Summa Theologiae, I, Q49, a1 and a2; I-II, Q9, a6; I-II, Q75, a3; I-II Q79, a1
 
In Part I, Q49, a1, Thomas explains that evil has no per se efficient cause, but it does always have a per accidens efficient cause. Every evil act or situation is good insofar as it exists, and something must cause per se that element of goodness. However, there need be no per se cause of the deficiency that is evil in the thing. When something good causes something bad, there is always some pre-existing deficiency of some kind, either in the agent or the patient, or both.
 
Since there is no defect in God, when God causes something that is evil, there must be some pre-existing defect in the patient. As Thomas puts it in reply 2, “And, likewise, whatever there is of being and action in a bad action, is reduced to God as the cause; whereas whatever defect is in it is not caused by God, but by the deficient secondary cause.”
 
When Thomas explains, in I-II Q9, a6, how God moves the human will, he points to two ways. First, God causes our nature, which includes the free will and its natural inclinations. And, second, God is absolute goodness—so every apparently good thing that moves the will does so because of its apparent participation in God’s goodness. In neither of these ways is God responsible for our sin.
 
In I-II Q75, a3, Thomas insists that nothing external to the will can necessitate sin. God is the cause of the movement of the will that is sin, but God (in His essence) does not necessitate that the will choose badly.
 
In I-II Q79, a1, “Whether God is a cause of sin,” Thomas summarizes his conclusions:
 
“Now God cannot be directly the cause of sin, either in Himself or in another, since every sin is a departure from the order which is to God as the end: whereas God inclines and turns all things to Himself as to their last end, as Dionysius states (Div. Nom. i): so that it is impossible that He should be either to Himself or to another the cause of departing from the order which is to Himself. Therefore, He cannot be directly the cause of sin.”

Summa Theologiae I, Q19, a4
 
Here Thomas is primarily interested in proving that God causes the universe by Will, and not by the necessity of His nature. His target is the Neo-Platonic tradition, who talked about finite things as “proceeding” from the One, rather than being intentionally created by God. Thomas argues that the first agent must act by intelligence and will and not simply by the necessity of His nature. Like all agents, God acts for the sake of an end. Since no end demands His choice, beyond His own existence, this means that all finite creatures are freely chosen for the sake of the proliferation of divine goodness.
 
Thomas appeals to Question 7, article 2, to the effect that God’s effects cannot be undetermined and indefinite. Thomas here anticipates a view recently defended by Notre Dame philosophy Peter van Inwagen. Van Inwagen proposes that God wills indeterminate facts, which the creation makes definite by chance. E.g., God could dictate that a certain coin should land either Heads or Tails, without choosing either option. When the coin falls one way or another, the determinate effect will exceed God’s intention in specificity. Similarly, van Inwagen imagines that the early history of the universe might be the result of chance filling in the indefiniteness of God’s decree. In Question 7, Thomas denies that God could give any created thing an indeterminate nature, since forms (both substantial and accidental) are always definite, and all forms come from God.
 
Is the Will of God always Fulfilled?
 
Summa Theologiae I, Q19, a6
 
No effect can escape the scope of God, who is the universal cause.
 
But, doesn’t God desire for all men to be saved? (Objection 1, appealing to I Timothy 2:4) Thomas’s best response is to distinguish between God’s “antecedent” and “consequent” will. Take some particular man, like Judas Iscariot. Does God want Judas to be saved? Yes—other things being equal, it is always better for a human to be saved rather than damned. But, things are not always equal. So, God’s antecedent will (His will about some result considered by itself, in isolation, other things’ being equal) is for all to be saved, but His consequent will (all things considered, with things not being equal) is only for the salvation of the elect.
 
What God wills antecedently need not happen, but what He wills consequently always happens.
 
Does God’s Will Abolish Contingency from the Created World?
 
If everything happens according to God’s will, in what sense can things be “contingent” in the created world. Things that God creates are absolutely contingent, in the sense that He could have chosen not to create them. But, given that He does will that a certain event or action happens, it is necessary that it should happen (as we saw in the last section). We might put it this way: our creaturely actions ever contingent in their actual circumstances (this is sometimes called historical contingency)? Is there any room for the creature to make its own contribution to what happens, a contribution that is in some sense independent of God’s will?
 
There is an important sense in which my action is free and undetermined in its actual circumstances (historically contingent). If I exercise free will, then my choice is not determined by the whole history of the world up until that point, including my current psychological state at the moment of acting. There are possible worlds in which exactly the same events and conditions obtain as obtain in the actual world, and in which I make a different choice. This doesn’t require changing the natures, powers, potentialities, or accidents of anything in creation, including myself.
 
But, of course, this does not make my free choice independent of God’s will. I cannot choose in a way that is contrary to the way that God wills that I choose. Doesn’t that make me unfree? I don’t think so. God’s will about my choice is also undetermined by the past. At the moment of my choice, both God’s will and my will are undetermined. They are jointly contingent—either of us could make either choice. My choice is necessarily caused by God’s will, but God’s will is not necessitated by the past. God’s will is free to cause me to act in either way that is open to me.
 
