25. The principles of an individual compound substance are matter and form; and there must be an agent, _i. e._, a mover which moves the object or the underlying matter until it prepares it to receive the form. This need not be the ultimate mover, but a proximate one having a particular function. The idea of Aristotle is that matter cannot move itself. This is the great principle which leads us to investigate into the existence of the first mover. Of these twenty-five propositions, Maimonides continues, some are clear after a little reflection, some again require many premises and proofs, but they are all proved in the Physics and Metaphysics of Aristotle and his commentators. My purpose here is, as I said, not to reproduce the writings of the philosophers. I will simply mention those principles which we must have for our purpose. I must add, however, one more proposition, which Aristotle thinks is true and more deserving of belief than anything else. We will grant him this by way of hypothesis until we explain what we intend to prove. The proposition is: 26. Time and motion are eternal and actual. Hence there must be a body moving eternally and existing actually. This is the matter constituting the substance of the heavenly bodies. Hence the heavens are not subject to genesis and decay, for their motion is eternal. This presupposes the possibility of accidental infinity (_cf._ above, p. 251). Aristotle regards this as true, though it does not seem to me that he claims he has proved it. His followers and commentators maintain that it is a necessary proposition and demonstrated. The Mutakallimun, on the other hand, think it is impossible that there should be an infinite number of states in succession (_cf._ _ibid._). It seems to me it is neither necessary nor impossible, but possible. This is, however, not the place to discuss it.[273] Now follows the classical proof of the existence of God from motion. It is in essence the same as that given by Ibn Daud, but much more elaborate. We shall try to simplify it as much as possible. The numbers in parentheses in the sequel refer to the preliminary propositions above given.