Thought Experiments, in Encyclopedia of Cognitive Science

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Nickerson RS (1998) Confirmation bias: a ubiquitous phenomenon in many guises. Review of General Psychology 2(2): 175±220. Stanovich KE (1999) Who is Rational? Studies of Individual Differences in Reasoning. Mahwah, NJ: Lawrence Erlbaum. Weisberg RW (1993) Creativity: Beyond the Myth of Genius. New York, NY: Freeman.

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Further Reading Halpern DF (1996) Thinking Critically about Critical Thinking. Mahwah, NJ: Lawrence Erlbaum. Shermer M (2001) The Borderlands of Science. New York, NY: Oxford University Press.

Intermediate article

Tamar Szabo Gendler, Syracuse University, Syracuse, New York, USA CONTENTS

What are thought experiments? Kinds of thought experiments Uses of thought experiments in philosophical cognitive science

Philosophical and empirical theories of thought experimental cognition Controversies and issues regarding the use of thought experiments

Thought experiment: to perform a thought experiment is to reason about an imaginary scenario with the aim of confirming or disconfirming some hypothesis or theory.

programmed computer might manifest understanding, Searle asks his reader to consider whether a person locked in a room with a sheaf of Chinese characters and a set of instructions enabling her to select certain batches of characters (`answers') when prompted by certain other batches of characters (`questions') would be properly credited with understanding Chinese. (Searle expects his reader to give a negative answer.) Other examples are presented and discussed below. Although Ernst Mach is generally credited with having coined the expression Gedankenexperiment in his 1897 essay of the same name, and although contemporary German, English, and French usage can be traced to Mach's writings, the expression Gedankenexperiment appears in the Danish Kantian È rsted's 1811 `Prolegomenon to the Hans Christian O General Theory of Nature', and a term for experiment with thoughts ± mit Gedanken experimentieren ± can be found in a 1793 entry to German polymath Georg Christoph Lichtenberg's `Common Place Book' (cf. Lichtenberg, 1793/1983; Mach, 1897; Mach, 1905/1976; Schildknecht, 1990, pp. 147ff; Witt-Hansen, 1976). In any case, use of the method antedated its labeling by several thousand years, having been employed by ancient and medieval philosophers and natural philosophers, and by scientists and philosophers in the early modern and contemporary

WHAT ARE THOUGHT EXPERIMENTS? To perform a thought experiment is to reason about an imaginary scenario with the aim of confirming or disconfirming some hypothesis or theory. In its original usage, the expression was reserved for cases intended to evoke intuitions about the physical world; more recently, it has also been used to refer to cases intended to evoke intuitions concerning the proper application of nearly any descriptive or evaluative concept. So, for instance, Galileo's famous refutation of the Aristotelian view that heavy bodies fall faster than light ones is a paradigmatic example of a scientific thought experiment concerning the physical world. Galileo asks his reader to imagine a heavier body strapped to a lighter one, and shows that the Aristotelian is committed to saying that the joined object will fall both faster and more slowly than the heavier body alone. By contrast, John Searle's (1980, 1984) case of the Chinese Room is a classic example of a philosophical thought experiment concerning the application of our concepts. In an effort to undermine the thesis that a suitably

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periods (for representative discussions, see Rescher, 1991; King, 1991; and other papers collected in Horowitz and Massey, 1991). After the publication of Mach's 1897 essay, the term itself seems to have taken roughly four decades to become widespread in scientific circles. (Despite his extensive reading of Mach, for instance, Einstein appears not to have used the expression in his own writings. In general, however, it is difficult to trace reliably the term's history, as later editions of works often interpolate it where it was not originally used.) Employment of the expression `thought experiment' in its philosophical sense seems to have begun sometime in the 1970s, and it was only in the last decade of the twentieth century that philosophical reference works began to include entries for the term. (For an extensive bibliography of the philosophical literature on thought experiment, see Gendler, 2000, pp. 229±250.) Given how broadly the term is used, it seems that nearly any imaginary example might reasonably be termed a `thought experiment'. As a matter of sociological fact, however, the expression tends to be reserved for cases involving a certain degree of visualization, complexity, or novelty. So, for instance, although they describe imaginary scenarios whose consideration may play some role in confirming or disconfirming some hypothesis or theory, simple examples in physics books (`a car travelling at 65 miles per hour strikes a concrete wall ¼') are rarely considered material for thought experiments, nor are their equally austere analogs in philosophy, psychology, linguistics, law, and so on.

