One traditional approach to describe a category is to define it by using sets of necessary and sufficient conditions. If an entity possesses these conditions, it is a member of the category. Take the category BIRD for example. If an animal has wings and feathers, lays eggs, and can fly, then it belongs to the category BIRD. The assumption that underlies this approach is that all members of a category are equal, and that the boundaries of categories are clear. This assumption runs into problems. Penguins and ostriches cannot fly, yet they are still birds. A bat has wings and can fly, but it is not a bird. What are the necessary and sufficient conditions for the category BIRD? How many such conditions must a member of the category possess?

　 The theory of prototypes proposed by Eleanor Rosch (1973, 1975) solves this logical problem by a notion of degree of membership. Through a number of experiments, she concludes that some members of a category are typical and central while others are peripheral. A chair, for example, was judged by her subjects (200 American college students) to be one of the prototypes of FURNITURE, a mirror less typical a member, and a fan among the peripheral members of the category. Labov (1973) studied the linguistic categorization of household receptacles like cups, mugs, bowls, and vases. Line drawings of these objects were shown to subjects, who were asked to name the depicted objects. The results, contrary to the expectations of classical theory, show that there was no clear dividing line between the judgment of cup and bowl. Rather the one category gradually merged into the other. These research findings suggest that category is gradable. This approach allows for marginal members (such as a whale, which is a peripheral member of MAMMAL and in the meantime resembles fish in some features). The theory of prototypes eliminates the dilemma of the traditional conception of category as it postulates that typical members may possess more attributes or features than non-typical members.