Summa contra Gentiles1.22 Chapter XXII. That in God existence and essence are the same

From what has been shown above, we may go on to prove that in God essence or quiddity is not distinct from His existence.

For it has been shown above that there is a thing which exists of itself necessarily, and this is God. Now necessary existence, if it belong to a quiddity which is not that existence itself, is either inconsistent with or repugnant to that quiddity, as per se existence is to the quiddity of whiteness, or else is consistent or akin thereto, for instance that whiteness exist in some other thing. In the former supposition it will not belong to that quiddity to exist per se necessarily, for instance it becomes not whiteness to exist per se. In the second hypothesis, either this existence must be dependent on the essence, or both of them on some other cause, or the essence on the existence. The first two are in contradiction with the very notion of necessary per se existence: for if it depend on something else, it no longer exists necessarily. From the third supposition it follows that this quiddity is added accidentally to the thing which exists per se necessarily: because whatever follows on the essence of a thing is accidental thereto. Therefore God has not an essence distinct from His existence.

Against this, however, it might be urged that this existence does not depend absolutely on this essence, and in such a way that it would not be at all unless the essence were: but that it depends as regards the conjunction 1 Ch. xvi. 2 Ch. xiii.

whereby they are united together. And thus this existence is per se necessary, while the conjunction is not per se necessary.

But this answer does not avoid the above impossibility. For if this existence can be understood without this essence, it will follow that this essence is related accidentally to this existence. Now this existence is that which exists per se necessarily. Therefore this essence is related accidentally to that which exists per se necessarily. Therefore it is not its quiddity. But God is that which exists per se necessarily. Therefore this existence is not God's essence, but something subsequent thereto. On the other hand if this existence cannot be understood apart from this essence, then this existence depends absolutely on that on which depends its conjunction with this essence: and thus the same conclusion follows.

Further. Each thing exists by its own existence. Wherefore that which is not its own existence does not exist per se necessarily. But God exists per se necessarily. Therefore God is His own existence.

Moreover. If God's existence is not His essence; and it cannot be a part of Him, since the divine essence is simple, as shown above it follows that this existence is something besides His essence. Now whatever is becoming to a thing besides its essence, is becoming to it through some cause: for those things which are not one per se, if they be united together, must needs be united through some cause. Therefore existence is becoming to that quiddity through some cause. Either, then, this cause is something essential to that thing, or the essence itself, or else it is some other thing. If the former; and the essence exists according to that existence; it follows that a thing is a cause of its own existence. But this is impossible, because according to the understanding the cause exists before the effect; and consequently if a thing is the cause of its own existence, it would be understood to exist before having existence, which is impossible:—unless it be understood that a thing is the cause of its own accidental existence, which is a relative existence. For this is not impossible: for we find an accidental being caused by the principles of its subject, before the substantial being of the subject is understood to exist. Now, however, we are speaking, not of accidental, but of substantial existence. If, on the other hand, existence be becoming to the essence, by reason of some other cause; then whatever acquires existence from another cause, is caused and is not the first cause: whereas God is the first cause, having no cause, as shown above. Wherefore this quiddity that acquires existence elsewhere is not the quiddity of God. Therefore it is necessary that God's existence be His own quiddity.

Moreover. Existence denotes a kind of actuality: since a thing is said to exist, not through being in potentiality, but through being in act. Now everything to which an act is becoming, and which is distinct from that act, is related thereto as potentiality to act: since act and potentiality are reciprocal terms. Accordingly, if the divine essence is distinct from its existence, it follows that His essence and existence are mutually related as potentiality and act. Now it has been proved that in God there is nothing of potentiality, and that He is pure act. Therefore God's essence is not distinct from His existence.

Again. Whatsoever cannot exist unless several things concur, is composite. Now no thing in which essence and existence are distinct from one another can exist except several things concur, to wit its essence and existence. Therefore every thing, in which essence and existence are distinct, is composite. But God is not composite, as proved above. Therefore God's existence is His essence.

Further. Everything exists through having existence. Therefore nothing the essence of which is not its existence, exists by its essence, but by participation of something, namely existence. Now that which exists by participation of something cannot be the first being, because that in which a thing participates in order to exist, is previous to that thing. But God is the first being, to which nothing is previous. Therefore God's essence is His existence.

This sublime truth Moses was taught by the Lord: for when he asked the Lord (Exod. iii. 13, 14): // the children of Israel should say to me: What is His name? what shall I say to them? the Lord answered: 1 am who am.... Thus shall thou say to the children of Israel: he who is hath sent me to you; thus declaring His own name to be: he who is. Now every name is appointed to signify the nature or essence of a thing. Wherefore it follows that God's very existence itself is His essence or nature.

Moreover. The Catholic doctors have professed this truth. For Hilary says (De Trin.): Existence is not an accident in God, but the subsisting truth, the abiding cause, and the natural property of His essence. And Boethius says (De Trin.) that the divine substance is existence itself, and all other existence proceeds therefrom.