In accordance with this proposition, Aristotle is compelled to assume that there exists actually a body with constant motion, viz., the fifth element. He therefore says that the heavens are not subject to genesis or destruction, because motion cannot be generated nor destroyed. He also holds that every motion must necessarily be preceded by another motion, either of the same or of a different kind. The belief that the locomotion of an animal is not preceded by another motion, is not true; for the animal is caused to move, after it had been in rest, by the intention to obtain those very things which bring about that locomotion. A change in its state of health, or some image, or some new idea can produce a desire to seek that which is conducive to its welfare and to avoid that which is contrary. Each of these three causes sets the living being in motion, and each of them is produced by various kinds of motion. Aristotle likewise asserts that everything which is created must, before its actual creation, have existed in potentiâ. By inferences drawn from this assertion he seeks to establish his proposition, viz., The thing that moves is finite, and its path finite; but it repeats the motion in its path an infinite number of times. This can only take place when the motion is circular, as has been stated in Proposition XIII. Hence follows also the existence of an infinite number of things which do not co-exist but follow one after the other. Aristotle frequently attempts to establish this proposition; but I believe that he did not consider his proofs to be conclusive. It appeared to him to be the most probable and acceptable proposition. His followers, however, and the commentators of his books, contend that it contains not only a probable but a demonstrative proof, and that it has, in fact, been fully established. On the other hand, the Mutakallemim try to prove that the proposition cannot be true, as, according to their opinion, it is impossible to conceive how an infinite number of things could even come into existence successively. They assume this impossibility as an axiom. I, however, think that this proposition is admissible, but neither demonstrative, as the commentators of Aristotle assert, nor, on the other hand, impossible, as the Mutakallemim say. We have no intention to explain here the proofs given by Aristotle, or to show our doubts concerning them, or to set forth our opinions on the creation of the universe. I here simply desire to mention those propositions which we shall require for the proof of the three principles stated above. Having thus quoted and admitted these propositions, I will now proceed to explain what may be inferred from them. CHAPTER I