Concept
κατηγορίαι · katēgoriai
The categories (katēgoriai) are Aristotle's list of the most general kinds under which everything that exists can fall — the ultimate headings of being and of predication. The word comes from katēgorein, 'to predicate' or, in the law courts, 'to accuse': a category is something asserted of a subject.
Aristotle counts ten: substance (ousia) — what a thing is — and nine kinds of attribute: quantity, quality, relation, place, time, position, state, action, and being-acted-upon. Of these, substance is primary; all the rest exist only as features of substances. To say of Socrates that he is a man names his substance; that he is pale, or in the marketplace, or older than Plato, names his qualities and relations.
The little treatise that opens Aristotle's logical works, the Categories, became one of the most studied books in history. Through Porphyry's introduction to it, it framed the logic and metaphysics of the entire later tradition, fixing the vocabulary in which philosophers would argue about substance, essence, and predication for two thousand years.
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