Aristotle’s Metaphysics (SEP) – Section 5 | What is Substance?
In the seventeen chapters that make up Book Ζ of the Metaphysics, Aristotle takes up the promised study of substance. He begins by reiterating and refining some of what he said in Γ: that ‘being’ is said in many ways, and that the primary sense of ‘being’ is the sense in which substances are beings. Here, however, he explicitly links the secondary senses of ‘being’ to the non-substance categories. The primacy of substance leads Aristotle to say that the age-old question ‘What is being?’ “is just the question ‘What is substance?’” (1028b4).
One might have thought that this question had already been answered in the Categories. There we were given, as examples of primary substances, an individual man or horse, and we learned that a primary substance is “what is neither in a subject nor said of a subject” (2a10). This would seem to provide us with both examples of, and criteria for being, primary substances. But in Metaphysics Ζ, Aristotle does not seem to take either the examples or the criteria for granted.
In Ζ.2 he recounts the various answers that have been given to the question of which things are substances—bodies (including plants, animals, the parts of plants and animals, the elements, the heavenly bodies), things more basic than bodies (surfaces, lines, and points), imperceptible things (such as Platonic Forms and mathematical objects)—and seems to regard them all as viable candidates at this point. He does not seem to doubt that the clearest examples of substances are perceptible ones, but leaves open the question whether there are others as well.
Before answering this question about examples, however, he says that we must first answer the question about criteria: what is it to be a substance (tên ousian prôton ti estin)? The negative criterion (“neither in a subject nor said of a subject”) of the Categories tells us only which things are substances. But even if we know that something is a substance, we must still say what makes it a substance—what the cause is of its being a substance. This is the question to which Aristotle next turns. To answer it is to identify, as Aristotle puts it, the substance of that thing.