The seeds of an answer to the question raised at the end of Chapter 7 – how do formal and material natures interact in the creation of a new substantial individual? – have already been sown inChapter 4 in connection with GC II 9. On the reading I defended there, while Aristotle agrees with those more sophisticated materialists who regard natural generation as the effect of the simple bodies and their inherent powers, he ultimately rejects their account on the grounds that the powers they give them are too instrumental (lian organikas) to serve as primary causes of the change (336a1–12). But Aristotle’s positive account in GC II 9 remains undeveloped in two important respects. First, he says very little about the nature of the efficient cause that was ‘vaguely dreamed of’ by all his predecessors but ‘definitely stated’ by none of them (335b7–8).