Of course, it is impossible for God to will that I choose A and for me to choose B, or vice versa. However, there are two live options: (i) I choose A and God wills that I choose A, or (ii) I choose B and God wills that I choose B. Is the choice between these two options in any sense “up to me”? I think it is. It is also up to God. The choice is 100% up to me and 100% up to God. The relation between God’s will and the will of a creature is a unique sort of relation, quite different from the relation between any two creatures. Crucially, although my choice of A (if I do choose A) is contingently caused by God’s will, but it isn’t caused by the fact that God wills that I choose A. The fact that I chose A and the fact that God willed that I choose A are the very same fact, looked at from two different perspectives. Facts cannot cause themselves, so my choice is not caused by God’s choice, nor is God’s choice caused by my choice.
 
Is my choice nonetheless explained asymmetrically by God’s choice? In some cases, Yes, and in other cases No. It may be that my choice plays some important role in God’s providence, in which case my free choice will be explainable in terms of that role (think of Mary’s sinlessness or Judas’s betrayal). But in most cases, God wills that I make a certain choice simply because that is the sort of choice that I would be inclined to make from time to time. God’s permissive willing is a kind of fiduciary willing: He wills what, putting Himself in our place, it makes sense for us to choose freely.
 
An analogy that I like to use is the relationship between a human author, like J. R. R. Tolkien, and one of his fictional characters, like Frodo. Does Frodo act with free will? Yes, I think he does. Does this make him independent of Tolkien’s choices? No. It’s not as if Tolkien had to go into a trance and let the spirit of Frodo move his pen or his typewriter in order to exercise his free will. Frodo decided freely to accept the One Ring, and Tolkien willed that Frodo should so decide. The two cases of free agency are compatible.
 
Article 12 is quite relevant to this point. In that article, Thomas distinguished five ways in which God wills things: prohibition, precept, counsel, operation, and permission. The most relevant distinction at this point is between operation and permission. If I choose to sin, God does not operationally will that I do so—instead, He permissively wills so. This does not mean that God wills only that I should sin or not sin, and then leave the result up to me. God cannot will something indefinite or indeterminate. Instead, God permissively wills that I sin, and He does not will consequently that I not sin. He does will antecedently that I not sin (He wills antecedently that each person abstain from sin on every occasion), but His antecedent will is not effective. His consequent will, even when that will is permissive, does necessitate the result.
 
The other three forms of God’s will, like His antecedent will, are not universally effective. God expresses what we ought to do and not do by way of prohibition, precept, and counsel. It is still up to us (and to God’s consequent, permissive will) whether we do what God commands.

Does God have a Will?
Summa Theologiae I, Q19, a1, and SCG I.72
 
For Thomas, this pretty easy. Everything that has intellect also has will. Every substance is striving in some sense to perfect its own nature. If a substance is intellectual, then it moves towards its own perfection with understanding. But to move with understanding is precisely to move voluntarily, by the exercise of will. This will be especially true of God, since He understands Himself perfectly.
 
However, there is a problem: God is unmovable, immutable. So, He cannot be said to move toward His own perfection. He is essentially His own perfection. But when a substance rests in its own perfection, that perfection remains its natural end. And when an intellectual substance rests in its own perfection, that resting is resting with understanding, which is by definition the state of delight. So, God perpetually delights in His own perfection. But delight is a function of the will, so God has a will.
 
In the SCG, Thomas also points out (in paragraph 8), that God, as first agent, is supremely free. Nothing can move Him or make Him do anything. To act with free will is essential to being free, so God must have a will.
 
Does God will other things than Himself?
 
Summa Theologiae I, Q19, a2, and SCG I.75
 
So, God will His own perfection, and so He wills Himself. Does He also will other things? Given that God is essentially in a state of perfection, which is also a state of perfect happiness in which His will is at rest, is there any room for Him to will anything else? He has no needs or deficiencies to fill. He can’t be any happier than He would be if He willed nothing but Himself. No additional object of the will can add to His happiness, if ‘adding’ implies a kind of increase. (Objection 3)
 
In addition, if God willed more than His own essence, then His will would be divisible. Or, at the very least, His act of willing would be complex. But God is simple. (Objections 1 and 4)
 
Moreover, if He did will something else, it seems that the value of that other thing could be said to “move” His will by attraction. But God is immovable. (Objection 2)
 
In response, Thomas appeals to the self-dispersive nature of goodness. Good things (including especially good people) have a natural tendency to spread that goodness to others, even if doing so doesn’t add to their own happiness. To good to others is a sufficient reason in itself for acting. Since God is maximally good, this dispersive nature is especially true of Him.
 
Thomas makes the point in a slightly different way in SCG, paragraphs 3 and 4. We’ve established that God loves Himself. Whatever loves itself also loves to some degree the things that resemble. Every actual thing resembles God to some degree, so God naturally loves all actual things.
 
God wills Himself as the sole proper and immediate object of His will, but He wills other things insofar as their existence is a fitting effect of His perfect goodness. Just as He understands all possible things by understanding His own essence, so He wills all actual things by willing His own perfection. There is a single act of will in God, just as there is a single act of understanding. And these two acts are the same act.
 
In response to objection 2, Thomas argues that God is “moved” only by His own goodness, and so His immovability is not compromised.
God doesn’t will things in order to add to His happiness. He wills them simply because and insofar as it is good that they exist.
 
