Studies of Similarity

Any event in the history of the organism is, in a sense, unique. Consequently, recognition, learning, and judgment presuppose an ability to categorize stimuli and classify situations by similarity As Quine (1969) puts it: There is nothing more basic to thought and language than our sense of similarity ; our sorting of things into kinds [p 1161 . Indeed, the notion of similarity that appears under such different names as proximity, resemblance, communality, representativeness, and psychological distance is fundamental to theories of perception, learning, and judgment This chapter outlines a new theoretical analysis of similarity and investigates some of its empirical consequences The theoretical analysis of similarity relations has been dominated by geometric models. Such models represent each object as a point in some coordinate space so that the metric distances between the points reflect the observed similarities between the respective objects In general, the space is assumed to be Euclidean, and the purpose of the analysis is to embed the objects in a space of minimum dimensionality on the basis of the observed similarities, see Shepard (1974) In a recent paper (Tversky, 1977), the first author challenged the dimensionalmetric assumptions that underlie the geometric approach to similarity and developed an alternative feature-theoretical approach to the analysis of similarity relations. In this approach, each object a is characterized by a set of features, denoted A, and the observed similarity of a to b, denoted s(a, b), is expressed as a function of their common and distinctive features (see Fig 4.1) That is, the observed similarity s(a, b) is expressed as a function of three arguments : A f1B, the features shared by a and b ;A B, the features of a that are not shared by b ; B A, the features of b that are not shared by a Thus the similarity between