SUBSTANCE
Descartes defines substance as “that which requires nothing but itself and God in order to exist.” The idea seems to be this: following the tendency of the Indo-European languages, any quality (adjective) or action (verb) is dependent on a thing (noun, or a pronoun referring to a noun). Even when it is difficult to find a real noun, we invent a noun (“thing”) or pronoun (“it”) to take its grammatical place. We say, for example, “It is raining,” but we couldn’t say what it is that is raining. Descartes says in the Meditations, “I am a thing that thinks,” and seems to conclude that there must be something (a mental substance) that is doing the thinking. The substance, following the Latin root of the word, “stands under” any qualities or actions that we might attribute to it. Because it underlies these qualities and actions, it is other than them, and thus can’t strictly be described; but Descartes thinks that we can designate substances by the kind of qualities they can have.

Thus, there are three kinds of substance:
1. Mind, characterized by thinking. “Thinking” is not just reasoning, but any action we would call “mental.” Descartes gives examples: a thing that thinks is a thing that “doubts, understands, affirms, denies, wills, refuses, and that also imagines and senses” (Meditation Two). It is probably significant that he left out remembering and anticipating.
2. Matter, characterized by extension. It takes up space, having a volume, the ability to move in space, and it occupies its space exclusively (that is, two things can’t be in the same place at the same time).
3. God, characterized by perfection. Contrary to some of his critics, Descartes does not claim to know exactly what perfection is. Leibniz and Spinoza both thought that a more precise definition was possible and necessary.

It is important to recognize that Descartes and his successors use the word “substance” in a different way than that used by Aristotle’s translators.