Thomas explains in SCG I.77 why God’s willing multiple objects is compatible with His absolute simplicity. God wills multiple things only insofar as they are included (“comprehended”) in His goodness, just as He knows multiple things by knowing Himself in His infinite power. In paragraph 5, Thomas argues that the multiplicity of objects of will is less of a threat to divine simplicity than multiple objects of understanding were, since the will acts for the sake of the goodness that is in the object, not for some value existing in itself. Hence, there is no obstacle to a simple will’s willing multiple objects

​Summa Theologiae I, Q14 Of God’s Knowledge
Article 1: Whether there is knowledge in God
 
Thomas relies heavily on the fact of God’s immateriality to prove that He is intelligent, i.e., a being with knowledge and understanding. In effect, he argues that any immaterial substance must be intelligent, because it is matter that it is the sole obstacle to intelligence. Remove matter, and you remove that obstacle.
 
One might criticize St. Thomas on the ground that he is confusing a necessary with a sufficient condition. Suppose that we grant that no material substance can be intelligent, because material substances can have only one substantial form, and they cannot have contrary accidents, while an intelligent being must be able to contain substantial forms other than its own, and must be capable of understanding (and, therefore, of containing) contrary accidents. But, from the premise No material thing is intelligent, it does not follow that Every immaterial thing is intelligent. An immaterial thing has one of the necessary conditions of intelligence (immateriality), but does it follow that it has a sufficient condition?
 
Thomas could argue that we do not know of any immaterial substances except for intelligent ones (angels, and the separated human soul). Perhaps we can’t even conceive of any such things. Nonetheless, it might be that such things are possible, even actual, lying beyond the bounds of our own understanding.
 
Of course, in God’s case, we know more than just that He is immaterial. We also know that He has the power to cause the existence of every possible kind of creature. Perhaps Thomas could argue that an immaterial substance can have an active power (like the power to cause things’ existence) only through the capacity for understanding and will. The forms of all possible creatures must pre-exist somehow in God, by virtue of the principle of proportionality. They cannot exist in God “naturally,” in the sense that God could actualize each of the forms in His own person, since many of the forms are mutually incompatible. Nothing can be black and white, or a blue whale and a daffodil. So, they must exist in God in some kind of “intentional” (non-natural way). And we could plausibly define understanding as simply being a thing that contains forms intentionally.
 
But what about unintelligent, material substances that have active powers? Take, for example, the sun’s power of heating the surface of the earth. As we saw in the Fifth Way, Thomas argues that such active powers cannot exist in unintelligent bodies except instrumentally, by virtue of existing primarily in some intelligent maker or user of the unintelligent thing. So, perhaps we could define understanding in this way:
 

• Substance x understands form F if and only if F exists in x intentionally and intrinsically (not by virtue of x’s being an instrument of some other substance).

 
And we could define ‘intentional’ existence of a form in a substance thus:
 

• Form F exists intentionally in substance x if and only F exists in x in some mode M such that it does not follow with metaphysical necessity that:  if some form G exists in some substance y in mode M, then G can be predicated truthfully of y.

 
Given these definitions, we can prove that God understands every form that could possibly exist naturally in any substance. Note that this argument relies heavily on God’s unity, as well as on His status as the necessary first cause of everything else. This helps to explain why Thomas postpones the discussion of God’s intelligence to this late point.
 
It’s also worth noting that Thomas does not appeal here to the Fifth Way at all. That suggests to me that he thought that this argument was stronger or more rigorous than the proof in the Fifth Way.
 
Summa Contra Gentiles I, Chapter 44 That God is Intelligent
 
In the SCG, Thomas offers six supplementary arguments. First, in paragraph 2, he appeals to the Aristotelian argument for God as Prime Mover. This argument seems to assume that the first Mobile thing is a self-mover, like the intelligent celestial spheres of Aristotle’s cosmology.
 
He offers an alternative version of this argument in paragraph 3, one that doesn’t rely on this outdated assumption. The Prime Mover must be responsible for all possible changes. This requires that the First Mover moves things through (per) some universal form, a form in a universal mode. This seems very similar to the interpretation I gave of the argument in the Summa Theologiae, an argument from a universal set of active powers to an understanding that is universal in scope.
 
In paragraph 4, Thomas argues that an intelligent being can never be the mere instrument of an unintelligent one. All intelligent creatures are mere instruments of God, so God must be intelligent. The first premise doesn’t seem obviously true to me. Couldn’t the sun heat the air around me by heating me? In response, Thomas could point out that it is my body, and not my soul, that is being instrumentalized in that case. The principle of proportionality might require that if God is the per se cause of changes in intellects, then He must be intellectual Himself.
 
Paragraph 5 is the appeal to God’s immateriality that I discussed above.
 
In paragraph 6, Thomas appeals to God’s containing all perfections. Assuming that intelligence is a perfection and not reducible to a set of merely physical and chemical powers, then God must be intelligent.
 
Paragraph 7 brings forward a version of the Fifth Way. At this stage he considers the argument worth mentioning, but he doesn’t put it forward as the first, much less the only, argument for God’s intelligence.
 
Paragraph 8 involves another appeal to God’s perfection. Thomas argues that when forms exist naturally in particular things, then they are imperfect. Thomas might have even more plausibly said that such forms are ‘finite’, since they are limited, either by matter or (in the case of angels) by some finite essence that distinguishes them from other agents. A form can exist in a perfect (and infinite) being only by existing in its intellect, as an object of understanding. All forms must exist in God, since otherwise He would be incapable of being the first cause of their natural instantiation. So, God must have the forms in His understanding.

