Physics (Aristotle/Wikisource translation)/Book VIII
Did motion ever begin, after a time of stillness? Will motion ever cease, with the result that nothing ever moves again? Or can't motion be created or destroyed? If so, motion would always have existed; it would be eternal. To all things that exist, motion would be an undying and never-ceasing heritage and legacy, a kind of life to all things composed by Nature.
Given these considerations, the primary immovable motor clearly cannot have any size, for its size would have to be either finite or infinite. Infinite size was shown to be impossible earlier in our Physics. A finite body cannot have infinite power, nor maintain a motion for an infinite time. However, the primary motor causes eternal motion through time unending. Therefore, the primary motor is indivisible, having neither parts nor size.