Book 12
2
The theory of the Ideas contains no peculiar treatment of the question; for the exponents of the theory call the Ideas numbers, and speak of the numbers now as though they were unlimited and now as though they were limited by the number 10Cf. Aristot. Met. 13.8.17, 20. This was a Pythagorean survival, cf. Vol. I. Introduction. xvi.; but as for why there should be just so many numbers, there is no explanation given with demonstrative accuracy.
3
We, however, must discuss the question on the basis of the assumptions and distinctions which we have already made.
3
The first principle and primary reality is immovable, both essentially and accidentally, but it excites the primary form of motion, which is one and eternal.
4
Now since that which is moved must be moved by something, and the prime mover must be essentially immovable, and eternal motion must be excited by something eternal, and one motion by some one thing; and since we can see that besides the simple spatial motion of the universei.e., the (apparent) diurnal revolution of the heavens.(which we hold to be excited by the primary immovable substance) there are other spatial motions—those of the planets—which are eternal (because a body which moves in a circle is eternal and is never at rest—this has been proved in our physical treatisesAristot. Physics 8.8, 9, Aristot. De Caelo 1.2, 2.3-8.); then each of these spatial motions must also be excited by a substance which is essentially immovable and eternal.
5
For the nature of the heavenly bodies is eternal, being a kind of substance; and that which moves is eternal and prior to the moved; and that which is prior to a substance must be a substance. It is therefore clear that there must be an equal number of substances, in nature eternal, essentially immovable, and without magnitude; for the reason already stated.Aristot. Met. 12.7.12, 13.
