volume two: symposia and invited papers || implications of incommensurability

Implications of IncommensurabilityAuthor(s): Philip KitcherSource: PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association,Vol. 1982, Volume Two: Symposia and Invited Papers (1982), pp. 689-703Published by: The University of Chicago Press on behalf of the Philosophy of Science AssociationStable URL: http://www.jstor.org/stable/192453 .

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Implications of Incommensurability

Philip Kitcher

University of Vermont

In 1962, Thomas Kuhn offered an account of the difficulties which attend the reception of revolutionary scientific ideas: "Communication across the revolutionary divide," he wrote, "is inevitably partial." (Kuhn 1962, p. 149). Eight years later, in the postscript to the second edition of The Structure of Scientific Revolutions, he sounded the same theme, characterizing the participants in revolutionary debates as people who attach their terms to nature differently, so that their communication is "inevitably only partial" (Kuhn 1970, p. 198). In his most recent discussion of these issues (Kuhn 1983), Kuhn has explained more clearly than before the nature of the differences in language which he takes to be present in revolutionary debates. In what follows, I shall try to put Kuhn's latest ideas into the context of his earlier work, and thereby explain my main thesis: if the Kuhn of today is right about conceptual change in science, then some of the claims made in 1962 and 1970 are incorrect, and the Kuhnian discussion of scieitific revolutions is far less radical than it initially ap- peared. Once this thesis has been explained, I shall attempt to argue for its truth. Finally, I shall try to defend the antecedent of my conditional thesis, by showing how Kuhn') present proposals about conceptual change may be further developed.

The concept of incommensurability entered the literature of philosophy of science as part of an account of how scientific revolutions occur. Kuhn, Feyerabend, and others were impressed by the fact that the episodes in which Ptolemaic astronomy gave way to Copernican astronomy, the phlogiston theory to Lavoisier's chemistry, creationist biology to Darwinian evolutionary theory, could not simply be viewed as transi- tions in which a shared body of evidence and shared canons of scientific confirmation forced the members of the scientific community to abandon the old theory in favor of the new. In one of his most provoc- ative chapters, Kuhn delineated three kinds of incommensurability and he argued that each of these three types of incommensurability gives rise to serious difficulty in the resolution of revolutionary debates.

PSA 1982, Volume 2, pp. 689-703 Copyright D 1983 by the Philosophy of Science Association



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Most fundamental is the type of incommensurability with which we are principally concerned here, conceptual incommensurability. Kuhn has claimed that differences in language divide the participants in revolutionary debates, so that they are inevitably at cross purposes. He adds to this the idea of observational incommensurability. Even if the protagonists could formulate their rival claims in a common language, they could not agree on a shared body of observational evidence against which those claims are to be judged. Finally, there is method- ological incommensurability. The methods and standards of evidence employed by the parties to a revolutionary debate are not constant. Opinions diverge about what problems are most important and about the constraints that are to govern adequate solutions. As a result, even given a common language for the formulation of rival claims and a shared body of observational evidence, reasonable disagreement could still persist.

These three types of incommensurability figure in a cumulative case for the thesis that is most often ascribed to Kuhn. Resolution of revolutions by rational means is impossible. The decision to accept a radically new theory can only be taken on faith. Because of Kuhn's talk of "conversion experiences" there are some grounds for the attri- bution, although I hasten to add that some neglected sentences in The Structure of Scientific Revolutions support fuhn's later disavowals of irrationalist views about scientific change. However, what concerns me here is not the idea that there are some scientific decisions that are inevitably insusceptible of rational reconstruction, but the early stages of the argument that has convinced many people of the correct- ness of this idea. In clarifying the notion of conceptual incommensurability, I think that Kuhn has shown it to be innocuous. When we im- port his present position back into the discussion of the resolution of revolutions, two of the difficulties alleged to attend that resolution lose much of their force. We can revert to the idea that full communication across the revolutionary divide is possible and that 4 rival claimants can appeal to a shared body of observational evidence.

