After having described the different theories, I will now proceed to show how Aristotle proved his theory, and what induced him to adopt it. CHAPTER XIV It is not necessary to repeat in every chapter that I write this treatise with the full knowledge of what you have studied; that I therefore need not quote the exact words of the philosophers; it will suffice to give an abstract of their views. I will, however, point out the methods which they employ, in the same manner as I have done when I discussed the theories of the Mutakallemim. No notice will be taken of the opinion of any philosopher but that of Aristotle; his opinions alone deserve to be criticized, and if our objections or doubts with regard to any of these be well founded, this must be the case in a far higher degree in respect to all other opponents of our fundamental principles. I now proceed to describe the methods of the philosophers. First Method.—According to Aristotle, motion, that is to say, motion par excellence, is eternal. For if the motion had a beginning, there must already have been some motion when it came into existence, for transition from potentiality into actuality, and from non-existence into existence, always implies motion; then that previous motion, the cause of the motion which follows, must be eternal, or else the series would have to be carried back ad infinitum. On the same principle he maintains that time is eternal, for time is related to and connected with motion: there is no motion except in time, and time can only be perceived by motion, as has been demonstrated by proof. By this argument Aristotle proves the eternity of the Universe. Second Method.—The First Substance common to the four elements is eternal. For if it had a beginning it would have come into existence from another substance; it would further be endowed with a form, as coming into existence is nothing but receiving Form. But we mean by “First Substance” a formless substance; it can therefore not have come into existence from another substance, and must be without beginning and without end; hence it is concluded that the Universe is eternal. Third Method.—The substance of the spheres contains no opposite elements; for circular motion includes no such opposite directions as are found in rectilinear motion. Whatever is destroyed, owes its destruction to the opposite elements it contains. The spheres contain no opposite elements; they are therefore indestructible, and because they are indestructible they are also without beginning. Aristotle thus assumes the axiom that everything that has had a beginning is destructible, and that everything destructible has had a beginning; that things without beginning are indestructible, and indestructible things are without beginning. Hence follows the Eternity of the Universe.