 What other type of structure could they have? A non-classical alternative that emerged in the second half of the twentieth century is the prototype theory (e.g., Hampton 2006). According to this theory, a lexical concept C doesn’t have definitional structure but has probabilistic structure in that something falls under C just in case it satisfies a sufficient number of properties encoded by C’s constituents. The prototype theory has its philosophical roots in Wittgenstein’s (1953/1958) famous remark that the things covered by a term often share a family resemblance, and it has its psychological roots in Eleanor Rosch’s groundbreaking experimental treatment of much the same idea (Rosch &amp; Mervis 1975, Rosch 1978). The prototype theory is especially at home in dealing with the typicality effects that were left unexplained by the classical theory. One standard strategy is to maintain that, on the prototype theory, categorization is to be understood as a similarity comparison process, where similarity is computed as a function of the number of constituents that two concepts hold in common. On this model, the reason apples are judged to be more typical than plums is that the concept APPLE shares more of its constituents with FRUIT. Likewise, this is why apples are judged to be a kind of fruit faster than plums are.