Given the position taken by the Kuhn of today, the problem of the rationality of scientific change looks very different from the way it has looked to many who have read the Kuhn of 1962 or 1970. Of the three cumulative difficulties only one remains. If there is a reason to worry about the rational resolution of revolutions, it must be found in the fact that the shared evidence and the shared standards of good science--that "set of commitments without which no man is a scientist" (Kuhn 1970, p. 42)--do not together suffice to determine a decision for the scientific community. Kuhn's discussion of decision-making in science thus identifies a type of problem with which philosophers of science are all too familiar, the general problem of the underdetermi- nation of theory by evidence. The problems are not quite the same, for, as Kuhn acutely notes, the typical scientific situation is not one in which we are asked to judge the merits of rival doctrines, both of which fit all the available evidence, but one in which we are asked to appraise two theories each of which accords only imperfectly with the



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data. The task for a proponent of the idea that revolutionary debates can be rationally resolved thus reduces to that of articulating a theory of the reckoning of costs and benefits, of potential problems and exciting prospects. This is no small task, but it is simpler than the talk of observational and conceptual incommensurability might have led us to expect.5

So far I have only tried to explain my central claim. When the doctrine of conceptual incommensurability is carefully probed--as Kuhn has probed it in his paper--the Kuhnian discussion of the resolution of revolutions seems to pose much less of a threat to orthodox empiricism than it did in its original version. I shall now try to defend this claim, showing first how Kuhn's current views allow for communication across the revolutionary divide and for shared observational evidence, and then concluding my remarks with an attempt to articulate further what I take to be Kuhn's current approach to conceptual incommensurability.

At the heart of Kuhn's paper is a distinction between translation and interpretation. Translation of a passage of an alien tongue occurs when one "tells the same story" in sentences of the home language. In constructing the translation, the home language remains unmodified: its stock of expressions is unaltered and the semantic features of old expressions (in particular the ways in which the referents of those expressions are determined) persist unchanged.6 By contrast, in interpretation, one gains access to the content of statements in the alien language by extending the resources of the home language, so that parts of the alien discourse are accommodated within the home language. Kuhn argues that the languages used by the rival protagonists in revolutionary debates are not intertranslatable, even though the user of one language can interpret the remarks made by the rival. He attempts to show that the proposals that have been offered for constructing translation manuals between such languages as the language of phlogistic chemistry and the language of Lavoisier's chemistry do not yield trans- lations. Although Lavoisier can interpret Priestley, he cannot trans- late Priestley's claims into his own idiom. Those who have tried to show how the translation might proceed have been guilty of confusing interpretation with translation.

As one of those so charged, I plead guilty. But I do not think that the offense is grave. Whether or not the scheme I offered (Kitcher 1978) for linking the languages of phlogistic chemistry and oxygen chemistry counts as a translation manual, it still serves as a vehicle by means of which Lavoisier and Priestley can gain access to one another's proposals. It still shows that communication--full communication--across the revolutionary divide is possible. In the paper which Kuhn discusses, (Kitcher 1978), I suggested that, from the perspective of oxygen chemistry, different tokens of key terms used by phlogiston theorists refer differently. 'Phlogiston' sometimes refers to hydrogen, but more typically, fails to refer. 'Dephlogisticated air' sometimes has its reference fixed as the substance which results when phlo-



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giston is removed from the air, and in these cases the token of 'dephlogisticated air' fails to refer. But other tokens of 'dephlogisticated air' refer to oxygen. Phlogiston chemists have false beliefs which dispose them to use the same term type in what, from the perspective of the oxygen theory, are radically different ways. If their ideas are to be rendered by an oxygen chemist then the differences in the semantic features of different tokens of the same type must be recognized.