By delaying this issue until Question 11 in the First Part of Summa Theologiae (and chapter 42 in the SCG, I), Thomas is indicating that it is easier to prove that there is at least one God than it is to prove that there is no more than one. It is also important to realize that the ‘one’ that we appeal to in proving God’s unity is not the number 1, but the concept of oneness that is convertible with being. God is supremely one precisely because He is supremely a being. We don’t count gods in the same way that we can count apples or doors. As we shall see, the nature of God excludes the very possibility of there being two or more gods. At the same time, since the oneness involved is not that of the numerical one, Thomas is leaving open the door for God’s being, in some sense, a plurality or multitude. This will help in working through both the problem of God’s ideas  and the Trinity.
 
In Summa Theologiae I, Q11, article 1, Thomas explains that the focal meaning of ‘one’ in ‘there is one God’ is that of being undivided. God is supremely indivisible, and this follows from His simplicity. Thomas argues that if oneness were not convertible with being, an infinite regress would result. If oneness is not equivalent to being, then it would have to be something that is added to being. But then we can ask what makes this addition one addition, and an infinite regress follows.
 
But what about multitudes, like crowds of people? If they exist, they must also be one. And, indeed, a crowd is one crowd. But how can something be both one and many? Isn’t that inconsistent? Thomas anticipates here an answer given much later by the German logician Gottlob Frege. A multitude is one in one way, and many in another. It is, for example, one crowd but many people. Thomas elaborates this point in article 2, again distinguishing between one as the principle of number and one as convertible with being. We can ask for the number of a crowd, and this question must appeal to some way of dividing the crowd—into families, or individuals, or human cells.  At the same time, a crowd must be, like anything that exists, in some more basic sense one thing. Some more recent metaphysicians, following the work of American logician George Boolos, have suggested that multitudes can exist without being one thing at all. Very large proper classes, like the class of all sets, for example, seems to be a real multitude that is not in any sense one thing. If this is right, it could create some difficulty for some of Thomas’s proofs (in article 3) for God’s oneness. Or, it might simply point to the fact that the oneness of God is consistent with His comprising a kind of multiplicity, so long as these things are not parts or attributes of God.
 
In objection 4, Thomas addresses the problem of the definition of one as undivided. Being undivided is a negative notion, signifying the absence of division. But being is perfectly positive. So, how can being be convertible with oneness? Thomas replies that division is prior to one only in the order of our understanding. Ontologically, being undivided is prior to being divided. It’s just that we are first aware of composite things before we are aware of their simple parts.
 
In article 3, Thomas offers three arguments for the oneness of God. The first argument appeals to God’s simplicity. God is made to be God by His divine nature, and that divine nature also makes Him exist as a particular being. For there to be two gods, there would have to be two divine natures, each of the same species. But for two natures to exist with the same species, there would have to be something responsible for making each distinct from the other. So, for example, two men can be two by virtue of being combined with two packets of prime matter. Two packets of prime matter have no actual nature of their own, and so they can be fundamentally or primitively distinct. The divine nature is an actual nature (it is maximally actual), and so two divine natures cannot be fundamentally distinct. Since God is identical to His own nature, there cannot be two instances of the divine nature, just as there cannot be two instances of a single angelic species.
 
In the second argument, Thomas appeals to God’s infinity. (This is a new argument, not present in the SCG.) In fact, he appeals to the infinity of God’s perfection, by which he means that nothing can be superior to God in perfection. Suppose that there were two such maximally perfect beings. In this argument, Thomas concedes (for the sake of argument) that there could be two distinct species of god. If there were two such species, something would have to differentiate them. One would have to have something that the other did not have. But this means that one would have to have some form of perfection that was lacking in the other. But God has all perfections. So, in fact, this argument actually appeals to God’s perfection, rather than His infinity.
 
Third, Thomas appeals to the apparent unity of the world. This is one of the relatively few cases in which Thomas appeals to some form of the Fifth Way—pointing to God as the cause of the world’s systematic harmony, the fact that the active and passive powers of the world’s created substances fit together in order to make a stable, scientifically intelligible universe. Thomas gives a more detailed version of this argument in Summa Contra Gentiles 1.42, paragraph 7.
 
In Summa Contra Gentiles 1.42, paragraph 5, Thomas also appeals to some details of Aristotle’s natural philosophy, especially the assumption that the movement of the heavenly spheres are regular and continuous. It’s not clear to me whether any of this can be salvaged, given the falsity of Aristotle’s astronomy.
 
In paragraph 8, Thomas argues that if there were two gods, at least one would have to be composite. But no composite being could be necessary through itself (as established in the Third Way). It’s not obvious here why one of the gods would have to be composite, but the two arguments that Thomas gives in the Summa Theologiae support this premise: either the two gods would belong to the same species, in which case each would have to have a part that individuates it from the other, or they would belong to two species, in which case each would have to add some differentia to their common genus.
 
Paragraphs 9-11 contain another interesting argument that appeals to God’s necessity of being. This argument involves a complex dilemma: if there were two necessary beings (each necessary per se), then either (1) the two differ by something required for the completion of the necessity of being, or (2) not.
 

