Rosch’s (1978) emphasizes that the prototypical instantiations of a given category are maximally distinct from each other. She states,
To increase the distinctiveness and flexibility of categories, categories tend to become defined in terms of prototypes or prototypical instances that contain the attributes most representatives of items inside and least representative of terms outside the category (1978, 30).
This is essentially a double characterization in that prototypicality is here defined both positively (most representative attributes) and negatively (least representative attributes). One implication of this definition is the fact that when we are dealing with two or more contrastive categories, non-prototypical usage of one of those categories will likely involve some of the prototypical attributes of that category, but also some prototypical attributes of another category–for example, distinguishing between what is a noun and what is a verb.*
The logical result of this fact is that it may be entirely possible for two contrasting categories to reflect near synonymy in some discourse contexts—contexts where the most representative attributes of their given category are dramatically downplayed. These sorts of non-contrasts between grammatical categories are prototype effects that arise from human cognition.
These facts are derived not merely from how categorization works for a limited set of items (e.g. lexemes), but how it functions for human cognition and reasoning in general. That is to say, all categorization is prototypical categorization. It is left to reason, then, that we ought to organize our grammar descriptions along similar lines, in a manner that most closely parallels basic principles of human cognition. A grammar that takes the nature of human categorization seriously will prioritize determining what the most representative and least representative attributes of a given grammatical category are and then also take those attributes as the standard for evaluating grammatical contrasts in the language description. The meaning of a given grammatical category must be grounded in most exemplar uses of that category. It is from here and here alone that non-prototypical usage may then be adequately evaluated and explained.**
*This, incidentally, is precisely the point of Hopper and Thompson (1984, 710), where they argue that the categorization of lexical items within a part-of-speech system should be grounded in discourse structure and usage.
**This refers, of course, to the formal grammatical description. In the analysis itself, evaluating what is and what is not prototypical usage is part of the process.