KINDS OF THOUGHT EXPERIMENTS Although a number of taxonomies for thought experiments have been proposed, none has become canonical. Perhaps the most widely accepted distinction is between scientific and philosophical thought experiments, though these categories are rarely made precise: scientific thought experiments are simply those concerning scientific subject matter, philosophical thought experiments those concerning nonscientific subject matter (cf., for instance, Horowitz and Massey, 1991; Sorensen, 1992). A more sharply focused version of the scientific/ nonscientific distinction is made by George Bealer (1998, pp. 207±208), who distinguishes imaginary cases that are used to evoke physical intuitions from those used to evoke intuitions about the application of nonphysical concepts. The former involve asking the reader to determine what would

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happen in a given imaginary scenario assuming that natural laws are held constant; the latter involve asking the reader to decide whether a particular scenario is logically or metaphysically possible, or whether a given concept applies to such a scenario. Bealer maintains that the term `thought experiment' should be reserved for cases of the former sort, roughly the class generally referred to as `scientific thought experiments'. Tamar Szabo Gendler (2000, pp. 25±27) suggests a slightly different taxonomy, distinguishing between factive and conceptual/valuational thought experiments. Factive thought experiments are those where the question asked is naturally described as `what would happen?'; conceptual/valuational thought experiments are those where the question asked is naturally described as `how should we describe or evaluate this outcome?' Thought experiments that are factive tend to be those involving scientific subject matter; thought experiments that are conceptual/valuational tend to be those involving philosophical subject matter. James Robert Brown (1991) provides a taxonomy of scientific thought experiments that has gained some currency in certain philosophy of science circles. Brown distinguishes between destructive and constructive thought experiments, subdividing the latter category into mediative, conjectural, and direct. Destructive thought experiments are those involving imaginary examples designed to raise difficulties for a particular theory; constructive thought experiments are those aimed at establishing a positive result. Within the class of constructive thought experiments, mediative thought experiments are those which facilitate the drawing of a conclusion from a specific, well-articulated theory; conjectural thought experiments are those where thinking about an imaginary scenario causes us to consider a phenomenon for which we then provide some sort of theoretical explanation; direct thought experiments are those that directly yield a well-established theory. Thought experiments that are simultaneously destructive and directconstructive Brown calls platonic, since, he claims, they give us a priori knowledge of nature. Other taxonomies have also been proposed, though like those described above, none has gained canonical status. Nicholas Rescher (1991), for instance, distinguishes between thought experiments that are explanatory and those that are refutatory, offering further subdivisions into six more precisely articulated methods. Sarah Thomason (1991) divides thought experiments in linguistics into two categories: those that identify what sort of evidence might be conclusive in testing a

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particular theory, and those that test linguistic hypotheses by providing introspective data (these might be called `experiments-in-thought'). D. A. Anapolitanos (1991) offers a six-celled taxonomy of thought experiments in mathematics; Richard Gale (1991) distinguishes thought experiments that yield clear-cut counterexamples from those that result in undecideable cases; Allen Janis (1991) distinguishes three ways in which thought experiments in physics might fail; Roy Sorensen (1992, pp. 197±202) classifies thought experiments on the basis of whether the corresponding actual experiment is gratuitous, unaffordable, or impossible; Pierre Duhem (1914/1954, p. 202) similarly distinguishes merely unperformed experiments, experiments which could not be performed with precision, physically unperformable experiments, and absurd experiments; and SoÈren HaÈggqvist (1996, pp. 136±159) and Kathleen Wilkes (1988, chap. 1) each present principles for distinguishing successful from unsuccessful thought experiments.

vision. Jackson asks what would happen if Mary were released from her confinement and shown a red object: would Mary learn anything new? Jackson (1982) expects his reader to agree that the answer is `yes', and concludes that what Mary has learned when she has learned what it is like to see red is a nonphysical fact.

