Kuhn, Incommensurability, and Cognitive Science Peter Barker University of Oklahoma and Danish Institute for Advanced Studies in the Humanities, Copenhagen. This paper continues my application of theories of concepts developed in cogni- tive psychology to clarify issues in Kuhn’s mature account of scientiªc change. I argue that incommensurability is typically neither global nor total, and that the corresponding form of scientiªc change occurs incrementally. Incom- mensurability can now be seen as a local phenomenon restricted to particular points in a conceptual framework represented by a set of nodes. The unaffected parts in the framework constitute the basis for continued communication be- tween the communities supporting alternative structures. The importance of a node is a measure of the severity of incommensurability introduced by replac- ing it. Such replacements occur incrementally so that changes like that from the conceptual structure of Aristotelian celestial physics to the conceptual structure of Newtonian celestial physics occur in small stages over time, and for each change it is in principle possible to identify the arguments and evi- dence that led historical actors to make the revisions. Thus the process of scientiªc change is a rational one, even when its beginning and end points are incommensurable conceptual structures. It is also apparent, from a detailed examination of the conceptual structure of astronomy at the time of Coperni- cus, that the kind of conceptual difªculty identiªed as incommensurability may occur within a single scientiªc tradition as well as between two rival traditions. An earlier version of this paper was presented at the conference “Kuhn Reconsidered”, at Virginia Tech, Blacksburg, VA, in March 2000. I would like to thank Hanne Andersen, Roger Ariew, Xiang Chen, Daniel Garber, Bernard R. Goldstein, and Stephen Wagner for comments on the present version. I would also like to express my appreciation for the sup- port of the University of Oklahoma’s sabbatical leave program, the Danish Institute for Advanced Studies in the Humanities, the University of Copenhagen, and Danmarks Nationalbank. Perspectives on Science 2001, vol. 9, no. 4 ©2002 by The Massachusetts Institute of Technology 433 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/106361401760375820 by guest on 25 September 2021 434 Kuhn, Incommensurability, and Cognitive Science 1. Introduction Kuhn’s philosophical account of scientiªc change met different responses from philosophers and non-philosophers. The concept of a paradigm rap- idly entered everyday speech, although the most important technical de- tails of the account that supported it, for example the concept of incommensurability, were passed over. Within the philosophical commu- nity the concept of incommensurability may have been the main barrier to the general acceptance of Kuhn’s work. Philosophers have consistently read the concept of a paradigm, and particularly the concept of incom- mensurability, as imposing a discontinuous, non-rational pattern on the history of science. In response to these criticisms, Kuhn radically revised his original claims about incommensurability, although these revisions never reached as wide an audience as his original work. At the same time, beginning in the Postscript to Structure of Scientiªc Revolutions (1970), Kuhn developed a nonstandard account of human concepts. The only ma- jor philosopher who shared such an account was Wittgenstein—one of Kuhn’s main inºuences (Barker, Chen and Andersen 2003). Wittgenstein’s account of concepts—the so-called family resemblance theory—has been even less popular with philosophers than Kuhn’s ac- count of science (Baker and Hacker 1980, pp. 320–343). Yet, at the same time that the philosophical world was ªrst rejecting Kuhn’s original work and then ignoring his revisions of it, an enormously inºuential movement in cognitive psychology and cognitive science was establishing a new con- sensus on the nature of human conceptual systems that directly supported Wittgenstein’s and Kuhn’s theories. The main features of this account, as it was developed by Kuhn, are that concepts are acquired by learning sim- ilarity and dissimilarity relations through ostension. This process estab- lishes classes that ªt the pattern of family resemblance introduced by Wittgenstein. The most important difference between this view and most commonly held philosophical accounts of concepts is the impossibility of deªning or analyzing concepts through a set of necessary and sufªcient conditions invoking other concepts. Most philosophers take it for granted that a concept only describable in family resemblance terms is defective, and that the preferred form of concept is one for which an exact deªnition in terms of necessary and sufªcient conditions can be speciªed. The dem- onstration by cognitive scientists that human beings cannot and do not use concepts in the way philosophers expected is a piece of bad news that has yet to be generally assimilated by the philosophical community. Kuhn’s work, however, and the extensions of it made possible through the application of methods from cognitive science, shows how scientiªc change and conceptual revision can take place in a world where family re- semblance concepts are the usual case. Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/106361401760375820 by guest on 25 September 2021 Perspectives on Science 435 Empirical research on human concepts in diverse populations as far apart geographically and culturally as the United States and New Guinea has revealed certain common factors in all human conceptual structures (Rosch 1973a, b; Rosch and Mervis 1975). In all cases, and even for the lowest level objects classiªed in terms of a given conceptual structure, ob- jects falling under concepts are graded as better or worse instances of the concept. The existence of this phenomenon, called the ‘graded structure’ of the concept, and its universality are the strongest arguments against the traditional philosophical analysis of concepts using necessary and sufªcient conditions. If it were possible to analyze concepts by means of necessary and sufªcient conditions, any and all of the subordinate concepts appearing in the analysis would have to be features of every object falling under the concept analyzed. Given no other basis for classifying objects at the lowest level in a conceptual scheme, all such objects would have to be treated as equally good examples of the concept. But empirical research across a wide range of cultures shows that all human concepts display graded structure. So whatever it is we do when we are using concepts, it is not simply reducible to operations by means of a list of necessary and sufªcient conditions corresponding to each concept (Barsalou 1993). In philosophy, and now also in psychology, it has become customary to identify concepts and the objects they designate as forming a family re- semblance class when they vary in the fashion described empirically as a graded structure (Rosch and Mervis 1975). The empirical conªrmation of the universality of graded structure may therefore be seen as conªrmation that a family resemblance account of concepts is correct. However, while Wittgenstein was most concerned to deploy the family resemblance ac- count of concepts to undermine the hold of the necessary and sufªcient condition picture, and advanced little in the way of a positive account, both Kuhn and cognitive psychologists developing family resemblance ac- counts of concepts have provided more speciªc and detailed analyses. Kuhn’s most important contribution to this discussion was to point out the signiªcance of two classes of relations in constituting the contrast classes of concepts and objects at the lowest level of a conceptual system. These are relations of similarity and dissimilarity (Hoyningen-Huene 1993, p. 72, n.34; Andersen, Barker and Chen 1996, pp. 351–356). While earlier attempts to understand conceptual systems have often taken similarity relations among objects as a starting point (Brewer 1993), no one before Kuhn seems to have recognized the signiªcance of dissimilarity relations in establishing conceptual structures. An important consequence of the family resemblance account made ex- plicit by Kuhn is that individuals using the same concepts may neverthe- less use wholly disjoint features to successfully classify objects as falling Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/106361401760375820 by guest on 25 September 2021 436 Kuhn, Incommensurability, and Cognitive Science under those concepts (Andersen, Barker and Chen 1996, p. 356). This possibility leads to one important class of cases of conceptual change. An anomaly may be an object that raises to the level of conscious recognition a divergence in the features employed to successfully classify objects. The anomaly itself will resist classiªcation, and thereby motivate revisions in the conceptual system (Chen, Andersen and Barker 1998, pp. 17–23). Al- though this is an important and powerful motive for conceptual change, it is not the only kind, and a motivation of a different sort will be discussed below when we consider Copernicus’s chief, announced objection to earlier astronomical theories. For both philosophers and non-philosophers, Kuhn’s mature work is largely unexplored territory (a conspicuous exception is Hoyningen- Huene 1993). But this work contains a well-developed theory of con- cepts that may be used to resolve longstanding