The unmoved mover, or God, is thus not body, for no body can move another body without being itself in motion at the same time. Besides, all body is finite, _i. e._, it has a finite magnitude. A body of infinite magnitude is an impossibility, as the very essence of body is that it must be bounded by surfaces. A finite body cannot have an infinite power, as Aristotle proves, though we need not at present go into the details of his proof. But a being which causes eternal motion in the world must have an infinite power to do this. Hence another proof that God is not corporeal. If God is not subject to motion, he is not subject to change of any kind, for change involves motion. As matter is at the basis of all change God is without matter, hence he is pure form, _i. e._, pure actuality without the least potentiality. This means that he is what he is wholly all the time; he has no capacities of being what he is at any time not. But if he is not corporeal, the nature of his actuality or activity must be Thought, pure thinking. And the content of his thought cannot vary from topic to topic, for this would be change, which is foreign to him. He must be eternally thinking the same thought; and the highest thought it must be. But the highest thought is himself; hence God is pure thought thinking himself, thought thinking thought. The universe is in the shape of a sphere with the earth stationary in the centre and the heavens revolving around it exactly as appears to us. The element earth is the heaviest, hence its place is below or, which is the same thing, in the centre. This is its natural place; and its natural motion when away from the centre is in a straight line toward the centre. Water is the next heaviest element and its natural place is just above earth; hence the water in the world occupies a position spherical in shape round about the earth, _i. e._, it forms a hollow sphere concentric with the earth. Next comes the hollow sphere of air concentric with the other two. Its natural motion when away from its place in the direction of the earth is in a straight line toward the circumference of the world, not however going beyond the sphere of the lightest element of all, namely, fire. This has its natural place outside of the other elements, also in the form of a hollow sphere concentric with the other three. Its natural motion is in a straight line away from the centre of the world and in the direction of the circumference. Our earth, water, air and fire are not really the elements in their purity. Each one has in it also mixtures of the other three elements, the one which gives it the name predominating.