Abstract: The Categories and Metaphysics Zeta (Book VII) are often thought to be incompatible because each posits different candidates for the title of primary substance or ousia. In the Categories, primary substance is the concrete individual thing, while in Zeta, it is the form or essence of the individual thing, which is now understood as a composite of form and matter. In this book, Wedin rejects the thesis of incompatibilism, and in its stead argues that the two treatises are concerned with different projects. Wedin argues that the theory of Metaphysics Zeta is meant to explain certain features of the theory of the Categories, and that it therefore presupposes the Categories theory of substance. The ousia of the Categories, what Wedin calls ‘c-substance’, remains a primary substance in the Metaphysics in the sense that its ontological priority is not called into question. Aristotle focuses on form in the Metaphysics because it explains the substantiality of c-substances: in other words, it has ‘explanatory’ primacy. Wedin's overall view of Metaphysics Zeta is that it is concerned with investigating what sort of thing form must be, given that form is the primary cause of the nature or being of a c-substance. The treatises, then, are not incompatible, because the explananda of Zeta are the primary substances of the Categories. Showing that this is so involves, firstly, setting out the theory of the Categories (Ch. 1–3), secondly, clarifying the sense in which ‘c-substance’ is the explanandum of Metaphysics Zeta (Ch. 4–5), and finally, examining in detail the explanatory theory of Zeta (Ch. 6—10). In the Introduction, Wedin announces his intention to consider only what he calls the ‘canonical chapters’ of Zeta, i.e. Z.1–6, 10–11, 13–17: Ch. 7–9 and 12 are mostly neglected, as Wedin believes that these chapters are not part of the original project of Zeta.

Keywords: Aristotle's theory of substance, Categories, composite, c-substance, essence, explanatory primacy, form, incompatibilism, matter, Metaphysics Zeta, ontological priority, ousia, primary substance, the subject of predications, the substance-of substance