Parfit's Fission Case

In the cognitive science literature, the term `thought experiment' is generally used to refer to some widely discussed imaginary case designed to evoke intuitions about the proper application of a concept such as `meaning' or `consciousness'. So, for instance, among the cases generally referred to as `thought experiments' are Frank Jackson's case of Mary the Color Scientist, Derek Parfit's case of fission, Hilary Putnam's case of Twin Earth, and John Searle's case of the Chinese Room. For whatever reason, discussions of zombies and inverted spectra are less commonly referred to as `thought experiments', though slight variations on them, such as Ned Block's case of inverted Earth, generally are. Each of these cases is described briefly below, followed by a discussion of some of their common features.

In an effort to undermine the view that personal identity is what properly underlies our concern for our future continuants, Derek Parfit (1984/1987) discusses a pair of cases involving brain transplants from an individual in whom all cognitively relevant features are realized in duplicate ± once in the left half of the brain, and once in the right. In the first scenario, the left half of the original person's brain is transplanted into the body of his decerebrated identical triplet, resulting in an individual qualitatively identical to the original in all bodily and psychological characteristics, while the right half of the original brain is destroyed. In the second scenario, both the left and right halves of the brain of the original individual are transplanted, each into the decerebrated body of one of his identical triplets, resulting in two individuals each qualitatively identical to the original in all bodily and psychological characteristics. Parfit suggests that the relation between the original individual and his successor in the first case is a relation of personal identity, and a fortiori is sufficient to render his prudential concern for that continuer rational. In the second case, the relation between the original individual and each of his two continuers is intrinsically identical to that in the first case; hence, contends Parfit, it is sufficient to render prudential concern for each of them rational. But a relation of identity does not hold between the original individual and both of his two continuers (since identity is a one±one relation). So, concludes Parfit, identity is not what matters in making prudential concern rational.

Jackson's Mary

Putnam's Twin Earth

In an effort to undermine the view that all facts are physical facts, Frank Jackson (1982) presents the example of Mary, a person who has never had color experiences, having been confined all her life to a black and white room and denied all access to color-involving visual stimuli. Mary is also a brilliant scientist who specializes in the neurophysiology of vision, and who knows all physical facts (including all neurological facts) about color

In an effort to show that an individual's social or physical environment is partly determinative or constitutive of that individual's mental states, Hilary Putnam (1975) presents the example of Twin Earth, a planet identical to Earth in all respects but one: the substance that plays the macro-role of Earthly water is not H2O, but a substance with a different chemical structure that Putnam calls XYZ. Putnam imagines two individuals:

USES OF THOUGHT EXPERIMENTS IN PHILOSOPHICAL COGNITIVE SCIENCE

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one on Earth named Oscar, and his molecule-formolecule Twin-Earth duplicate Twin-Oscar. Putnam holds that when Oscar says water he refers to water (that is, H2O), but that when Twin-Oscar says water he refers to twin-water (that is, XYZ). So, concludes Putnam, reference is at least partly determined by physical environment. (See also Burge, 1979 for a number of parallel cases). (See Externalism)

Searle's Chinese Room See the description in opening section.

Zombies In an effort to bring out certain issues related to the nature of conscious experience and the plausibility of physicalism, numerous philosophers have discussed the case of zombies, beings molecule-formolecule identical to human beings but who lack all conscious experience (cf. Kirk, 1974; Dennett, 1991; Chalmers, 1996). On the basis of such cases, some have concluded that consciousness cannot be fully explained in physical terms.

