Aristotle posits 10 categories of existing things: substance, quantity, quality, relation, place, time, position, doing, having, and being affected. Each of these terms was defined by Aristotle in pretty much the same way we would define it today, the one exception being substance. For Aristotle there were primary and secondary substances. A primary substance was a whole thing, such as a man or a dog. A secondary substance was a quality of that thing, such as rationality or loyalty.

To take the rest in turn: quantity is the number of something, a mathematical amount or measure; relation is a connection or comparison between things, such as above, below, before, or after; place refers to where a physical thing is; time is both the passage of events and a specific time on a clock or a calendar; position refers to how something is oriented, for example, right side up or upside down; doing refers to action, such as playing the harp or curing the sick; having refers to both the possession of a thing other than the possessor (for example, your wallet), or to something that is happening to you, such as having a good time; being affected refers to the effect of one thing on another, for example, your being affected by heat when you put your hand in the flame of a candle.

Aristotle’s main unit of existence was primary substances. A primary substance is a specific thing, such as a cow in a field, a dog, or a tree outside the Lyceum. Secondary substances are the groups to which the primary substances belong, such as bovines, canines, or plants. Primary substances have accidents—which are changing qualities that we would call attributes—that can only exist in them; for example, tallness, fatness, furriness, or greenness. Our scientific knowledge is all about secondary substances, which have no real existence of their own but are abstractions in our mind based on the common nature of members of groups of similar primary substances.