(CG 2/91). That there are Subsistent Intelligences not united with Bodies (Book 2: God The Origin of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

INTELLIGENCES subsisting apart are subsistent essences. Now the definition of a thing being the mark of its essence, is the mark of its species. Subsistent essences therefore are subsistent species.

2. Difference in point of form begets difference of species, while difference in point of matter begets difference in number. But intelligences subsisting apart have nothing whatever of matter about them. Therefore it is impossible for them to be several in one species.

4. The multiplication of species adds more nobility and perfection to the universe than the multiplication of individuals in the same species. But the perfection of the universe consists principally in intelligences subsisting apart. Therefore it makes more for the perfection of the universe that there should be many intelligences different in species than many different in number in the same species.