In his Pysics 8.5, first having confirmed the thesis `All moved is moved by something', Aristotle says that there are two ways. (1)Something which moves is moved not by itself but by something another, or (2)by itself. Then if all moved is moved only in (1) way, it would result in the infinite regress of a series of efficient causes. Therefore the series of efficient causes must stop some time at the moved in (2) way.
Secondly having shown the impossibility of the self-mover moving itself as a whole, Aristotle says that one part of the self-mover must cause a motion and the other part must be moved. For this three cases can be thought. (a)By the two parts moving each other, or (b)by more than one part moving itself and the other part not moving, or (c)one part moving the other part. Having shown the impossibility of (a) and (b), Aristotle examines the possibility of (c).
Finally Aristotle concludes that the self-mover moves itself as a whole not because its one part is the self-mover but because its one part moves and the other is moved. However he admits plainly that it is unclear in animals how the moving part of them and the moved part must be distinguished. Generally in this part of the Pysics Aristotle has been thought to criticize the Platonic conception of the soul. And in its conclusion he demands that those who suport the Platonic conception of the soul should make clear how the soul as a motion moving itself functions in animals.
In any case Aristotle showed how the self-mover can move itself. He describes it thoroughly as the series of motions caused by the contact of bodies. Even though the soul is not such a body and it is not clear how the body and the soul must be distinguished, as far as we think of animals as a whole to be the self-mover, we cannot help reach the same conclusion as Aristotle's.