Philosophical categories are classes, genera, or types supposed to mark necessary divisions within our conceptual scheme, divisions that we must recognize if we are to make literal sense in our discourse about the world. To say that two entities belong to different categories is to say that they have literally nothing in common, that we cannot apply the same descriptive terms to both unless we speak metaphorically or equivocally.

Aristotelian Theory

The word category was first used as a technical term in philosophy by Aristotle. In his short treatise called Categories, he held that every uncombined expression signifies (denotes, refers to) one or more things falling in at least one of the following ten classes: substance, quantity, quality, relation, place, time, posture, state, action, and passion. By "uncombined expression" Aristotle meant an expression considered apart from its combination with other expressions in a sentence, and he intended his...