In the Physics Book VIII, Aristotle begins his argument by defining motion as the “actuality of the moveable insofar as it is moveable” (Phys. 251a10). The moveable as moveable must therefore contain within itself some potency determined to some act. That which is altered must first be alterable, for instance. The second principle for which he argues is that whatever is in motion is moved by something. This premise is proven through a consideration of both act and potency. Potency as such cannot be in act or else it would not be in potency. That which is what it is cannot at the same time become what it is because something cannot become what it already is. No potency can bring itself to act without first being in act, which would mean that it was not in potency.
What we now have is a series of movers being moved by something or a series of potentialities being brought into actuality by something which is already actual. According to Aristotle, this series cannot go on infinitely because if it did there would be no first mover and, hence, no motion at all. The series must cease or, more accurately, begin with some first mover that actualizes potentialities without itself being brought from potency to act. In other words, the series must begin with some mover that is itself unmoved. “For it is impossible to have an infinite series of movers each of which initiates motion and is moved by the agency of something else; for there is no first term in an infinite series” (Phys. 256a15-20). Even in what Aristotle calls self-movers, living things, it is more precise to say that motion is initiated in one part by another part. These self-moved movers are also subject to generation and corruption and therefore cannot be the unqualified first movers in any series since, for Aristotle, motion is continuous. The motion of self-moved movers, then, must also regress to some unmoved mover.