• Assuming (2). (Paragraph 9) If not, then they are different by something accidental. This accident must have a cause (since all accidental additions must be caused), which is either (a) the essence of the necessary thing, or (b) some external thing. We can rule out (a), since both necessary beings would have the same accident, if it is a proper accident determined by their common essence. If (b), then the existence of two necessary beings depends on the action of this third thing. But then the existence of each thing would also depend on the third thing, and so neither could be necessary in itself.
•  
• Assuming (1). (Paragraph 10) That which differentiates the two necessary beings would be either (a) something included in the common nature of necessary being, or (b) something that differentiates the two natures into two species. Case (a) is impossible because in that case, the factor will be present in both necessary beings. In case (b), the common nature must be made actual by each of the two differentiae, in which case, each necessary being would derive its existence from something else. Hence, the two things cannot be necessary in themselves, nor could each be simple and identical to its own act of existence. This complex argument seems to be mirrored in the first two arguments in the Summa Theologiae. The SCG helps us to see that these are really two parts of a single argument, based on a dilemma (either one species of god or two).

 
Paragraph 12 involves a very simple argument, based on the thesis that God is identical to His nature and to His act of existence. If there were two gods, then each god would have the same divine nature. But this divine nature would then have to be identical to two distinct acts of existence. But nothing can be identical to two distinct things.
 
Paragraphs 13 and 14 contain another complex argument.
 

1. If x is a per se necessary being, then x’s necessary being belongs to x insofar as it is x. That is, the necessity of x is grounded by x’s being x.
2. Suppose there were two per se necessary beings, x and y. Then x’s being x would also consist in x’s not being y, since if x and y are distinct, x’s being x depends on its not being y.
3. Consequently, x’s necessity would belong to it insofar as x is distinct from y.
4. This means that x’s necessity depends on the existence of y.
5. Consequently, x’s necessity would not be per se. Contradiction
6. So, there cannot be two per se necessary beings.

 
Thomas defends premise 1 in paragraph 14. If x’s necessary being depends only on x, then x’s necessary being must belong to x insofar as it is x.
 
In paragraph 18, Thomas appeals to the fact that God is supreme being. Since being is convertible with oneness, God must be supremely one, and so undivided. This applies equally well to the divine nature.
 
Thomas also appeals in paragraph 20 to the superiority of monarchy as the form of government. Since God is the perfect governor of the universe, God must be one.

As I have argued, Thomas proposes that every actual being has its own act of existence (actus essendi). An act of existence is a real being, although it is not a substance. In this sense, the act of existence is something like an accident, since accidents are also real but non-substantial beings. Nonetheless, an act of existence is not an accident of its substance, since it is causally prior to that substance. The act of existence gives the substance actual existence. Substances do have (by virtue of their essences) a kind of being (potential being) that is independent of the act of existence. It is potential beings that make up the structure of mere possibilities (including Leibniz's 'possible worlds').
 
Do acts of existence themselves exist, and, if so, do we face an infinite regress, with each act of existence requiring a further act to give it existence? I think acts of existence do exist, but they exist in a way that is different from the way that substances and accidents exist. They exist simply by being acts of existence, while substances and accidents require distinct acts to give them actual existence. Substances and accidents can have potential existence, which is why they can be contained in mere possibilities. Acts of existence cannot have potential existence: they are intrinsically actual. Hence, no mere possibility contains any act of existence.
 
An act of existence must give whatever actuality a thing has. Hence, acts of existence, as such, are infinite, while all substances and accidents are finite. If acts of existence as such were finite in any way, then there would be possible entities that could not be actualized. But that is a contradiction in terms: to be a possible entity is to be possibly actualized. Hence, acts of existence must have the power to actualize everything, to the outermost limits of possibility. They must be intrinsically infinite.
 
Furthermore, if any act of existence were finite as an individual, then this finitude would define a particular kind of being. In other words, the act of existence would have a kind of essence built into it. But if the act includes such an essence, then it would make sense for the act to exist only in potentiality. But, as we've seen, this is impossible. Every act of existence must be actual.
 
Finally, existence itself can have no limit, since a limit implies some possible thing beyond the limit, but nothing can exist “outside” of existence. A limit is something that receives existence, and that limits the existence it receives. Existence itself cannot be or have a limit.
 
God is a pure act of existence, without any associated essence. Hence, God is absolutely infinite. He must possess every possible perfection without any limit whatsoever.
 
Now, infinity seems to be something negative—the lack of finite boundaries or limitations. However, when infinity is combined with God’s perfection, we do get something positive. God must have each perfection to a greater degree than any possible creature.
 
Infinity is, for Thomas, a complicated matter. For Aristotle, infinity is generally a bad thing. Being not-finite means lacking sharp boundaries. A non-finite thing is somewhat amorphous and shapeless, lacking any sharp definition. Shadows, clouds, or crowds would all be examples of in-finite, somewhat vague entities. Clearly none of this applies to God. God is definitely what He is, with no vagueness or amorphousness.
 
In addition, Aristotle generally associates infinity with potentiality. Nothing in the material world is ever actually infinite. A real line segment (say, one on the surface of a box), for example, does not contain an actual infinity of points. Instead, each of its internal points exists only potentially, since the line segment can be divided in any of an infinite number of quantitatively different ways. Similarly, the future is potentially but not actually infinite. We will never reach a day that is infinitely many days after this one. God, in contrast, is pure actuality. So, it seems that God’s essence and infinity should be incompatible.
 