Inverted Spectrum and Inverted Earth In an effort to illuminate various issues relating to the status of qualia, materialism, behaviorism and consciousness, numerous philosophers have employed a case first introduced by John Locke (1689/1975 at II:XXXII:15). In its simplest form, the Inverted Spectrum example hypothesizes an individual whose visual experience on seeing, say, yellow is qualitatively identical to the visual experience of a normal person seeing, say, blue. Variations on the case abound. For instance, in arguing against certain representationalist and functionalist accounts of qualia, Ned Block (1990) introduces the example of Inverted Earth, a planet whose colors are inverted, so that grass on Inverted Earth is red and the sky on Inverted Earth is yellow. A person is transported to Inverted Earth, and given color-inverting contact lenses that cause everything on Inverted Earth to appear to her to be normally colored (cf. also Shoemaker, 1982; Chalmers, 1996). (See Functionalism; Materialism; Qualia)

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described; (b) an intuition concerning the scenario is presented with the assumption that it will be endorsed, or some argument is presented for why a particular evaluation of the scenario is correct; and (c) this intuition or evaluation is then taken as a datum in understanding something about cases beyond the scenario. So, for instance, in the case of Twin Earth, the imaginary scenario described posits the existence of the planet on which something qualitatively identical to water has the chemical structure XYZ; the intuition Putnam expects the scenario to evoke is that speakers of English and speakers of Twin-English refer to something different by their use of the word water; and the larger lesson is that `meanings [or at least reference] ain't just in the head' (Putnam, 1975, p. 227, italics omitted). In the case of fission, the imaginary scenario posits a pair of cases where the relations between the earlier and later individual(s) are qualitatively indistinguishable, but differ in their identity properties; Parfit's arguments aim to show that this gives us a case where prudential concern for a nonidentical continuer is rational; the larger lesson is that `personal identity is not what matters' (Parfit 1984/1987, p. 255, italics omitted). Challenges to particular thought experiments may come at any of these three levels: (a0 ) incoherence criticisms: the scenario described is in some sense incoherent; (b0 ) misleading intuition/ unsound argument criticisms: although the scenario described is coherent, the intuition it generates is unreliable or the argument establishing the correct evaluation of the scenario is unsound; or (c0 ) inapplicability criticisms: although the scenario described is coherent and the evaluation of the scenario correct, the conclusion drawn on its basis is mistaken. So, for example, some have argued (a0 ) that fission is biologically or physically or conceptually impossible; others (b0 ) that though the scenario described is coherent, it does not present us with a case where someone would bear a relation of rational prudential concern to a nonidentical continuer; and others (c0 ) that though the scenario presents a case where someone would bear a relation of rational prudential concern to a nonidentical continuer, this does not show that identity is not what matters for rational prudential concern in ordinary cases.

Discussion of Common Features of the Above

PHILOSOPHICAL AND EMPIRICAL THEORIES OF THOUGHT EXPERIMENTAL COGNITION

Appellation notwithstanding, such cases tend to share the following features: (a) a scenario is

Perhaps the most perplexing question raised by the technique of thought experiment is the epistemic

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puzzle articulated sharply by Thomas Kuhn: `How, relying exclusively on familiar data, can a thought experiment lead to new knowledge?' (Kuhn, 1964/ 1977, p. 241). The question can be broken in two: (a) how can thought experiments lead to beliefs that are properly classified as new? (b) how can thought experiments lead to beliefs that are properly classified as knowledge? Classic rationalist discussions answer both questions simultaneously by suggesting that in certain cases, thought experimental reasoning can lead to rational insight and thereby give access to a priori truths (for a modern defence see Brown, 1991). Classic empiricist answers, such as Mach's, suggest that thought experiments provide access to unsystematized empirical knowledge itself acquired through experience or evolution. The justification for beliefs formed thereby is thus parasitic on the basic knowledge; their novelty is a consequence of its having been previously unavailable in propositional form. Kuhn's own answer is that thought experiments work by forcing a simultaneous rethinking of conceptual structures and the information they contain, and in this way are able to yield beliefs that are both novel and justified. Recent discussions of thought experimental cognition have tended to focus on whether the structured contemplation of imaginary examples produces distinctive sorts of cognitive access, rendering thought experiment epistemically indispensable. In a series of articles, John Norton (1991, 1996) has argued against this position, defending instead the view that thought experiments are arguments of a certain sort. Norton's view has been widely discussed and criticized by those who, following Mach (1905/1976, 1933/1960), hold that at least some knowledge accessed by thought experiment is nonpropositional or nonconceptual, and that contemplation of imaginary cases gives us access to that knowledge in a way that argument alone cannot (cf. Arthur, 1999; Brown, 1991; Gendler, 2000, chap. 2). Some, such as Nancy Nersessian (1993) and Nenad Miscevic (1992), have tried to make Mach's notion more precise by assimilating the technique of thought experiment to recent psychological work on mental modeling. Others, such as Daniel Dennett (1984), have suggested that many philosophical thought experiments are best understood as `intuition pumps'. Yet others stress the parallels between thought experiments and actual experiments, contending that similar explanations can be offered for the utility of each (cf. Sorensen, 1992; Gooding, 1992).

