Ch 13 "God Not a Genus" From the "Compendium of Theology" by St. Thomas Aquinas

Caput 13 Quod impossibile est Deum esse genus alicuius

CHAPTER 13 GOD NOT A GENUS

Ulterius autem ostendendum est, quod neque possibile est Deum esse genus. Ex genere enim habetur quid est res, non autem rem esse: nam per differentias specificas constituitur res in proprio esse; sed hoc quod Deus est, est ipsum esse. Impossibile est ergo quod sit genus.

We go on to show that God cannot be a genus. What a thing is, but not that it is, comes from its genus; the thing is established in its proper existence by specific differences. But that which God is, is very existence itself. Therefore He cannot be a genus.

Item. Omne genus differentiis aliquibus dividitur. Ipsius autem esse non est accipere aliquas differentias: differentiae enim non participant genus nisi per accidens, inquantum species constitutae per differentias genus participant. Non potest autem esse aliqua differentia quae non participet esse, quia non ens nullius est differentia. Impossibile est igitur quod Deus sit genus de multis speciebus praedicatum.

Moreover, every genus is divided by some differences. But no differences can be apprehended in very existence itself. For differences do not share in genus except indirectly, so far as the species that are constituted by differences share in a genus. But there cannot be any difference that does not share in existence, since non-being is not the specific difference of anything. Accordingly God cannot be a genus predicated of a number of species.