 In Ζ.17 Aristotle proposes a new point of departure in his effort to say what sort of a thing substance is. The new idea is that a substance is a “starting-point and cause” (archê kai aitia, 1041a9) of being. Before looking at the details of his account, we will need to make a brief detour into Aristotle’s theory of causes. The relevant texts are Physics II.3, Posterior Analytics II.11, and Metaphysics Α.3 and Δ.2. See also the entry on  Aristotle’s natural philosophy  and Section 2 of the entry on  Aristotle’s psychology.