 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.