Categories

A short treatise, placed first in the collection of Aristotle's logical works later called the Organon, that classifies the kinds of things there are. It begins by distinguishing words used in the same sense, in different senses, and in derived senses. It then separates what is said of a subject from what is in a subject, giving a fourfold division of beings. Next comes the famous list of ten categories: substance, quantity, quality, relation, place, time, position, having, acting and being acted on. The individual, such as a particular man or horse, is primary substance; species and genera, such as man and animal, are secondary substances, and everything else depends on primary substances to exist. Substance has no contrary, does not come in degrees, and can take on contrary qualities while staying one and the same. Chapters follow on quantity, relatives and quality, and a final part, whose authenticity has been questioned, on opposites, priority and change.

It sets out the basic grammar of Western ontology, the distinction between substances and their properties, and for centuries it was the first philosophy text students read.