But, of course, God’s infinity is not a quantitative infinity. He is not infinite by having an infinite number of parts, or by filling (in a physical way) an infinite volume of space or an infinite duration of time. The kind of finitude that material substances have is itself foreign to God. My form is made finite by my matter, since my matter is quantitatively limited prior to my generation. At the same time, my matter is also made finite by my form, since it is form that gives a definite size and shape to my matter. Thus, finitude emerges in the cooperation of matter and form.
 
There is nothing corresponding to matter in God. Hence, there is nothing that can put a limit to His “form”. And, conversely, there is nothing that could be limited by His form. Hence, God is in-finite.
 
When we turn to article 2 of Question 7, we see that creatures can be “relatively” infinite, but only God can be absolutely infinite. Matter, for example, is relatively infinite, in the sense that it is capable of taking on any substantial form for a material substance. A material substance can be relatively finite with respect to its possible accidents, like shape. Angels have no matter, and yet they too can be only relatively infinite, since their being (esse) is “received and contracted into a determinate nature.” Only God is pure, absolute being, and so only He can be absolutely infinite.
 
God’s infinity is an infinity of active power and maximum nobility. He and He alone has the full power of being itself.
 
In the Summa Contra Gentiles, I I.43, Thomas talks of God having an infinite “spiritual” magnitude. We shouldn’t read too much into the word ‘spiritual’ (spiritualis). The term is used in two quite different ways in scholastic philosophy. It can mean purely intellectual substances, like God, the angels, and the separated soul. But it can also mean the more ethereal and subtle aspects of the physical world. Light, for example, is sometimes classified as spiritual. So too are the ‘animal spirits’ which scholastic philosophers took to be circulating through the body (like our modern electrical impulses of the nervous system).
 
Thomas identifies two dimensions of spiritual magnitude: active power, and the goodness or completeness (perfection) of one’s nature. Clearly, God is infinite in both these respects.
 
In natural magnitudes, as we’ve seen, infinity is a kind of privation, a lack of definition and form. In God’s case, it is a pure, non-primitive negation, a simple absence of boundaries and limits, without any implication that those boundaries and limits ought to be there.
 
Things that have limits have those limits either due to the definition of their nature (reflected in their genus and species) or from something into which they are received. So, anything whose existence and essence are distinct must have a limit: the essence will have limits due to its definition, and the act of existence will be limited by being received by something limited (the essence). God has no definition or genus, and His existence is not received into any distinct nature. Hence, God’s existence is infinite.
 
Thomas also argues that God is infinite in the sense that He cannot be exceeded by anything else. Since He has no passive potentiality, nothing can be more actual than He is. (SCG 1.43, paragraphs 6-7, 9-11) He argues, in somewhat Platonic fashion, that nothing could be better than a being that is its own goodness (par. 9). He appeals to something like Anselm’s conception of God as the greatest conceivable thing in par. 10, arguing that there must be a greatest of all intelligible beings. These are interesting, but they fall short of showing that God is infinite, as opposed to simply the greatest of all finite beings.
 
In par. 11, Thomas argues that we can think of something greater than any finite being, and yet our intellect cannot think of something greater than its own cause. So, God (our intellect’s cause) must be infinite. (This is close to one of Descartes’ arguments for God in the Meditations, a sort of hybrid of the cosmological and ontological arguments.)

I have argued that we can deduce that God is a being of pure actuality, without passive potentialities, from the fact that He is absolutely the First Cause. Joe Schmid has challenged me on this inference, rightly pointing out that the implication is not immediate. I will try here to make the connection more nearly evident.
 
To do so, I will have to introduce some technical machinery. First, I will need the concept of a logical moment. If agent A causes some effect E, then we can identify two logical moments, even if the action and the effect are temporally simultaneous. If agent A causes E, then agent A acts at one logical moment M1, and the effect is first in actuality at a posterior moment M2. The relation of priority/posteriority between logical moments is transitive and asymmetric. No logical moment is prior to itself, and no moment is prior to any moment that is prior to it.
 
We have proven that there are absolutely uncaused events. These events must belong to logical moments that are absolutely primal—i.e., logical moments that are posterior to no logical moments.
 
The second technical device that I need is the definition of a nature-constituted disjunction of properties. Let D be a (possibly infinite) disjunction of monadic properties. Then D is a nature-constituted disjunction for individual x if and only D is a minimal disjunction such that it is essential to x to have some property in D. That is, there is no disjunction D* whose disjuncts form a proper subset of the disjuncts of D and such that it is essential to x to have some property from D*.
 
Let’s say that an individual x has a full complement of properties at logical moment M just in case x has at M every property that is essential to x, and, for every nature-constituted disjunction  D of properties for x, x has a property from D at M.
 
Now I’m in a position to propose a basic principle about causality:
 

• The Completeness of Agents (COA). Necessarily, if agent x acts at logical moment M to produce some effect y, then x has a full complement of properties at M.

 
The rationale for COA is this: in order to act at a logical moment, an agent must actually exist in that moment. But an agent cannot actually exist at a moment without possessing a full complement of properties at that moment. If it didn’t possess such a full complement, it would fall short of actuality and would thereby be disqualified from acting.
 
