explanatory principle H i s part of what the theory asserts about D; and, as a general theoretical principle, it expresses a nomic claim.1° 5. In sum. then, I think that rational and dispositional explanations do conform to the covering-law conception of explanation. As for the general claim to be made for that conception, I would persist in limiting it to the thesis that any adequate scientrfic explanation accounts for its explanandum phenomenon by subsuming it under general laws or theoretical principles. I do not wish to assert that all empirical phenomena admit of scientific explanation; for what empirical phenomena are explainable depends upon the nomic structure of our world: and the tightness and reach of the nomic net cannot be determined by philosophical analysis. Another reason for my position will be indicated in Section 6. Addis suggests another claim, stronger than the one I have put forward, but weaker than the one just spurned; it asserts: "There is no good reason to believe that any event occurs which cannot be given an explanation of the covering-law type". I certainly agree with Addis in the view, suggested at the end of his paper, that arguments that have been offered to prove the absolute unexplainability of certain aspects of the world are flawed; indeed, I think that no conclusive proof of this kind is possible. But Addis's thesis seems to me open to question. Consider the probabilistic laws that specify the half-lives of radioactive elements. They may be said, in the sense of the inductive-statistical form of the coveringlaw model, to explain such facts as that, of a certain initial amount of the element RaZz3,only very close to half is left after a period of 11.2 days -its half-life; that close to one-quarter remains after 22.4 days, etc. But, one may argue, these laws cannot account for certain other kinds of event: they cannot explain, for example, why atom a, decayed exactly two hours after the start of the test, or why atom a, survived for exactly 200 hours and 41 minutes; for to events thus characterized, the decay law ascribes zero probability. " An explanation of individual events of this kind would presumably require laws of strictly universal form. But suppose there are grounds for doubt that there are universal laws governing such events: would this not constitute a good reason to believe that there are events which cannot be explained ? Now, just such grounds

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for doubting the explainability of the individual decay events, and of various other kinds of elementary event, would seem to be provided by the probabilistic aspects of quantum theory and by the various considerations that throw into question the possibility of supplementing the theoretical system by a hidden-variable theory which would restore a form of determinism to the level of elementary events. 6. On closer examination, the idea of the explainability of an event proves to be frustratingly obscure. Events which form the subject of any explanation are always events-under-a-sentential-description; the explanandum of an explanatory account is a sentence describing the occurrence to be explained, not an expression naming it. Perhaps, then, the thesis that every event can be explained should be construed as claiming that for every true sentence E describing an event in some language L, there exists a set of true sentences expressible in L which jointly explain the event described by E. But thus construed, the thesis is surely false: the expressive means of L may be too limited for the formulation of suitable explanatory laws or theories. So one would have to claim, perhaps, that L can always be extended or modified in ways that permit an explanation of the given event, or that there is some other language in which the event can be described and explained. But this proposal needs supplementation by an account of the conditions under which a sentence E' in a new language L' can be said to describe the same event as E in L : and this project faces familiar difficulties. Analogous problems stand in the way of a sharp formulation of the principle of causality and of the idea of universal determinism. The thesis that all events are explainable is very closely related to these two ideas; and like them, it had perhaps best be viewed, not as an elusively comprehensive claim about the world, but as a heuristic maxim for scientific inquiry, calling for unflagging efforts to find explanatory principles where none are known so far. Princeton Universitj NOTES For fuller details: see C. G. Hempel, Aspects of Scientific Explanation und Other Essays in the Philosophy of Science (Free Press, New York, 1965), pp. 469.472.

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See G . Ryle, The Concept of Mind (Hutchinson's University Library, London, 1949), pp. 88-90; note Ryle's remark "The imputation of a motive for a particular action is ... the subsumption of an episode proposition under a law-like proposition." @. 90) The character of dispositional explanations and of law-like sentences is discussed in some detail in Hempel, op, cit., pp. 457463. These considerations are closely akin to ideas developed by Quine - for example, in "Carnap and Logical Truth", in P. A. Schilpp (ed.), The Philosophy of Rudo(f Carnap (Open Court, La Salle, Illinois, 1963), pp. 385406. This characterization is used by Ryle, who then goes on to stress that there are "many dispositions the actualisations of which can take a wide and perhaps unlimited variety of shapes" (op. cit., pp. 43-44). The issue is examined more fully in Hempel, op. cit., pp. 472477. See A. Pap, Anal.vtische Erkenntnistheorie (Wien, Springer, 1955),pp. 140-142 (reference to Kaila on p. 141), and the amplified discussion in A. Pap, An Introduction to the Philosophy of Science (Free Press, New York, 1962) pp. 278-284. ' R. Carnap, "Testability and Meaning", Philosophy ofscience 3 (1936), pp. 419471 and 4 (1937), pp. 1 4 0 ; see pp. 439441. Cf. N. Goodman, Fact, Fiction, andForecast (2nd edition, The Bobbs-Merrill Company, Indianapolis, 1965), p. 41. Pap, An Introduction to the Philosophy of Science, pp. 281-282. Pap's analysis also differs from Kaila's and Addis's by invoking causal implication where the other two authors rely on the material conditional. 'O A series of illuminating and suggestive observations on the issues touched upon in this section will be found on pp. 4-15 of W. V. Quine, The Roots of Reference (Open Court, La Salle, Illinois, 1974). '' The point is not affected by the fact that limitations of measurement do not permit the assignment of an event to an instant, but only to a finite time interval, and that the decay law yields some finite probability for the latter case. The point at issue hinges on the logical form of the explanatory laws: limitations of measurement notwithstanding, strictly universal laws do permit the deduction of consequences concerning what happens at some particular instant. Moreover, even the disintegration of a given atom during a specified time interval may have an extremely small probability according to the decay law depending on the length and location of the interval; accordingly, the law tells us that the decay was almost certain not to occur during that interval: and this hardly qualifies as an explanation of why it did occur all the same. My earlier analyses of statistical explanation do not qualify the statistical arguments associated with such cases as explanatory; but important alternatives to my way of viewing this issue have since been proposed by R. C. Jeffrey in 'Statistical Explanation us. Statistical Inference' and by W. C. Salmon in 'Statistical Explanation'; both of these studies are included in W. C. Salmon (ed.) Statistical Explanation and Statistical Relevance (University of Pittsburgh Press, Pittsburgh, 1971). I doubt, however, that Addis would take comfort from these views in regard to his thesis about explainability. -