One way to try to recognize these differences would be to try to construct a context-sensitive translation. Given a passage in the language of the phlogiston theory, we would endeavor to replace tokens of the difficult terms with coreferential tokens in the language of the oxygen theory, bearing in mind that we might have to treat differently different tokens of the same type. Some occurrences of 'dephlogisticated air' would give way to occurrences of 'the substance that remains when the substance emitted in combustion is removed from the air'; others would give way to occurrences of 'oxygen'. Kuhn thinks that the result would not meet the standards of adequate translation. The main problem, quite evidently, is that, in replacing different tokens of the same (phlogistic) type with different expressions of the language of oxygen chemistry, we lose sight of the connection among the referents of those tokens which the language of the phlogiston theory takes for granted. Two points should be made here. First, for some scientific purposes, the loss of that connection can be accepted with equanimity. When Priestley talks lyrically about the way in which some laboratory mice frolicked in dephlogisticated air, neither we nor Lavoisier omit anything crucial by rendering his observations as remarks about the survival of the animals in oxygen. In this context, we can ignore Priestley's mistaken theoretical account about how what we call "oxygen" is formed.7 Second, even if we concede to Kuhn that the attempted translation does not satisfy the canons that govern translation, the scheme that generated it still provides a vehicle for entering Priest- ley's language. Assume that we abandon the idea of translating Priest- ley, adopting Kuhn's suggestion that 'phlogiston' and compound terms containing 'phlogiston' (or one of its cognates) be added to the home language. Then our specification of the referents of the tokens of these terms that occur in the passage we wish to understand can serve as a prefatory gloss to the passage in question, a gloss which renders that passage comprehensible.

Let us now imagine Lavoisier reading a work of phlogistic chemistry. Recognizing that different tokens of crucial terms may refer differently, he constructs the context-sensitive specification of referents that I would favor. When he reads that "Dephlogisticated air supports combustion longer than ordinary air" he is on guard. But he can disentangle Priestley's remark, identifying a part with which he agrees and a part with which he disagrees. Context convinces him that Priestley's current token of 'dephlogisticated air' refers to oxygen.8 Thus Priestley has enunciated the truth that oxygen supports combustion longer than ordinary air. However, the use of the term 'dephlogisti-



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cated air' signals a difference of belief. Priestley believes that the substance on which he performed his combustion experiment--the substance Lavoisier calls "oxygen"--was obtained by removing from the air the substance that is emitted in combustion. Lavoisier disagrees with this account because he claims that there is no substance that is emitted in combustion.

Given this scenario, I do not see what problems of communication attend this particular exchange in this particular revolution. Moreover, the historical record suggests that something close to this scenario may well have occurred. In the writings of Cavendish and Lavoisier we find passages which reveal sensitivity to differences of usage and which set out guidelines for identifying the referents of terms used by the other side.9 Yet it may be thought that my example is atypical. Perhaps other exchanges in this, or in other, revolutions will not lend themselves to the treatment that I have offered.

My own investigation of other cases, specifically those of Bturidan's term 'impetus' and of the twentieth century term 'gene', suggest to me that the semantic phenomena I find in the language of the phlogiston theory are not peculiar to that case. However, some of Kuhn's remarks suggest that there is a special source of difficulty for the interpretative enterprise. He claims that some terms of past scientific languages "constitute an interrelated or interdefined set that must be acquired together, as a whole before any of them can be used, applied to rnatural phenomena." (Kuhn 1983, p. 676). He cites as examples the terms 'element' and 'principle' from the language of phlogistic chemistry, and Newton's terms 'force' and 'mass'. Do these examples show that the procedure of interpretation I have outlined will not always be applicable?

I think that they do not. Notice first that the endeavor of commu- nicating across the revolutionary divide, as I have described it, re- quires the user of one language to be able to specify the referents of tokens used by the other. The questions whether Newton's terms 'force' and 'mass' must be acquired together, or whether Einstein's terms 'force' and 'mass' must be acquired together, are irrelevant to this endeavor. What is at issue is whether the user of the language of Einsteinian physics can specify the referents of tokens of 'mass' and 'force' as used in the language of Newtonian physics. Or, to switch to a less complicated example,10 the issue is whether Lavoisier and Priestley can specify the referents of one another's tokens of 'element' and 'principle'.

A look at the writings of those involved in the chemical revolution reveals that this case is no different from that surrounding the use of 'phlogiston' and its cognates. Lavoisier is clearly able to recognize that, from his perspective, Priestley sometimes counts as different elements different states of aggregation of the same substance, but that, at other times Priestley uses 'element' to refer to the set of things that Lavoisier would count as elements, to wit, indecomposable



