The first and more manifest way, however, is that which is taken from the part of motion. For it is certain, and is clear to the senses, that some things are moved in this world. And everything that is moved is moved by another. For nothing is moved except insofar as it is in potentiality to that to which it is moved, but it is moved by something insofar as it is in actuality. For to move is nothing other than to bring something from potentiality into actuality, and from potentiality nothing can be brought into actuality except through some being in actuality, just as the actual heat of fire makes wood, which is potentially hot, become actually hot, and in this way it moves and alters it. However, it is not possible for the same thing to be at the same time in act and potentiality with respect to the same thing, but only with respect to different things. For that which is hot in act cannot at the same time be hot in potentiality, but it is at the same time cold in potentiality. It is therefore impossible for the same thing, in the same respect and in the same way, to be both a mover and a moved, or for it to move itself. Therefore, everything that is moved must be moved by another. If, then, that by which it is moved is itself moved, it must also be moved by another, and that one by another. But we cannot proceed to infinity in this way, because there would then be no first mover; and consequently, no other mover either, because secondary movers do not move except insofar as they are moved by the first mover, just as a staff does not move except insofar as it is moved by the hand. Therefore, it is necessary to arrive at some first mover, which is moved by no one, and this all men understand to be God.