Does the God of classical theism exist? The principal reason for thinking that God fits the profile defined by classical theism is this: we have good metaphysical grounds for believing both that some entity that fits this profile exist, and we have good theological grounds for believing that, if such an entity exists, it must be identical to God, that is, to the God of the Bible, the God of Abraham, Isaac, and Jacob. 
 
So, what is the profile of the God of classical theism? There are four crucial characteristics:
 
1.God is absolutely the first cause of all causable things.
2.God is atemporal.
3.God lacks any intrinsic, passive potentiality.
4.God has no distinct nature and no distinct act of existence: He is identical to His own act of existence, which is also His nature or essence.
 
It is noteworthy that this list does not contain any of the traditional omnis: omniscience, omnipotence, or omnibenevolence. I haven’t included personality on the list, nor such properties as having knowledge or will. I haven’t included God’s infinity or perfection or uniqueness. This is not because classical theism is silent on any of these points, but because, in the classical-theist tradition, all these divine characteristics follow from the four that I have listed. They are, from an epistemological point of view (that is, in the order of human understanding), secondary characteristics of the God of classical theism. These secondary characteristics are not points of controversy between classical and non-classical theists; in contrast, all of the primary characteristics on the list are quite controversial.
 
There are, therefore, two tasks which the classical theologian must complete: first, to argue successfully that some entity satisfies the four primary characteristics, and, second, to demonstrate that any being with the primary characteristics must also have the familiar secondary characteristics.

This is the crux of Thomas’s whole natural theology—it is the main conclusion toward which the first Four Ways point, and it is the fulcrum from which Aquinas moves our theology toward God’s perfection and infinity. In this respect, Aquinas’s natural theology is unique. I don’t know of anyone, earlier or later, who proceeds in the same way. Duns Scotus, for example, drops the First Way entirely and relies primarily on God’s infinity, deduced from his version of the Second, Third, and Fourth Ways.
 
In his masterful The Metaphysics of Theism, Norman Kretzmann elucidates Thomas’s arguments in Book I of the Summa Contra Gentiles. In the chapter on simplicity, Kretzmann distinguishes two “interpretations” of Thomas’s claim that God’s essence is His existence: the cautious and the bold. On the cautious interpretation, we claim only that God’s essence entails (all by itself) that God exists. On the bold interpretation, we claim that God’s essence is identical to His act of existence. Kretzmann recognizes that Aquinas clearly endorses the bold interpretation. The only question is whether his arguments support this bolder and stronger claim.
 
Kretzmann begins with the argument that he calls G6, which is a version of Aquinas’s Third Way. So, let’s assume that God is that thing which is necessary per se and not through another. Kretzmann suggests that a being whose essence entailed its existence would qualify as necessary per se. He compares the existence of God with the existence of certain mathematical entities, like the number zero or the empty set, whose nature seems to guarantee that they “exist” in some sense in every world. Yet clearly in none of these cases are the essences identical to their act of existence.
 
Kretzmann thinks that Aquinas’s strongest argument for the bolder claim is the argument from potentiality and actuality. That is, if we think of essences as representing the potential existence of something, and the act of existence as the actuality of that potential, then essences must be thought of as passively receiving existence from something else. Since a thing can’t exist until its essence has received such existence, nothing of this sort could be uncaused. Therefore, since God is uncaused, His essence cannot receive existence. So, He could exist only if His essence already was an act of existence.
 
Of course, this presupposes that we have accepted the essence/existence to potentiality/actuality correspondence. This turns, I think, on seeing Thomas’s theory as a theory of actuality. It is acts of existence that actualize possibilities. Mere essences, sans such acts, are thus mere potentialities for existence.
 
In addition, one could question Kretzmann’s claim that something could be necessary per se by having an existence that is “entailed by” but not identical to its essence. His examples are mathematical, and one could argue that such mathematical things derive their necessary existence from God. If God were not the ultimate, necessarily existing Mathematician, could things like numbers or sets really exist?
 
And, in fact, do numbers and sets exist at all, in the relevant sense. We are looking for something exists necessarily and without cause, and which exists with the causal power to create other things. Mathematical objects are causally inert. Alexius Meinong suggested that they merely “subsist” in a shadowy realm of mental objects, in contrast to the full-bored existence of God and concrete creatures. Arguably, the existence of something could be explained by an essence only by being caused by it. So, if God is uncaused, His existence cannot be explained or “entailed” by His essence, unless they are one and the same thing.
 
1. God is identical to His own Essence (SCG I.21 and STh Q3 a3)
 
The first step in Aquinas’s argument is to suggest that God is, in a way, identical to His own form, as are angels. In fact, this already follows from the fact that God contains no matter, as Thomas argues in the Summa Theologiae. Without matter, there is nothing in God to individuate Him from other things with the same kind of form. Hence, God must be identical to His form simpliciter.
 
In the SCG, Thomas also appeals to the fact that God lacks composition. If God were not identical to His own essence, then there would have to be something (matter or accidents) that have been added to His essence, resulting in His having “parts” of a kind (i.e., metaphysical parts).
 