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substances. 1 Some tokens of 'principle' are intended to refer to entities which are assumed to modify the elementary substances out of which bodies are composed, thereby enabling these bodies to take on special properties, and these tokens fail to refer. Other tokens of 'principle' refer to a nonempty set of substances, including such things as hydrogen, mistakenly identified as phlogiston, the "principle of combustion". To be sure, the situation with respect to the terms 'element' and 'principle' is slightly more complicated than the case of 'phlogiston' and related terms, in that there are more connections-- mistaken connections, from Lavoisier's point of view--that a phlogistic chemist is disposed to make. Nonetheless, the fundamental ingredients are the same. I conclude that Kuhn has offered no reason for thinking that the interpretative strategy I have outlined will sometimes break down.12

I have been arguing that if the Kuhrn of today is right about the phenomenon of incominensurability of concepts, then the rational resolution of revolutions is much less problematic than it has often been taken to be.13 In the rest of this paper, I want to defend the antecedent of this conditional: I shall try to show how Kuhn is right in claiming that the conceptual incommensurability that occurs in the history of science is best understood by recognizing how different lanl- guages divide the same world differently.

Kuhn uses his distinction between translation and interpretation to build a case for the idea that our understanding of linguistic change in science must attend to more than merely the shifts of reference. He claims that if we are to understand the differences between the languages scientists use at different times, then we must recognize changes not only in reference but in the ways in which reference is fixed.14 So far, Kuhn seems to me to be completely correct, and I diverge from him only in separating the idea of the way in which the reference of a term is fixed from the idea of a technique (or criterion) which a speaker uses to fix the reference of a term.15

At the bottom of Kuhn's latest approach to the topic of incommensurability is a simple and conmpelling idea. When we look at the language in use among scientists at a particular time, we may find that for some important expression types there is a variety of ways in which the reference of tokens of those types can be fixed, and that the varied em- ployment of tokens of these types presupposes connections that later scientists will reject. So, from the perspective of the scientific language in use at later times, the former usage of the key terms will be mistaken, and there will be no term in the later language which is used in the same variety of ways as the old expressions.

One way to articulate this idea would be to suppose that the reference of the expressions in question is always fixed by description. On this view, incommensurability arises when different descriptions are used to fix the reference of the terms in the old language, different descriptions which the users of the old language take to be equivalent



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but which are subsequently believed to be nonequivalent. But there are good reasons, advanced by Kripke, Putnam, and Donnellan (Kripke 1972, Putnam 1975, Donnellan 1970), to believe that this is not a generally correct account of the fixing of reference. Nevertheless, we can ac- commodate the arguments of Kripke et al. while still doing justice to Kuhn's compelling idea.

Instead of thinking of the referent of a token of a term as fixed in virtue of the speaker's present disposition to supply a descriptive characterization of the referent of the term, we can view reference as fixed through the causal history that leads up to the production of the token. On most occasions of speech our dominant intention is to conform to the linguistic practices of our fellows, and the referents of our tokens are fixed through causal chains that lead back to earlier events in which some expression was attached to nature either through ostension or through explicit description. I suggest that a general way to understand the ways in which terms have their reference fixed, and so to do justice to Kuhn's idea of incommensurability, is to asso- ciate with every token an event, which initiates the causal chain cul- minating in the production of the token and through which the referent of the token is determined. Some initiating events are events in which the referent of an expression is fixed by giving an explicit description. Others are events in which a term is attached to a kind or thing present on that occasion. The idea of a variety of ways in which the referent of a crucial scientific term can be fixed can now be understood as follows. We are to think of a term (a term type), as being associated at a given time with a set of events, the events that could initiate the production of tokens of the term by the people who use the term at that time. Kuhn's fundamental intuition can then be glossed by saying that the set of events associated with a term type varies over time, that it does so in ways that change the reference of the term because certain connections among events that were formerly accepted are later explicitly discarded.

