Good metaphysical arguments don’t operate in a vacuum. They occur within a theoretical framework provided by a successful, time-tested research program. The oldest and most successful research program in metaphysics is that of the so-called perennial philosophy, beginning with Plato and Aristotle, extended by the Neo-Platonists, and developed in Western scholastic philosophy. At the core of this program is the distinction between two modes of being, potential and actual, along with a commitment to a strong principle of proportionate causation, that is, the principle that the greater the effect, the greater the cause must be. Many contemporary philosophers have defended this program (myself included).
 
Another important assumption of the perennial philosophy concerns the dependent nature of time. Time is not merely a static dimension within which events and states can be located. Such a Block Universe picture of time would leave us with many inexplicable data, including the irreversible direction of time and causation, the fixity of the past and the openness of the future, the basis of the Second Law of Thermodynamics and other irreversible laws, and our universal experience of the passage of time. Aristotle provides a much more satisfying account of time in Book III of his Physics: time is simply the measure of change. Change is the more fundamental phenomenon, and the distinctive characteristics of time are derivable from the nature of change.
 
This hypothesis requires that change itself not be given a real definition in terms of time. That is, we must reject Bertrand Russell’s At-At theory of change (Russell 1922, Lecture VI), according to which a thing x undergoes change just in case it has one feature at some time t1 and a contrary feature at some later time t2. Instead, we must define change as Aristotle does. A thing x is undergoing change just in case there is some feature F of such a kind that x has a potentiality for F-ness that is in some degree of partial actualization. This definition does not make any reference to moments of time or their temporal relations of earlier and later. However, it does entail that if some entity x is undergoing change with respect to F-ness, there must be earlier and later times of such a kind that x is progressively closer to F at the successively later moments of time. Partial actualization requires at least two distinct modes of being (i.e., instants of time), one in which x is (still) only potentially F, and another in which it is actually F. In fact, there must be an infinite number of such instances, each with a different degree of actuality of x’s F-ness, with the full actualization of x’s potential for F-ness occurring in exactly one of these. The direction of time is determined by the prior direction of change: if x’s potentiality for F-ness is partially actualized, and this partial actualization corresponds to a set of moments, then the later moments in that set must be ones in which x is closer to being F.
 
Now let’s add to this picture the assumption that all change must have a cause. Another word for ‘change’ in this context is ‘passion’. Let’s assume, then, that every passion has a corresponding action. On this picture, the action of the agent is the cause; the passion of the patient is the effect. Causation always involves two or more substances. Here we will also introduce a principle of proportionate causality. It is obvious, for example, that actual change or passion can only result from actual action, involving an actually existing agent and an actually possessed active power. A merely potential event cannot be the actual cause of any actual change. A merely potential agent cannot act.
 
If we were to abandon this principle of ontologically proportionate causality, we would have no explanation for the asymmetry and irreflexivity of causation. That is, we couldn’t explain why a given passion couldn’t be its own cause, promoting itself from mere potentiality to actuality. This would be tantamount to rejecting the causal principle altogether.

It is important to recognize that accepting this Aristotelian framework does not depend on deciding the A Theory/B Theory issue. In particular, it does not depend in any way on so radical a thesis as Presentism—the view that everything that is actual is actual at the present time. It is compatible with Aristotelianism that there be infinitely many different modes of actuality, one for each moment of time, past, present and future. All that is required  is the assumption that to effect a change that is actual in mode t, the agent must have a power that is actual in that same mode, i.e., at that same time.
 
This viewpoint would not be consistent with a non-Aristotelian version of the B Theory—one in which every event at every moment of time is actual in exactly the same way. On such a Block Universe model, there is no room for defining change as the actualization of a specific potential. Such a model, as J. M. E. McTaggart long ago noted, fails to take seriously the reality of change.
 

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.

​The key to a dynamic (Aristotelian) B Theory of time is a notion of relative potentiality. Some things are potential relative to one event or point in time but no potential relative to others. So, for example, my winning the Noble prize in 2020 is potential relative to some point in my life (say 1960) but clearly it is potential relative to the present (Dec. 31, 2020). We can use relative potentiality to define relative actuality: an event E is actual relative to time t if it is absolutely actual and nothing is potential relative to t that is not also potential relative to E.
 
But what can it mean to say that one event is potential relative to a time or another event? Isn't potentiality a simple property and not a binary relation? The right B-Theoretic answer, I think, is to say that 'being' is said in many ways (as Aristotle says in Metaphysics Gamma 2). There are many ways of 'being' potential. These different modes of being correspond to different moments in time.
 
How do substances actualize potentialities? If a substance has an active power at t, and some patient has at all times prior to t the potentiality relative to those times of being changed in the appropriate way, then it is possible for t to be a time at which the agent exercises its power on the patient, actualizes the patient's potentiality. Suppose this involves the patient's becoming A (where A is some contingent accident). The patient's being A at t is absolutely or eternally actual. Nonetheless, it is actual because of the agent's exercise of its active power at t, and because of the patient's potentiality for being A in the preceding interval of time. Thus, some actual facts are causally explained by others, which corresponds to their location in time.
 
Formally, we can model this dynamic B Theory by using a series of trees. Each tree has a trunk (representing the past and present) and a set of branches, representing the potential futures. Each moment of time has its own tree, representing what is potential and what is actual relative to that time. One moment of time is later than another just in case it's tree is smaller, in the sense that the later tree will have a longer trunk and fewer branches. As time passes, branches (representing relative potentialities) fall off the tree (to use Storrs McCall's vivid image). We can then define two kinds of future tense: that which might yet happen (with reference to the present moment's future branches) and that which will actually happen (with reference to future trees).