We start with something that is known, namely, the motion we see in the sublunar world, the motion which is involved in all the processes of genesis and decay and change generally. This motion must have a mover (25). This mover must have another mover to move it, and this would lead us to infinity, which is impossible (3). We find, however, that all motion here below ends with the motion of the heaven. Let us take an example. The wind is blowing through an opening in the wall. I take a stone and stop up the hole. Here the stone is moved by the hand, the hand by the tendons, the tendons by the nerves, the nerves by the veins, the veins by the natural heat, the natural heat by the animal soul, the animal soul by a purpose, namely, to stop the hole from which the wind comes, the purpose by the wind, the wind by the motion of the heavenly sphere. But this is not the end. The sphere must also have a mover (17). This mover is either outside the sphere it moves or within it. If it is something outside, it is either again a body like the sphere, or an incorporeal thing, a "Separate Intelligence." If the mover of the sphere is something within the sphere, two alternatives are again possible. The internal moving power of the sphere may be a corporeal force extended throughout the body of the sphere and divisible with it like heat, or an indivisible power like soul or intellect (10, 11). We thus have four possibilities in all. The mover of the heavenly sphere may be (a) a body external to the sphere; (b) a separate incorporeal substance; (c) an internal corporeal power divisible with the division of the sphere; (d) an internal indivisible power. Of these four, (a) is impossible. For if the mover of the sphere is another body, it is likewise in motion (9) and must have another to move it, which, if a body, must have another, and so on _ad infinitum_, which is impossible (2). The third hypothesis, (c), is likewise impossible. For as the sphere is a body it is finite (1), and its power is also finite (12), since it is divisible with the body of the sphere (11). Hence it cannot move infinitely (26). Nor can we adopt the last alternative, (d). For a soul residing within the sphere could not alone be the cause of continuous motion. For a soul that moves its body is itself in motion _per accidens_ (6); and whatever moves _per accidens_ must necessarily sometime stop (8), and with it the thing set in motion by it will stop also. There is thus only one alternative left, (b), _viz._, that the cause of the motion of the sphere is a "separate" (_i. e._, incorporeal) power, which is itself not subject to motion either _per se_ or _per accidens_; hence it is indivisible and unchangeable (7, 5). This is God. He cannot be two or more, for "separate" essences which are not body are not subject to number unless one is cause and the other effect (16). It follows, too, that he is not subject to time, for there is no time without motion (15).