rather, it demonstrates the opposite of their position. For motion must have a mover, and all motions ultimately lead back to a first mover. They concede this. That first mover, from which the motion of everything else proceeds, must either be in motion or not be in motion. If it is not in motion, this entails motion proceeding from something that is not in motion, which contradicts sensory experience and reason.

For an effect must correspond to its cause. If the effect comes into being successively, its cause cannot remain in one unchanged state. As you yourselves have said, nothing can come into being from such a cause after not having existed. Indeed, continuous coming into being from such a cause is even more clearly impossible than a new occurrence proceeding from it, for the former entails more instances of the impossible than the latter.

For if it is said, “It is known by plain reason that, for anything that was not acting, a cause must arise that necessitates its acting; and that if an agent remains in the same state it was in before acting, it will do nothing and nothing will come into being from it,” the reply to them is:

This very truth, known by plain reason, entails that nothing can come into being from that cause at a later time which did not exist at an earlier time, unless some new factor has arisen in the cause itself. If nothing new arises in it, nothing new comes into being from it. Thus, to maintain that things continually come into being from it without anything new arising in it is to maintain that impossibilities continually occur. For whenever anything comes into being, the essence is turned, at the moment of its occurrence, into what it was before its occurrence. Previously, it was impossible for that thing to come into being from it; so now, likewise, it remains impossible for that thing to come into being from it.