We can make the discussion a little more precise by using some terminology that I have introduced elsewhere. Let us say that the reference potential of a term type for a scientific community at a particular time is the set of events that could initiate the tokens of that type produced by members of the community at that time.16 Scien- tific terms often have complex reference potentials, that is, there are a number of different events that can initiate production of tokens of the terms in question. From the perspective of the users of the terms, the multiplicity of initiating events is likely to seem unproblematic (even if it should be explicitly recognized). Some of the initiating events are observations or experiments involving a particular kind or thing, and the event serves to fix reference to that kind or to that thing. Others are events in which an object, or set of objects, is singled out by description. For the users of the term, these events are taken to identify the same referent. They believe that the same kind or thing is present on all the occasions of observation and ex-



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periment, that the same kind or thing is singled out by all the descriptions. Let us call these beliefs, whether or not they are explicitly formulated, the theoretical presuppositions of the term. Later theorists may well want to reject the theoretical presuppositions formerly associated with a term, because, from their perspective, different entities are referred to by different tokens of the same type. They will modify scientific language so that the different referents are explicitly differentiated, and there will be no expression in the new language whose reference potential matches that of the old term. Incommensurability lies precisely in the mismatch of reference potentials. More exactly, I shall say that a language L is incommensurable with a language L with respect to a term t just in case there is no expression in L' whose reference potential is the same as the reference potential of the term t in L, and where this situation results from the fact that the users of L' reject one or more of the theoretical presuppositions of the term t in L. I hope that this definition provides a development of Kuhn's idea that incommensurable languages divide the world differently.17

Two examples may help both to clarify this account and to show how it applies to the historical episodes which have moved Kuhn (and others) to talk of incommensurability. Consider Priestley's term 'dephlogisticated air'. Many tokens of this term produced by Priestley and other phlogistonians have their referents fixed through a causal chain initiated by the event in which Stahl explicitly specified phlogiston as the substance emitted in combustion. These tokens fail to refer. But, after Priestley had isolated oxygen and misidentified it, many subsequent tokens of 'dephlogisticated air' had their reference fixed through causal chains initiated by encounters with oxygen. Those tokens refer to oxygen. Phlogistonians engaged in linguistic practices which enabled them to produce tokens of 'dephlogisticated air' initiated in these two different ways because they were confident that the gas isolated from the red substance obtained by burning mercury was dephlogisticated air. From Lavoisier's perspective this is quite wrong, and Lavoisier's language contains no expression with the reference potential of the phlogistonian term 'dephlogisticated air'.

A more complicated example concerns the use of the term 'gene' between 1915 and 1955 (see my 1982 for more details). During this peri- od, the referent of 'gene' was sometimes fixed by one of a wide variety of descriptions, sometimes fixed through particular breeding experiments. One explicit specification, used by Sturtevant and other members of the Morgan group, takes genes to be chromosomal segments which can be separated by recombination in breeding experiments. On the basis of this criterion and some experimental results, Sturtevant sup- posed that there are multiple alleles at the white/eosin locus in Drosophila, since the results he used failed to show segregation among the chromosomal segments involved. These experiments served as a para- digm for some later discussions of multiple allelism, and the tokens of 'gene' produced in these discussions have their referents fixed through the original experiments: the eosin allele constitutes a single



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gene. But, as we now recognize, the eosin allele does not meet Sturte- vant's official recombination criterion, so that, from the perspective of post-Benzerian genetics, the referents of some earlier tokens of 'gene' are fixed to distinct entities. This is one of the many reasons why the language of genetics developed in the 1950s is incommensurable with earlier languages with respect to the term 'gene'.

On the account that I have sketched, it is possible to do justice to the phenomena of conceptual change in science, while recognizing the insights of recent philosophical work on the topic of reference. There is no need to suppose that the users of a term must always have some reference-fixing technique which they can always trot out to identify what they are talking about. They produce tokens in ways that reflect their general beliefs about what situations involve the same entities and what descriptions pick out the same things. These beliefs need not be formulated explicitly. Speakers operate under some very general intentions: to conform to the usage of their fellows, to refer to the genuine kinds that exist in nature, and sometimes to refer to what satisfies a particular description. On different occasions of usage, different intentions predominate, and according to the dominant intention and as the facts about the world external to the speaker dictate, so the reference of the token produced is determined. When we look to the language of past science, we sometimes find our predecessors refer- ring to the kinds that we identify--oxygen, say, or cistrons--because we understand them as intending to refer to the kinds that they have encountered, irrespective of their beliefs about membership in these kinds. But it is not always so, and there are occasions on which we see their tokens as produced in accordance with an intention to refer to that which meets a particular description. When the kinds we recog- nizecut across the sets marked out by the descriptions, incommensurability enters.