CONTROVERSIES AND ISSUES REGARDING THE USE OF THOUGHT EXPERIMENTS Controversies concerning thought experiments can be divided into two main categories: controversies about the standard interpretations of particular thought experiments, and controversies about the utility of the methodology itself. Even those whose concern is with the methodology itself, however, tend to be opposed to the use of far-fetched examples, rather than to the technique of reasoning about imaginary cases as such. Controversies about particular thought experiments are generally expressions of substantive philosophical disagreements. For instance, debates about what, if anything, Mary learns when she leaves the black-and-white room; about whether the person locked in the Chinese Room understands Chinese and if not, what that shows; about whether zombies are negatively conceivable (not a priori incoherent) or positively conceivable (verified by a clearly and distinctly conceivable scenario), and if so what that implies about the status of physicalism or the nature of consciousness ± each involves conducting a significant philosophical debate primarily through discussion of a particular imaginary case. Similarly, debates about the proper understanding of particular scientific thought experiments ± for instance, Einstein and Bohr's 1930 debate concerning the clock-in-the-box ± can also be understood along these lines (cf. Bohr, 1949). Occasionally, however, disagreements about a particular case are better understood as disputes about the methodology of thought experiment; this is particularly striking in debates about whether the concept of personal identity is sufficiently far-reaching to deliver reliable intuitions about fission cases. In general, uneasiness with the methodology of thought experiment tends to be focused on thought experiments involving far-fetched cases, though there are certain strands of Marxist thought that stress the importance of focusing on the actual rather than the hypothetical, and strains of moral particularism that suggest that no situation may stand in as surrogate for another (cf. Dancy, 1985). Far more typical, however, are discussions such as those of Kathleen Wilkes (1988), who expresses misgivings about the use of wildly fantastic imaginary cases in discussions of personal identity on the grounds that the intuitions they evoke are unreliable as guides to our actual conceptual commitments. W. V. O. Quine (1972) expresses similar reservations, claiming that our concepts are

