SUMMA THEOLOGICA. Thomas Aquinas
His anger.
Reply to Objection 3: Forms which can be received in matter are individualized by matter, which cannot be in another as in a subject since it is the first underlying subject; although form of itself, unless something else prevents it, can be received by many. But that form which cannot be received in matter, but is self-subsisting, is individualized precisely because it cannot be received in a subject; and such a form is God. Hence it does not follow that matter exists in God.
Whether God is the same as His essence or nature?
Objection 1: It seems that God is not the same as His essence or nature. For nothing can be in itself. But the substance or nature of God---i.e. the Godhead---is said to be in God. Therefore it seems that God is not the same as His essence or nature.
Objection 2: Further, the effect is assimilated to its cause; for every agent produces its like. But in created things the "suppositum" is not identical with its nature; for a man is not the same as his humanity. Therefore God is not the same as His Godhead.
On the contrary, It is said of God that He is life itself, and not only that He is a living thing: "I am the way, the truth, and the life" (Jn. 14:6). Now the relation between Godhead and God is the same as the relation between life and a living thing. Therefore God is His very Godhead.
I answer that, God is the same as His essence or nature. To understand this, it must be noted that in things composed of matter and form, the nature or essence must differ from the "suppositum," because the essence or nature connotes only what is included in the definition of the species; as, humanity connotes all that is included in the definition of man, for it is by this that man is man, and it is this that humanity signifies, that, namely, whereby man is man. Now individual matter, with all the individualizing accidents, is not included in the definition of the species. For this particular flesh, these bones, this blackness or whiteness, etc., are not included in the definition of a man. Therefore this flesh, these bones, and the accidental qualities distinguishing this particular matter, are not included in humanity; and yet they are included in the thing which is man. Hence the thing which is a man has something more in it than has humanity. Consequently humanity and a man are not wholly identical; but humanity is taken to mean the formal part of a man, because the principles whereby a thing is defined are regarded as the formal constituent in regard to the individualizing matter. On the other hand, in things not composed of matter and form, in which individualization is not due to individual matter---that is to say, to "this" matter---the very forms being individualized of themselves---it is necessary the forms themselves should be subsisting "supposita." Therefore "suppositum" and nature in them are identified. Since God then is not composed of matter and form, He must be His own Godhead, His own Life, and whatever else is thus predicated of Him.
Reply to Objection 1: We can speak of simple things only as though they were like the composite things from which we derive our knowledge. Therefore in speaking of God, we use concrete nouns to signify His subsistence, because with us only those things subsist which are composite; and we use abstract nouns to signify His simplicity. In saying therefore that Godhead, or life, or the like are in God, we indicate the composite way in which our intellect understands, but not that there is any composition in God.
Reply to Objection 2: The effects of God do not imitate Him perfectly, but only as far as they are able; and the imitation is here defective, precisely because what is simple and one, can only be represented by divers things; consequently, composition is accidental to them, and therefore, in them "suppositum" is not the same as nature.
Whether essence and existence are the same in God?
Objection 1: It seems that essence and existence are not the same in God. For if it be so, then the divine being has nothing added to it. Now being to which no addition is made is universal being which is predicated of all things. Therefore it follows that God is being in general which can be predicated of everything. But this is false: "For men gave the incommunicable name to stones and wood" (Wis. 14:21). Therefore God's existence is not His essence.
Objection 2: Further, we can know "whether" God exists as said above ( Question [2], Article [2]); but we cannot know "what" He is. Therefore God's existence is not the same as His essence---that is, as His quiddity or nature.
On the contrary, Hilary says (Trin. vii): "In God existence is not an accidental quality, but subsisting truth." Therefore what subsists in God is His existence.
I answer that, God is not only His own essence, as shown in the preceding article, but also His own existence. This may be shown in several ways. First, whatever a thing has besides its essence must be caused either by the constituent principles of that essence (like a property that necessarily accompanies the species---as the faculty of laughing is proper to a man---and is caused by the constituent principles of the species), or by some exterior agent---as heat is caused in water by fire. Therefore, if the existence of a thing differs from its essence, this existence must be caused either by some exterior agent or by its essential principles. Now it is impossible for a thing's existence to be caused by its essential constituent principles, for nothing can be the sufficient cause of its own existence, if its existence is caused. Therefore that thing, whose existence differs from its essence, must have its existence caused by another. But this cannot be true of God; because we call God the first efficient cause. Therefore it is impossible that in God His existence should differ from His essence. Secondly, existence is that which makes every form or nature actual; for goodness and humanity are spoken of as actual, only because they are spoken of as existing. Therefore existence must be compared to essence, if the latter is a distinct reality, as actuality to potentiality. Therefore, since in God there is no potentiality, as shown above ( Article [1]), it follows that in Him essence does not differ from existence. Therefore His essence is His existence. Thirdly, because, just as that which has fire, but is not itself fire, is on fire by participation; so that which has existence but is not existence, is a being by participation. But God is His own essence, as shown above ( Article [3]) if, therefore, He is not His own existence He will be not essential, but participated being. He will not therefore be the first being---which is absurd. Therefore God is His own existence, and not merely His own essence.
Reply to Objection 1: A thing that has nothing added to it can be of two kinds. Either its essence precludes any addition; thus, for example, it is of the essence of an irrational animal to be without reason. Or we may understand a thing to have nothing added to it, inasmuch as its essence does not require that anything should be added to it; thus the genus animal is without reason, because it is not of the essence of animal in general to have reason; but neither is it to lack reason. And so the divine being has nothing added to it in the first sense; whereas universal being has nothing added to it in the second sense.
Reply to Objection 2: "To be" can mean either of two things. It may mean the act of essence, or it may mean the composition of a proposition effected by the mind in joining a predicate to a subject. Taking "to be" in the first sense, we cannot understand God's existence nor His essence; but only in the second sense. We know that this proposition which we form about God when we say "God is," is true; and this we know from His effects ( Question [2], Article [2]).
Whether God is contained in a genus?
Objection 1: It seems that God is contained in a genus. For a substance is a being that subsists of itself. But this is especially true of God. Therefore God is in a genus of substance.
Objection 2: Further, nothing can be measured