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] It will be seen that the first three arguments prove only that the world in the form which it has now is not eternal. The possibility is not yet excluded of an eternal matter out of which the world proceeded or was made. The fourth argument proves a great deal. It shows that nothing which is subject to time can be eternal, hence not even prime matter. God can be eternal because he is not subject to time. Time, as we shall see later, cannot exist without motion and moving things, hence before the world there was no time, and the fourth argument does not apply to premundane existence. To complete the first three arguments Saadia therefore proceeds to show that the world, which we now know came to be in time, must have been made by someone (since nothing can make itself), and that too out of nothing, and not out of a pre-existing eternal matter. If an eternal matter existed before the world, the explanation of the origin of the world is open to two possibilities. One is that there is nothing outside of this matter and the world which came from it. This is absurd, for it would mean that an unintelligent dead thing is the cause of intelligence and life in the universe. We must therefore have recourse to the other alternative that someone, an intelligent being, made the world out of the primitive, eternal matter. This is also impossible. For if the matter is eternal like the maker of the world, it is independent of him, and would not be obedient to his will to adapt itself to his purpose. He could therefore not make the world out of it. The only alternative left now is that the author of the universe is an intelligent being, and that nothing outside of him is eternal. He alone is responsible for the existence of the world, which was at one time nothing. Whether he first created a matter and then from it the universe, or whether he made the world outright, is of secondary importance.[56]