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indeterminate in their application when we consider such bizarre cases. Others have offered parallel arguments from a Wittgensteinian perspective (e.g. Gale, 1991). In recent years, two other areas of related interest have begun to be explored, both of which raise concerns for the reliability of thought experiment as a methodology. As before, the implications of these investigations will need to be considered on a case-by-case basis. Following the work of psychologists such as Daniel Kahneman and Amos Tverksy, demonstrating that human reasoning is, in a wide range of cases, subject to apparently intractable cognitive illusions, a number of philosophers and psychologists have begun to consider whether intuition itself is reliable in the ways that thought experimental reasoning seems to presuppose (see, for instance, the papers collected in DePaul and Ramsey, 1998). In a related vein, though for reasons arising from a concern with the nature of modality and the relation between epistemology and metaphysics, a number of philosophers have begun to rethink the relation between conceivability and possibility (see, for instance, papers collected in Gendler and Hawthorne, forthcoming). If, as some suggest, what we can conceive (or fail to conceive) is unreliable as a guide to what is genuinely possible, or if we lack a reliable sense of what we are capable of conceiving, then reasoning about imaginary scenarios may be an ineffective means of confirming or disconfirming certain hypotheses or theories. References Anapolitanos DA (1991) Thought experiments and conceivability conditions in mathematics. In: Horowitz and Massey (eds) Thought Experiments in Science and Philosophy, pp. 87±97. Savage, MD: Rowman and Littlefield. Arthur R (1999) On thought experiments as a priori science. International Studies in the Philosophy of Science 13(3): 215±229. Bealer G (1998) Intuition and the autonomy of philosophy. In: DePaul M and Ramsey W (eds) Rethinking Intuition: The Psychology of Intuition and its Role in Philosophical Inquiry. New York: Rowman and Littlefield. Block N (1990) Inverted Earth. Philosophical Perspectives 4: 53±79. Bohr N (1949) Discussions with Einstein on epistemological problems in atomic physics. In: Schilpp PA (ed.) Albert Einstein: Philosopher-Scientist, pp. 199±242. La Salle, IL: Open Court. Brown JR (1991) The Laboratory of the Mind: Thought Experiments in the Natural Sciences. New York and London: Routledge.

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Burge T (1979) Individualism and the mental. In: French P et al. (eds) Midwest Studies in Philosophy: Studies in Metaphysics, pp. 73±122. Minneapolis, MN: University of Minnesota Press. Chalmers D (1996) The Conscious Mind. New York and Oxford, UK: Oxford University Press. Dancy J (1985) The role of imaginary cases in ethics. Pacific Philosophical Quarterly 66: 141±153. Dennett D (1984) Elbow Room. Cambridge, MA: MIT Press. Dennett D (1991) Consciousness Explained. Boston, MA: Little, Brown. DePaul MR and Ramsey W(eds) (1998) Rethinking Intuition: The Psychology of Intuition and its Role in Philosophical Inquiry. Lanham, MD: Rowman and Littlefield. Duhem P (1914/1954) The Aim and Structure of Physical Theory, translated by P Wiener. Princeton, NJ: Princeton University Press. Gale R (1991) On some pernicious thought experiments. In: Horowitz and Massey (eds) Thought Experiments in Science and Philosophy, pp. 297±304. Savage, MD: Rowman and Littlefield. Gendler TS (2000) Thought Experiment: On the Powers and Limits of Imaginary Cases. New York, NY: Garland Press. Gendler TS and Hawthorne JP (eds) (forthcoming) Imagination, Conceivability, and Possibility. Oxford, UK: Oxford University Press. Gooding DC (1992) The cognitive turn, or, why do thought experiments work? In: Giere R (ed.) Cognitive Models of Science, pp. 45±76. Minneapolis, MN: University of Minnesota Press. HaÈggqvist S (1996) Thought Experiments in Philosophy. Stockholm, Sweden: Almqvist and Wiksell. Horowitz T and Massey G (eds) (1991) Thought Experiments in Science and Philosophy. Savage, MD: Rowman and Littlefield. Janis AI (1991) Can thought experiments fail? In: Horowitz and Massey (eds) Thought Experiments in Science and Philosophy, pp. 113±118. Savage, MD: Rowman and Littlefield. King P (1991) Mediaeval thought-experiments: the metamethodology of mediaeval science. In: Horowitz and Massey (eds), pp. 43±64. Kirk R (1974) Zombies vs. materialists. Proceedings of the Aristotelian Society 48 (supplement): pp. 135±152. Kuhn T (1964/1977) A function for thought experiments. Reprinted in The Essential Tension. Chicago, IL: University of Chicago Press. Jackson F (1982) Epiphenomenal qualia. Philosophical Quarterly 32: 127±136. Lichtenberg GC (1983) Schriften und Briefe: SudelbuÈcher, Fragmente, Fabeln, Verse (Erster Band) (ed.) FH Mautner. Frankfurt, Germany: Insel Verlag. Locke J (1689/1975) An Essay Concerning Human Understanding (ed.) PH Nidditch. New York, NY: Oxford University Press. È ber Gedankenexperimente. Poskes Mach E (1897) U Zeitschrift fuÈr den physikalischen und chemischen Unterricht, January 1897, pp. 1±5.

