Here are some of his proofs. The world is finite in magnitude. For the world consists of the earth, which is in the centre, and the heavens surrounding it on all sides. This shows that the earth is finite, for an infinite body cannot be surrounded. But the heavens are finite too, for they make a complete revolution in twenty-four hours. If they were infinite it would take an infinite time to complete a revolution. A finite body cannot have an infinite power. This Saadia regards as self-evident, though Aristotle, from whom this statement is derived, gives the proof. Hence the force or power within the world which keeps it going is finite and must one day be exhausted. But this shows also that it could not have gone on from eternity. Hence the world came to be in time.[52]
Another proof is based on the composite character of all things in heaven and earth. Minerals, plants and animals are made up of parts and elements. The heavens consist of spheres, one within the other. The spheres are studded with stars. But composition implies a time when the composition took place. In other words, the parts must have been there first and somebody put them together. Hence the world as we see it now is not eternal.[53]
A special form of composition, which is universal, is that of substance and accident. Plants and animals are born (or sprout), grow and decay. These manifestations are the accidents of the plant or animal's substance. The heavenly bodies have various motions, lights and colors as their accidents. But these accidents are not eternal, since they come and go. Hence the substances bearing the accidents, without which they cannot exist, are also temporal like them. Hence our world is not eternal.[54]
Finally, past time itself cannot be eternal. For this would mean that an infinite time has actually elapsed down to our day. But this is a contradiction in terms. What is already accomplished cannot be infinite. Infinity is possible only as a potentiality, for example, we may speak of a given length as infinitely divisible. This merely means that one may mentally continue dividing it forever, but we can never say that one has actually made an infinite number of divisions. Therefore not merely the world, but even time must have begun to be.[55]