prototype theory a theory according to which human cognition involves the deployment of ‘categories’ organized around stereotypical exemplars. Prototype theory differs from traditional theories that take the concepts with which we think to be individuated by means of boundary-specifying necessary and sufficient conditions. Advocates of prototypes hold that our concept of bird, for instance, consists in an indefinitely bounded conceptual ‘space’ in which robins and sparrows are central, and chickens and penguins are peripheral – though the category may be differently organized in different cultures or groups. Rather than being all-ornothing, category membership is a matter of degree. This conception of categories was originally inspired by the notion, developed in a different context by Wittgenstein, of family resemblance. Prototypes were first discussed in detail and given empirical credibility in the work of Eleanor Rosch (see, e.g., ‘On the Internal Structure of Perceptual and Semantic Categories,’ 1973). See also ARTIFICIAL INTELLI- GENCE , PHILOSOPHY OF MIND , WITTGENSTEI. J.F.H.