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È ber Gedankenexperimente. Mach E (1905/1976) U Erkenntnis und Irrtum, Leipzig, 1905, pp. 183±199. Reprinted as: On thought experiment. Knowledge and Error (translation of 1926 edition of Erkenntnis und Irrtum by TJ McCormack and P Foulkes). Dordrecht, the Netherlands: Reidel, 1976, pp. 134±147. Mach E (1933/1960) The Science of Mechanics, 9th edn, translated by T McCormack. La Salle, IL: Open Court Publishers. Miscevic N (1992) Mental models and thought experiments. International Studies in the Philosophy of Science 6(3): 215±226. Nersessian N (1993) In the theoretician's laboratory: thought experiment as mental modeling. Proceedings of the Philosophy of Science Association, vol. 2, pp. 291±301. Norton J (1991) Thought experiments in Einstein's work. In: Horowitz and Massey (eds) Thought Experiments in Science and Philosophy, pp. 129±148. Savage, MD: Rowman and Littlefield. Norton J (1996) Are thought experiments just what you thought? Canadian Journal of Philosophy 26(3): 333±366. Parfit D (1984/1987) Reasons and Persons. Oxford, UK: Oxford University Press. Putnam H (1975) The Meaning of `Meaning'. Reprinted in Mind, Language and Reality: Collected Papers Volume 2, pp. 215±271. Cambridge, UK: Cambridge University Press. Quine WVO (1972) Review of Identity and Individuation. Journal of Philosophy 69(16): 488±497. Rescher N (1991) Thought experiments in pre-Socratic philosophy. In: Horowitz and Massey (eds) Thought Experiments in Science and Philosophy, pp. 31±41. Savage, MD: Rowman and Littlefield. Schildknecht C (1990) Philosophische Masken: Literarische Formen der Philosophie bei Platon, Descartes, Wolff und Lichtenberg. Stuttgart, Germany: Metzler. Searle J (1980) Minds, brains and programs. Behavioral and Brain Sciences 3: pp. 417±424. [Peer commentary, pp. 425±449; reply by Searle, pp. 450±456.

Searle J (1984) Minds, Brains and Science. Cambridge, MA: Harvard University Press. Shoemaker S (1982) The inverted spectrum. Journal of Philosophy 79(7): 357±381. Sorensen R (1992) Thought Experiments. New York and London: Oxford University Press. Thomason S (1991) Thought experiments in linguistics. In: Horowitz and Massey (eds) Thought Experiments in Science and Philosophy, pp. 247±257. Savage, MD: Rowman and Littlefield. Wilkes K (1988) Real People: Personal Identity without Thought Experiments. Oxford, UK: Clarendon Press. È rsted, Immanuel Kant, and Witt-Hansen J (1976) H.C. O the thought experiment. Danish Yearbook of Philosophy 13: 48±65.

Further Reading Bunzl M (1996) The logic of thought experiments. Synthese 106(2): 227±240. Cargile J (1987) Definitions and counterexamples. Philosophy 62: 179±193. Fodor JA (1971) On knowing what we would say. In: Rosenberg JF and Travis C (eds) Readings in the Philosophy of Language, pp. 198±212. Englewood Cliffs, NJ: Prentice-Hall. Gentner D and Stevens AL (eds) (1983) Mental Models. Hillsdale, NJ: Lawrence Erlbaum Associates. Giere R (ed.) (1992) Cognitive Models of Science. Minneapolis, MN: University of Minnesota Press. Hintikka J (1999) The emperor's new intuitions. Journal of Philosophy 96(3): 127±147. Miller FD and Smith N (eds) (1989) Thought Probes: Philosophy through Science Fiction Literature. Englewood Cliffs, NJ: Prentice-Hall. Popper K (1959) On the use and misuse of imaginary experiments, especially in quantum theory. The Logic of Scientific Discovery. New York: Basic Books.