Summa contra Gentiles1.24 Chapter XXIV. That the divine being cannot be specified by the addi- tion of any substantial difference

Again. From what we have said above, it can be shown that we cannot add anything to the divine being so as to specify it by an essential specification, as a genus is specified by differences. For it is impossible that a thing be in act unless there be also all those things whereby its substantial being is specified: for an animal cannot be in act unless it be either a rational or an irrational animal. Wherefore also the Platonists who postulated ideas, did not postulate per se existing ideas of genera, which derive specification from essential differences, but they postulated per se existing ideas of the species alone, which need not to be specified by essential differences. If, then, the divine being- can receive an essential specification from something added to it, that being will not be in act without something added to it. But God's very being is His substance as shown above. Therefore the divine substance cannot be in act without some addition: the contrary of which has been shown above.

Again. Whatever needs something added to it, in order to exist, is in potentiality to that thing. But the divine substance is not in potentiality in any way, as proved above: and God's substance is His being. Therefore His being cannot receive essential specification from something added to it.

Moreover. Whatever makes a thing to be in act, and is intrinsic to that thing, is either the whole essence thereof or part of its essence. Now that Which specifies a thing by an essential specification, makes a thing to be in act, and is intrinsic to the thing specified: otherwise the latter could not be specified essentially thereby. Therefore it must be either the very essence or part of the essence of that thing. But if something be added to the divine being, it cannot be the whole essence of God, for it has already been proved that God's existence is not distinct from His essence. Therefore it follows that it is a part of the divine essence: and thus God would be composed of essential parts, the contrary of which was proved above.

Again. That which is added to a thing by way of essential specification, does not constitute the notion of that thing, but only makes it to be in act: for rational added to animal makes animal to be in act, but does not constitute the notion of an animal as such: because the difference does not enter into the definition of the genus. Now if something be added to God to specify Him with an essential specification, it must give that to which it is added the notion of its proper quiddity or nature: since what is added thus, gives the thing actual being. Now this, namely actual being, is the divine essence itself, as shown above. It follows, therefore, that nothing can be added to the divine being to give it an essential specification, as a difference specifies a genus.