 As I noted earlier, taken on its own terms, GA has a reasonably clear structure. Here again, as with PA II-IV, I will begin with a brief sketch of its overall structure, before focusing narrowly on its method as applied to one problem. It begins (715a1–18) by noting that it will explain two sets of facts left unexplained in PA: the differences among the parts related to generation; and the generation of the parts generally. Though he acknowledges that some animals do not have males and females, he treats sexual generation as his central topic. Initially he provides a theoretically ‘neutral’ account of the distinction: the male is what generates in another, the female what generates in itself (716a14–15). It is not until late in book I that he begins to develop his own detailed theory of their distinctive contributions to generation.