It should now be clear why conceptual incommensurability is so common. Scientists constantly attempt to give descriptive specifications of what they are talking about, and, when these specifications seem successful, they take on the role of sometimes determining the referent. So apparently unproblematic but faulty connections become built in to linguistic usage. The development of the concept of the gene in the twentieth century reveals this phenomenon on a large scale. That development also shows that conceptual incommensurability is not con- fined to those episodes in the history of science that Kuhn has hailed as revolutions. If there is indeed a sharp division between those scientific changes that constitute revolutions and those that do not, then conceptual incommensurability does not furnish us with a criterion for drawing it.18

Finally, my articulation of the notion of conceptual incommensurability shows why this phenomenon is epistemologically innocuous. Even when matters go awry, even when the assumed connections among the events in the reference potential of an expression are faulty, later workers can recognize that this is so, and, by attending to context,



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they can disentangle what their predecessors are saying. Oxygen theorists can preface their discussions with glosses that identify the reference potential of Priestley's term 'dephlogisticated air', and they can pin down the referent of each token of the term that occurs in a phlogistic text. Thus full communication across the revolutionary divide may be achieved. There are no devastating implications of incommensurability. There are no "thoughts which lie too deep for words."19

Notes

1 As Mary Hesse rightly reminded me in her comments at the symposium

at which the papers printed here were originally given, Kuhn's discussion of scientific revolutions changed the character of philosophy of science. If it should turn out (as Hesse does not in fact concede) that the type of empiricist approach recommended in my discussion can be coherently elaborated and defended, we should not forget that Kuhn's work has profoundly affected the character of the empiricist framework within which the account of the growth of scientific knowledge is to be given. I am very grateful to Hesse for making this point explicit. In arguing that some of Kuhn's challenges to empiricism can be met, I do not mean to lose sight of the fact that those challenges have been enormously important to the development of a more sophisticated empiricist philosophy of science.

2 Kuhn has been so often and so badly misinterpreted by critics and

would-be allies, that any commentator on a new paper of his is bound to be concerned about beginning a new tradition of misrepresentation. My aim in this paper is to discuss a number of theses, at least some of which are not Kuhn's but only attributed to Kuhn (either by well- wishers or by opponents). Those which I know to be claims which Kuhn is popularly believed to believe--but which he does not in fact believe--will not be attributed to him; however, it is possible that, in some cases, I have ascribed to Kuhn a doctrine that he does not hold; if so, I hope that his reply to these comments will clarify the issue.

3See, for example, the explicit references to the role of arguments in revolutionary debates in (Kuhn 1970, pp. 155, 157, 158). The last paragraph on p. 158 is especially revealing, as is the whole of (Kuhn 1977)-

I shall not have the space to defend this latter claim. It is a relatively simple corollary of an adequate understanding of conceptual change.

5Of course, several papers by Imre Lakatos (collected in Lakatos 1978) and Larry Laudan's (1977) address the problem of the rationality of scientific change in much the way that I have presented it. But I think that it is worth noting that, when the irrelevant complications



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due to conceptual and observational incommensurability have been cleared away, traditional philosophical discussions of theory under- determination become relevant to this problem.

I draw this condition for linguistic stability from (Kuhn 1983, p. 672), where Kuhn suggests that translation does not alter "the way in which ... referents ... are determined." However, as I suggest below, (footnote 14), I am no longer confident about the emphasis that Kuhn wants to place on this condition.

Kuhn evidently disagrees. He charges me (1983, p. 685) with wrongly supposing that some of Priestley's claims were true, others false. I see no reason to believe that this is an error. Given our ability to specify the referents of the tokens of Priestley's words, we are able to identify the truth or falsehood of his sentence-tokens. Success with reference generates success with truth. By contrast, I have no idea what it would be to evaluate Priestley's theory "as a whole", in the way that Kuhn apparently recommends. (It seems to me that Priest- ley was partly right, and partly wrong--and that we can say where he was right and where he was wrong.)

8The contextual clues might include Priestley's description of how the substance under investigation was prepared.

See Cavendish (1784-85) especially pp. 35-38, and Lavoisier (1777).

