CategoriesAristotle

The Categories is the opening work of Aristotle's logical writings, the Organon, and it addresses the most basic question in ontology: what kinds of things are there? Aristotle distinguishes ten categories, the highest genera under which anything that exists can be classified: substance, quantity, quality, relation, place, time, position, state, action, and affection.

Substance holds a privileged place. Primary substances are individual things (this man, this horse); secondary substances are the species and genera they belong to (man, animal). Everything else, every quality, quantity, or relation, exists only as said of or present in a substance. Without individual substances, nothing else would exist. This asymmetry between substance and accident structures Aristotelian metaphysics and persists through Scholastic philosophy into early modern debates about the nature of attributes and their bearers.

The treatise also introduces the theory of opposites (contradictories, contraries, privation and possession, relatives) and the notion of priority in its several senses. These distinctions are not merely classificatory; they establish the conceptual vocabulary that Aristotle deploys throughout his philosophy. The Categories is brief and appears introductory, but the problems it raises, about what substance is, how universals relate to particulars, and whether the list of categories is exhaustive, generated centuries of commentary from Porphyry through Aquinas to Kant.