How does Aristotle argue that motion is eternal? How does this relate to the unmoved mover?

Aristotle does not argue on the subject but only consider true the doctrines of Thales and Democritus that things are made of atoms that goes together while a thing is being generated then separate themselves when the thing perish out of being. He don't argue that this motion of generation and perishment goes to eternity because he said that while every motion have a cause, there must be an ultimate cause that does not move itself (or it would be a paradox), because the chain of causes would go at infinitum and it is inconceivable.