"...no two ideas are ever exactly the same, which is the proposition we started to prove. The proposition is more important theoretically than it at first sight appears." William James, 1890, p. 235.
Traditional theories of categorization concentrate on the stability of Cognition, how it is that people perform the same cognitive act over and over despite varying local circumstances. For example, each time a person encounters a frog, whether in a pond or in a comic strip, the encountered object is understood to be the same kind of thing -- a frog. Traditional theory explains this stability by positing stable mental representations. According to this view, the reason that people understand the frog in the pond and the frog in the comic strip to be the same sort of thing is chat in both cases they access the same representation, the same concept of frog.