I need one further principle:
 

• The Groundedness of Accidents (GA). Necessarily, if an individual x has property P as an intrinsic, natural property, then there is some nature-constituted disjunction D of x’s essence such that P is one of the disjuncts of D.

 
In order for a property to be an intrinsic and natural property of a thing, it must be determinate of some determinable property that is essential to that thing. A thing can’t just take on intrinsic properties willy-nilly. An intrinsic property must fulfill some essential role dictated by the thing’s nature. The requirements of a thing’s nature lay out the possibilities for a thing’s intrinsic character.
 
Let’s suppose that God is one of the agents acting at a primal moment. Suppose for contradiction that God has some intrinsic property P contingently. By GA, there must be some disjunction D such that D is a nature-constituted disjunction for God, and P is a disjunct of D. If D were a trivial disjunction (with only one disjunct), then P would be an essential property of God and so not contingent. So, D must be a non-trivial disjunction.
 
Let M be a primal moment at which God acts. By COA, we know that God possesses a full complement of properties, relative to the divine nature, at that primal moment M. So, God must possess some member of D at M. But every member of D is such that, if God possesses it at M, He must possess it contingently. Consequently, God must have some contingent intrinsic property at M. Call this property Q.
 
Now, either it is necessary that God possess Q at M, or it is contingent that He do so. Both are impossible.
 
If it is necessary that God possess Q at M, there must be some explanation of this necessity. A logical moment is not a thing in its own right, but simply a node in the causal network of the world. So, if it is necessary that God possess Q at M, this must be a result of God’s essence, and a result of God’s essence alone, since there are no prior facts to appeal to. But if God’s possessing Q is a result of God’s essence alone, then He must possess Q necessarily and not contingently.
 
There is a second reason for ruling out this first horn of the dilemma: a reason based on St. Thomas's First Way. Suppose that it is necessary for God to have Q at the primal moment M. Then, since Q is contingent, it must be possible for God to have some contrary property at some posterior logical moment. If so, this would mean that God is intrinsically changeable. But an intrinsically changeable being exists within time, not outside it. And a being that is inside time cannot be causally responsible for the propagation of time, as the Prime Mover must be.
 
If, alternatively, it is contingent that God possess Q at M, then this fact (by the PSR or principle of causality) must have a cause. But that is inconsistent with our assumption that M is a primal moment, and so no fact at M can have a cause.
 
Hence, God cannot possess any contingent intrinsic properties at any logical moment. He must be a being of pure actuality, with no passive potentiality.
 
There is a complication that is introduced by the fact of the Incarnation. The Second Person of the Trinity has two complete natures, one divine and one human. Hence, we must distinguish between the properties that God the Son has qua God and those He has qua man. Qua God, the Son cannot have any contingent intrinsic properties, for the reason given above. Qua man, the Son has many contingent intrinsic properties. In fact, having a human nature is one of those contingent properties that the Son has qua man. So, in this sense, and only in this sense, God can have contingent intrinsic properties, namely, by assuming a second nature.

In the Summa Theologiae Q4, a1, Thomas gives just one simple argument for concluding that God is perfect: namely, the fact that imperfection always implies some lack of actuality, and God is actuality in its absolute fullness. His reply to objection 3 is also noteworthy. Objection 3 observes that ‘existence’ seems radically imperfect, since things can exist no matter how far they are from perfection. Since God’s nature is simple, unqualified existence, He would have to be the most imperfect thing of all.
 
In the Reply, Thomas rejects this greatest common factor conception of ‘existence’. It is true that the formula ‘X exists’ can be true, no matter how minimal X’s grip on existence may be. (Thomas calls this the ‘formal principle of existence’, by which he seems to mean its logical or semantical role.) However, Thomas urges us to focus instead on the metaphysical role that is played by acts of existence (actus essendi). It is the act of existence that gives each thing whatever degree of being it has. The act is limited by the nature or essence of the thing, not vice versa. Acts of existence are the source of reality and actuality. Hence, God, as the supreme act of existence, must contain eminently every possible degree of being, so that He can give to each individual act of being all the actuality it needs to do its work, given its associated essence.
 
In article 2, Thomas argues that God possesses all the perfections had by creatures. This is the crucial point at which we depart from purely negative theology. He gives two arguments: one from God’s place as First Cause, and the other from God’s essence being His pure existence. First, since God is the cause of all the perfections in any creature (not only in the actual world, but also in any alternative possibility), God must possess in an eminent form each of those perfections. This is an appeal to the principle of proportionate causality which we’ve seen repeatedly since the First Way. As Thomas explains, the fact that the sun is the cause of heat does not entail that the sun is formally hot. But it does entail that it contains heat eminently—that it has some attribute that is, by virtue of its greater nobility of being, capable of making other things formally hot. Similarly, God is the cause of all heat, but He is not himself formally hot. How, then, is He eminently hot? He is so, as we shall see, by being an Intellect, knowing the form of heat as one of His possible creations. This is the same sense in which an architect is eminently a house.
 
