Thomas Aquinas, Compendium of Theology

We infer from the above that God is not contained as a species within any genus. Species is constituted by specific difference added to genus. Hence the essence of any species possesses something over and above its genus. But existence itself, ipsum esse, which is God's essence, does not comprise within itself any factor that is added to some other factor. Accordingly God is not a species of any genus.

Furthermore, since genus potentially contains specific differences, in every being composed of genus and differences, act is commingled with potency. But we have shown that God is pure act without any commingling of potency. (Cf. chap. 9.) Therefore His essence is not composed of genus and differences; and so He is not in any genus.