10The case of 'mass' is more tricky because it is controversial whether Newton's usage of this term really cuts across Einsteinian usage in the way that Kuhn (and others) has (have) suggested. In an extremely interesting paper, (Field 1973), Hartry Field suggested that Kuhn's claims about the Newtonian and Einsteinian terms 'mass' could be reconstructed using the semantical concept of partial denotation. Newton's term 'mass' partially denotes proper mass and partially denotes relativistic mass. John Earman (1977) countered by arguing that four-dimensional reformulations of Newtonian theory suggest that Newton's term 'mass' fully refers to proper mass. If Earman is right, then this particular case does not furnish an example of incommensurability, and neither the approach adopted by Field nor the account that would parallel my treatment of Priestley's language is needed to cope with it. I should add that my general approach to the reference of scientific terms allows for cases in which a token partially denotes two different referents, but I know of no cases that require Field's semantic apparatus.

11 There are complications here. Although Lavoisier thinks of ele-

ments as indecomposable substances, he sometimes adds an extra condition. Elements are substances that largely determine the properties of the bodies in which they occur. This is a concession to the chemical tradition, a tradition that includes Priestley.

It is possible that some cases require that the referents of some



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tokens of some terms be specified by something analogous to David Lewis's procedure for defining theoretical terms. That is, a user of L picks out a Ramsey sentence which represents the theory of the speaker of L' and, on this basis, specifies the referents of the terms of L-. But it is important to note that the "observational terms" figuring in the Ramsey sentence may be any nonlogical expressions of L, and that this is sufficient for the Lewis procedure (given in Lewis 1970) to provide the user of L with specifications of the referents of the terms of L'. Hence I don't think that there is much basis for Kuhn's worry about nonexistence of unique realizations, and I believe that his objection to the use of the Lewis procedure reduces to the complaint that it will not allow for the simple kinds of substitutions required for translation. If this is correct then it merely reinforces the main point I have been making: even if there are not procedures for translation, there are techniques of successful interpretation, and this is what is significant.

As already noted, (ft. 4), I think that the fact that observational incommensurability is innocuous is a straightforward corollary of an adequate understanding of conceptual incommensurability.

14This interpretative claim was prompted by the original version of the paper. There, Kuhn laid much more emphasis on modes of determining reference. Since I believe that this approach is fruitful for understanding conceptual change in science, I have attempted to articulate it in the discussion that follows. However, in his comments at the symposium, and in the final version of the paper, Kuhn seems to hanker after a notion of incommensurability which is different from that which I try to reconstruct. My position is thus best read as a disjunction: if Kuhn is content with the concept of incommensurability introduced in the text, then I think he is right, that the concept applies to a phenomenon of genuine interest that is found repeatedly in the history of science--but I think that phenomenon belies the radical claims that others have drawn from his discussions of it; if he wants more, then I think he is wrong, for the reason that there is nothing more to be had. Hence my future attributions to Kuhn can be viewed as an attempt at charity--an attempt that he may want to reject as misguided.

15 Again, although this view of the fixing of reference is suggested

by phraseology that Kuhn uses in many of his writings (including the original version of the present paper) I am not sure that Kuhn current- ly holds it. However, the issue needs to be dealt with explicitly because the view is prevalent in discussions of conceptual change in science.

A more detailed account is given in my (1978) and (1982). 17

The definition that I have given acknowledges a difference between two ways in which mismatch of reference potential can arise. Refine- ments of reference-fixing technique occur when new initiating events are added to the reference potential of an old term. These do not pro-



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duce incommensurability because no theoretical presupposition of the old term is rejected.

In (Kuhn 1962, 1970) it seemed that conceptual incommensurability would be found only between languages separated by a revolution. Un- less the use of 'revolution' is radically extended, this will not be so. Episodes which I think of as relatively small advances in genetics led to the type of incommensurability I have delineated. In any case, I suspect that Kuhn is no longer wedded to the enterprise of distin- guishing sharply between revolutions and normal scientific changes and that, even if he were, he would not want to use conceptual incommensurability as a criterion of revolutionary change.

I am very grateful to Thomas Kuhn for his patience in attempting to explain his views on this issue to me on numerous occasions. I hope that I have not misunderstood him, but, if I have, then I trust that my misinterpretation may give him a way to place his own ideas in sharper relief.



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