 A subject, Aristotle tells us, is “that of which the other things are said, but which is itself not further said of any other thing” (1028b36). This characterization of a subject is reminiscent of the language of the Categories, which tells us that a primary substance is not predicated of anything else, whereas other things are predicated of it. Candidate (iv) thus seems to reiterate the Categories criterion for being a substance. But there are two reasons to be wary of drawing this conclusion. First, whereas the subject criterion of the Categories told us that substances were the ultimate subjects of predication, the subject criterion envisaged here is supposed to tell us what the substance of something is. So what it would tell us is that if x is a substance, then the substance of x—that which makes x a substance—is a subject that x is predicated of. Second, as his next comment makes clear, Aristotle has in mind something other than this Categories idea. For the subject that he here envisages, he says, is either matter or form or the compound of matter and form. These are concepts from Aristotle’s Physics, and none of them figured in the ontology of the Categories. To appreciate the issues Aristotle is raising here, we must briefly compare his treatment of the notion of a subject in the Physics with that in the Categories.