 In Metaphysics Λ.8 Aristotle initially takes a step in the direction of such a bipartite picture. He asks how many substantial activities are required to explain astronomical phenomena, such as the movements of the stars and planets, and answers that there must be forty-nine of them (1074a16). But these forty-nine are coordinated with each other so as to form a system. And what enables them to do so, and to constitute a single heaven, is that there is a single prime mover of all of them: