We proceed to the fifth article thus:

1. It seems that God does belong to a genus. For "substance" means self-subsistent being, and this is pre-eminently applicable to God. God therefore belongs to the genus "substance."

2. Again, each thing is measured by what belongs to its own genus. Thus lengths are measured by length, and numbers by number. Now the commentator on 10 Metaph. says that God is the measure of all substances. God therefore belongs to the genus "substance."

On the other hand: we think of a genus as prior to what it contains. But there is nothing prior to God, whether in reality or in the understanding. Therefore God does not belong to any genus.

I answer: a thing may belong to a genus in two ways. It may belong to it absolutely and properly, as does a species which the genus contains. Or it may be reducible to a genus, as are principles and privations. Point and unity, for example, are reducible to the genus "quantity" as principles of it, while blindness, and all privation, are reducible to the genus of their habits. But God does not belong to a genus in either of these ways.

There are three proofs that God cannot be a species of any genus. First, a species is made up of a genus and a difference. Now that from which the difference which constitutes a species is derived is always related to that from which the genus is derived as the actual to the potential. Thus "animal" is concretely derived from "sensitive nature," a thing being called animal because it has a sensitive nature, while "the rational" is derived from "intellectual nature," since the rational is that which has an intellectual nature. The intellectual is then related to the sensitive as the actual to the potential. This is likewise clear in other things. It is therefore impossible that God should belong to a genus as a species of it, since in God there is no adjunction of the potential with the actual.

Secondly, it was proved in the preceding article that God's existence is his essence. Hence if God belonged to any genus, this genus would have to be "being," since a genus indicates the essence of a thing, and is predicated because of what the thing is. But the philosopher proves that "being" cannot be the genus of anything (3 Metaph., text 10), since every genus includes differences which are external to its essence, and there are no differences external to being, since "not-being" cannot be a difference. It follows from this that God cannot belong to a genus.

Thirdly, all things which belong to one genus agree in their "what," or the essence of their genus, which is predicated of them because of what they are. But they differ in point of existence, since the existence of a man is not the same as that of a horse, nor the existence of one man the same as that of another. Existence and essence are thus bound to be different in anything which belongs to a genus. But they are not different in God, as we proved in the preceding article. This makes it plain that God does not belong to a genus as a species.

It is clear from the foregoing that God has neither genus nor differences, and that there is no definition of God, nor any way of demonstrating him except through his effects. For definition is by means of genus and difference, and definitioa is the means of demonstration.

That God does not belong to a genus as a principle reducible to it is obvious from the fact that a principle which is reducible to a genus does not extend beyond that genus. The point, for example, is the principle of continuous quantity only, and the unit of discrete quantity only. But God is the ground of all existence, as we shall prove in Q.44, Art.1. Consequently, he is not contained in any genus as a principle.

On the first point: the term "substance" signifies more than self-subsistent being, for we have shown above that "being" cannot by itself be a genus. It signifies an essence which has the ability to exist, i.e., which can exist through itself, but whose existence is not identical with its essence. This makes it plain that God does not belong to the genus "substance."

On the second point: this objection argues from the measure of proportion. God is not in this way the measure of anything. He is said to be the measure of all things because all things have existence in so far as they are like him.