Theories of categorization have typically focused on the internal structure of categories. This paper is concerned with the external structure of categories. In particular, we suggest that many categories specify the relational role that is played by category members. To support this claim, we distinguish between traditional feature-based categories, relational categories (which specify a relational structure) and role-governed categories (which specify that an item plays a particular role within a relational structure). After discussing the relationship among these types of categories, we discuss the implications of this view for the study of category learning and category use.

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