Summa summae S. Thomae sive compendium theologiae

St. Thomas, q. 11.

[Note 1. One does not add any reality over and above being, but only the negation of division. One signifies nothing other than undivided being, as appears from this very fact, that one is convertible with being¹. Now this transcendental one is opposed privatively to multitude², because it signifies the privation of division. But quantitative one, which is the principle of number and is what is treated here, is opposed to multitude relatively, because quantitative unity stands to multitude as measure to the thing measured; for multitude is measured by unity repeated.

Note 2. When we ask whether God is one, the question is not about positive and specific unity, by which something is undivided being in itself and divided from any other of the same species, but about numerical and exclusive unity, by which another being of the same nature is excluded — in the sense in which the sun is one. We ask, therefore, whether God is sole and the only God, such that, besides him, there is no other God.

I say: God is one.

This is against polytheism. Blind and insane paganism, wholly given over to idolatry, went so far that, according to the testimony of Eusebius, book V of the Preparation for the Gospel, chapter 15, Hesiod numbered thirty thousand gods.

Among Christians, however, not a few heretics have attacked the unity of God — especially those who admitted two eternal principles, independent and equal: one good, the author of invisible things; the other evil, the cause of visible things. Such were the Gnostics, in the second century; the Marcionites, so called from Marcion, in the same century; the Valentinians, from Valentinus, in the same century. He set up two principles in such a way that he called one depth and the other silence, male and female, from whose combination came forth thirty aeons, that is, thirty ages. The Manichaeans, in the third century, so called from Manes or Manichaeus, their leader. In later centuries the Albigensians and others were followers of these. The Tritheists, other enemies of the unity of God, toward the end of the sixth century, said that the three divine persons were three gods.

Against all these the proposition is proved.

1. Against those who accept the Old Testament, such as were very many Jews according to Annat in the Apparatus, book VII, art. 1 — it is proved from that text of Deut., VI, 4: Hear, Israel: the Lord our God is one Lord. And ibid., XXXII, 39: See that I am he alone, and there is no other God besides me.

Against the heretics who accept the New Testament, it is proved from that text of Mark, XII, 32: Well, Master, you have said truly that God is one, and there is no other besides him. And John, XVII, 3: This is eternal life, that they may know you, the only true God. And 1 Cor., VIII, 4 and 6: We know that an idol is nothing in the world, and that there is no God but one... Yet for us there is one God. To these and other similar texts are added the professions of faith in the Nicene, Constantinopolitan, and Athanasian creeds, and the consensus of all the Fathers in their writings.

Against the pagans and all others whatsoever, it is proved by a threefold argument³.

The first argument is drawn from the supreme simplicity of God. Singularity belongs to the essence or nature of God. But it is repugnant for a being essentially singular to be multiple. Therefore. — The minor is clear, because singularity cannot be multiplied; otherwise, if it could be multiplied, it would be common to many, and would then no longer be singular: thus, if the singularity or haecceity by which Socrates is this man belonged to the essence of human nature, as rationality does, there would be only one man in the world. — Proof of the major. God is singular, or this God, through that by which he is God. But he is essentially God. Therefore he is essentially this God, that is, singularity belongs to his essence. — Proof of the major; because, if he were God through one thing, and this God through another, the divine nature would have to be contracted by a haecceity or individual difference, and so there would be in God a certain composition, and not supreme simplicity. Therefore.

The second argument is drawn from the infinite perfection of God. God is an infinitely perfect being. But there cannot be two or more infinitely perfect beings. Therefore. — Proof of the minor. If there were two infinitely perfect beings, they would have to differ from each other. But that by which they would differ would be either an imperfection — and this cannot be said, because an infinitely perfect being admits no imperfection — or it would be a perfection, and neither can this be said, since otherwise the one would lack a perfection that the other would have, and thus that first one would not be infinitely perfect. Therefore. — Akin to this is this other argument. God is the highest being, the highest good. But what is highest has neither anything greater nor anything equal to it, otherwise it would not be highest. Therefore, one.

The third argument is drawn from the unity and order of the universe. By the name "God" we understand the supreme ruler and governor of the world. But the governor of the world is one alone. Therefore God is one alone. — Proof of the minor. The diverse and unequal beings of the world, some higher, some lower, are so ordered that some serve and are coordinated with others by a fixed law. But they cannot come together in this way into one order unless they are ordered by one, because it belongs to one thing per se to be the cause of one, and many things are not the cause of one except accidentally, namely insofar as they are in some way one.

This argument is pressed further. If there were two or more gods, either one would suffice for the governance of the whole universe, such that he alone would do entirely and in every way what the other does; or he would not suffice, but would need the help of the other, such that one would do one thing or in one way, and the other would do another thing or in another way. If the first, then that other is superfluous; but nothing is more absurd than to posit a superfluous God. If the second, then neither is God, because neither is omnipotent, neither is the maker of all things, and the ordering of things — which is the most perfect thing in the whole universe — is an accidental effect, namely, from many causes fortuitously concurring into one.

Likewise, either one would necessarily agree with the other in the governance of the universe, such that he could not dissent from the other, or he would be free and could dissent from him. If the first, then he is not God, since he is dependent on the other and subject to him. If the second, in the case where he should dissent and wish to dispose of the things of the universe in another way, either one would prevail in this war and subject all creatures to himself, and thus the other, unwarlike and conquered, would not be God; or neither would prevail, but each would subject to himself a certain portion of creatures, and thus neither would be God, because neither would be omnipotent, neither would subject all creatures to himself, and the order of the universe would be dissolved. Therefore it is manifestly necessary to posit a single governor of the world.

As for what might be objected concerning the three divine persons, this does not stand in the way, because they govern through one and the same nature, one will, and one and the same omnipotence.]