He also argues (in par. 5) that, if something is not identical to its form, then that form is a kind of cause of the thing. The thing would then depend for its existence on the form. Since God depends on nothing, He must be identical to His own form, and so to His own essence.
 
In par. 6, he appeals to the potency/act distinction, arguing that whatever is outside an essence is a mere potentiality that is actualized by the essence (or form). I think the picture is that the parts of things (matter, accidents) that are not identical to a form receive their existence through the form. But, since God contains no passive potentialities (chapter 16), He must be pure form/essence.
 
2. God is identical to His own act of Existence (SCG I.22, STh Q3 a4, De Ente par. 80-3)
 
In the Summa Theologiae, Aquinas proves that God’s essence is identical to His existence. Since we have already established that God is identical to His essence, this entails that God is also identical to His act of existence.
 
The main argument appeals to causation. If a thing has something beyond its essence, then this thing must be caused either by that essence or by something else. So, if God has existence as something “beyond” His essence, then either that existence is caused by the essence or by something else. However, God’s existence is absolutely uncaused. So, God’s existence cannot be “beyond” his essence. As Kretzmann pointed out, this argument doesn’t show that God’s existence must be identical to His essence. It could be something entailed by His essence without being identical to it or caused by it.
 
Thomas goes on in article 4 to argue that the existence of a thing cannot be entailed by its essence, since this would mean that the thing had caused itself to exist. For the essence to do any causing, the thing would have to already exist. Hence, the essence of a thing cannot cause the existence of that very thing.
 
But could the essence “entail” the existence without causing it? Aquinas is assuming (reasonably) that something can explain or entail the existence of a concrete thing with causal power only by causing it to exist. Hence, the essence of a thing cannot be the ultimate explanation for its existence.
 
Aquinas’s second argument is the appeal to potency and act that I discussed above.
 
In his third Summa Theologiae argument, Aquinas argues that if something has existence but is not its own existence, then it enjoys only “participated” (i.e., caused) existence. This is I think an appeal to the Fourth Way. The highest form of existence would be to be simply identical to pure, unadulterated existence. This would be possible only if one’s essence was identical to one’s existence.
 
In SCG I.22, paragraph 2, Aquinas argues in the following way. First, he shows that God’s essence must be compatible with existence. Given that, Aquinas argues that there are just three alternatives: either God’s existence depends on His essence, or both depend on some third thing, or the essence must depend on the existence. One might push back: why couldn’t both the existence and essence be independent, uncaused things? I think that Aquinas would argue that, whenever essence and existence are distinct, then the two must be dependent on each other, or on some third thing. An essence depends on the existence in order to exist, and the existence depends on the essence for its identity, tied as that is to the character of the thing whose existence it is.
 
If God’s existence depended on His essence or some third thing, then God’s existence couldn’t be uncaused, contrary to the Second and Third Way. So the case to consider is that in which God’s essence depends on His existence. However, if God’s essence could depend on God’s existence, then we could make sense of God’s existence without considering His essence. Here again we have to turn to the actuality/potentiality distinction. If essence and act of existence are distinct, then the act of existence must actualize the possibility represented by the essence. The act of existence cannot both bring into being a possibility and then actualize that some possibility. An act of existence cannot do anything prior to its actualizing of an essence—it is only after actualizing an essence that it can be said to have a nature that could bring about anything.

I've recently discovered that my views on prime matter as the universal individuated can provide new support for the real distinction between essence and existence. As I've explained in earlier posts, I believe that packets of prime matter are the ground for any numerical distinction among substances of the same species, whether those substances exist at the same time or at different times. Imagine, for example, a sempiternal world with an infinite past and infinite future. Let's suppose that Nietzsche's eternal recurrence is realized in this world, so that there are an infinite number of distinct but indiscernible Napoleon Bonapartes, one for each cycle. Each temporal stage one each Bonaparte has its own, unique packet of prime matter. It is the distinctness of these packets that ground the distinction between different Bonapartes, and it is the distinction between the Bonapartes that distinguishes the accidents of time during which they exist.
 
This individuating role of prime matter extends also to merely potential beings. Consider, for example, some merely potential pair of identical twin daughters of my wife and me (we did not in fact have any such twins). There are surely some potentially existing pairs of twins, and, within each pair, each twin must be numerically distinct from the other. This numerical distinction between potentially existing daughters must be grounded by the numerical distinctness of the potentially existing packets of prime matter associated with each daughter at each moment of her potential existence.
 
Now, each of these potentially existing daughters is potentially a human being. This modal fact about the twins must be grounded in the potential existence of two substantial forms of the human species. Thus, there must be potential substantial forms as well as actual ones. What is the difference between the two sorts of forms? The actual forms transmit actual existence to their substances, while merely potential forms do not. Hence, there must be acts of existence (actus essendi) which are present in the one case and absent in the other. Thus, we get a real distinction between essence (form + matter) and existence (acts of esse).
 
Those (like Averroës) who reject the real distinction must suppose that every form is an actual form, a source of actual existence. This means that they must deny that merely potential beings are in any sense real. In particular, they must deny that there are potentially existing twin daughters (as described above). They must say, instead, that there could have been such daughters. However, they cannot provide any ground or truthmaker for such a modal truth.