is another classic piece on categorization:
http://en.wikipedia.org/wiki/Sophist_%28dialogue%29

"The definition of the sophist aims at verbal explanation and requires knowledge of the nature of the kinds, as well as of their ability of blending."

Plato goes into "Titanomachia", attempt to define basic categories like being, non-being, movement, rest, sameness, difference, and finds out that all those categories are interdependent (cf. de Saussure's general linguistics). Plato's conclusion is to characterize being as 'dynamis'.