The atomists suppose that the origin of everything is the random movement of matter and that from this source comes all of the order and life in the physical world. Plato argues that this cannot be how it happened, that instead life and thought must have been first, the origin of everything. He contends that the atomists have, mistakenly, reversed the true order of things. Book X. Plato's proof of the priority of design in the world. He is going to claim that atheism is provably false.
All motions are either a.
Spontaneous, things that move by themselves. The first , original, motion must have been spontaneous, not communicated. If atoms move by themselves, then they are alive, not inanimate. In either case, something living started the world in the first place, not something non-living. The visible motions of the physical world are orderly and regular, in particular, the movements of the heavens.