(CG 1/024). The Existence of God cannot he characterised by the addition of?... (Book 1: Of God As He Is In Himself) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

EVERYTHING that is in any genus has something in it whereby the nature of the genus is characterised and reduced to species: for there is nothing in the genus that is not in some species of it. But this is impossible in God, as has been shown in the previous chapter.

2. If God is in any genus, He is either in the genus of accident or the genus of substance. He is not in the genus of accident, for an accident cannot be the first being and the first cause. Again, He cannot be in the genus of substance: for the substance that is a genus is not mere existence: otherwise every substance would be its own existence, since the idea of the genus is maintained in all that is contained under the genus: at that rate no substance would be caused by another, which is impossible (Chap. XIII, XV). But God is mere existence: therefore He is not in any genus.

3. Whatever is in a genus differs in point of existence from other things that are in the same genus: otherwise genus would not be predicated of several things. But all things that are in the same genus must agree in the quiddity, or essence, of the genus: because of them all genus is predicated so as to answer the question what (quid) each thing is. Therefore the existence of each thing that exists in a genus is something over and above the quiddity of the genus. But that is impossible in God.

4. Everything is placed in a genus by reason of its quiddity. But the quiddity of God is His own mere (full) existence. Now a thing is not ranked in a genus on the score of mere (bare) existence: otherwise 'being,' in the sense of mere (bare) existence, would be a genus. But that 'being' cannot be a genus is proved in this way. If 'being' were a genus, some differentia would have to be found to reduce it to species. But no differentia participates in its genus: I mean, genus is never comprehended in the idea of the differentia: because at that rate genus would be put twice over in the definition of the species. Differentia then must be something over and above what is understood in the idea of genus. Now nothing can be over and above what is understood by the idea of 'being'; since 'being' enters into the conceivability of all things whereof it is predicated, and thus can be limited by no differentia.

Hence it is also apparent that God cannot be defined, because every definition is by genus and differentias. It is apparent also that there can be no demonstration of God except through some effect of His production: because the principle of demonstration is a definition of the thing defined.