or as a composition of the subject from the essence; or a quantitative composition, like the way a body is composed—according to one school—from matter and form, or—according to another—from discrete atoms, so that He is not divisible, whether in actuality or even in imagination, nor divisible into matter and form; or as a composition of quiddity and existence. These five—or six, or even seven—modes of composition, as their technical vocabulary defines them, they emphatically deny of Him.
They go on to say: Unclouded reason bears witness that existence is of two kinds—what is necessary in itself and what is contingent in itself. For every contingent being, one of its two alternatives is tipped in favour of existence only by a determiner. The series of contingents must therefore either regress to infinity or terminate in a being whose existence is necessary in itself and not contingent. Were the series infinite, nothing contingent would ever come to be, because the existence of every contingent depends on the prior existence of its determiner, and that is impossible. The sequence must therefore culminate in a Necessary Existent in itself.
Such a Necessary Existent cannot, within His essence, possess principles that combine to constitute Him, nor quantitative parts such as matter and form, nor the genus-and-differentia structure; for those principles would have to precede the very essence of the Necessary Existent, making Him necessary through them rather than in Himself—contradicting the premise. That is impossible.
They employ another argument—different from the one the questioner mentioned, yet of the same nature and founded on the same root—to deny the existence of two Necessary Existents. If, they contend, we were to posit two beings each necessary in existence, they would share necessity of existence, and one would have to be distinguished from the other by a property peculiar to it. Necessity of existence would then be common, serving as a genus, while the exclusive property would serve as the differentia; the essence of each would thus be composed of the two. Those parts would have to be prior in essence to the whole, rendering the whole posterior to them and therefore necessary through them, not through itself—another contradiction.
Accordingly, the species “Necessary Existent,” whether taken without qualification or with any possible specification, can be only one. His existence is His unity, and the necessity of His being is pure reality