 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.