So far, we have not really moved very far from a negative theology, since to say that God is eminently F is really to say little more than that He is the cause of F-ness. The second argument is, I think, the more fruitful. God’s existence is unlimited by any distinct form or essence. He is thereby a case of pure, unqualified existence. For the moment, let’s define a perfection as a property that doesn’t explicitly entail that its bearer is not God (not the First Cause, not identical to its own act of existence). So, for example, being finite, being created, having a nature, having passive potentiality—none of these would count as perfections, since they all entail explicitly that their bearer is not the First Cause. In contrast, being powerful, loving, knowing things—none of these explicitly rule out being God.
 
Say that F is such a perfection. If God lacked the possibility of being F, then being itself would lack this possibility. If F were not absolutely impossible, and if F does not explicitly rule out being God, then it would have to be possible for God to be F. There could be then no explanation of the impossibility of God’s being F: F itself does not rule this out, and God has no nature but existence, so if His nature rules it out, there must be a fundamental incompatibility between existing and being F. So, God must be possibly F.
 
But God lacks all passive potentiality. Hence, it is impossible for God to be merely potentially F. If He can be F, He must actually be F. Consequently, God must possess every perfection.
 
The basic premise of this second argument is that impossibility must have some explanation. If F and G are both purely positive, then there could be no logical inconsistency between them. Hence, is must be possible for F and G to be combined. Existence as such is purely positive. Hence, existence can be combined with any purely positive property. Since God’s nature is nothing but existence, it must be possible for God to have any purely positive property. Since God is a being of pure actuality, He must have in actuality every purely positive property.
 
We saw, when considering the Third Way, that Thomas seemed to assume that the possible existence of something that doesn’t actually exist needs to have an explanation. There is some prima facie tension between that principle and the principle relied on here. Suppose that there is no explanation for the possible existence of p, and no explanation for the impossibility of p. Is p then possible or not? The Third Way would say No, and the argument from perfection would say Yes. But p can’t be both possible and impossible.
 
The two principles can be reconciled in the following way. The Third Way appeals to the Nihil ex Nihilo principle:
 
(NEN) If x does not actually exist, then x’s existence is possible only if there is something that actually exists with the actual power to cause x to exist.
 
The second perfection argument depends on the principle that logically consistent combination is always possible:
 
(LCC) It is impossible to combine two properties only if there is some logical inconsistency between their essential internal structure or definition.
 
Are the two principles consistent? Consider some x that does not actually exist. Now consider two properties: actually existing, and being x. These are clearly not logically inconsistent in their internal structure or definition. So, by LCC, it must be possible to combine them. This means that x’s existence is possible. By NEN, it follows that something has the power to cause x to exist. But this is true: God exists and has this power. What’s interesting is that these two principles by themselves seem to entail that in every possible world, something exists with the power to cause the existence of anything that doesn’t exist in that world. This isn’t quite a proof of God’s existence, but it’s close. This does suggest that many atheists will find either NEN or LCC implausible. One could mitigate this problem by restricting LCC in such a way that it doesn’t apply to properties like being x (some particular thing). We might limit LCC to potentially general properties, properties that could in principle be instantiated by more than one thing. Then the atheist could consistently accept both principles.

A central thesis of Thomas’s natural theology is the claim that God is a being of pure actuality—meaning that God is utterly lacking in passive potentiality. This follows quickly from the fact that God is absolutely the first cause, in the sense of being the cause even of other necessities. God is the only being that is necessary in Himself. In order to act as first cause, God must be in His very essence fully equipped with a complete complement of intrinsic properties. If He were not, then there would be a component of contingency or at most conditional necessity in His intrinsic state as the first cause, but any such component would require a still more primary cause, contradicting God’s status as absolutely first.
 
God lacks only passive potentialities: He possesses active potentialities (i.e., active powers) to the greatest possible degree. God can cause anything that is metaphysically causable—His active power is without limit. If it were limited, then this limit would require some cause, again contradicting God’s status as first cause.
 
God is, moreover, maximally free. He was free, in particular, to create nothing at all, or to create any cosmos that would be a fitting expression of His nature. There are many such possible cosmoses. Ours is just one. Hence, our cosmos is thoroughly contingent.
 
How is the contingent exercise of active power on God’s part consistent with His lack of passive potentiality? In Aristotelian metaphysics, there is no conflict, since the exercise of active power is an action, and an action takes place in the patient, not in the agent. God does not have to modify Himself in order to exercise His active power. He does not have to deliberate or plan, and His intentional action requires no internal representation in His mind. The truthmaker for God’s intentionally creating creature x is simply the existence of x itself. No difference internal to God is required to differentiate worlds in which God creates x from worlds in which He does not.
 
But, a critic may respond, isn’t God identical to His own action, given Thomas’s strong doctrine of divine simplicity? If so, the objection goes, since God exists necessarily, His action must exist necessarily, in which case everything that God creates must also exist necessarily.
 
It is true that Thomas embraces the thesis that God is identical to His own action. In Summa Contra Gentiles, I, chapter 45 and in Summa Theologiae I, Q14, article 4, Thomas argues that God’s act of understanding is identical to His essence. Since He is identical to His own essence, God is identical to His act of understanding. In Part II of the Summa Contra Gentiles, Thomas claims both that God’s power is identical to His own substance, and that His action is identical to His power.
 
Ordinarily, the act of a power is distinct from that power. So, if I whistle a tune, my particular act of whistling is obviously distinct from my power of whistling. However, in God’s case, His act of understanding is identical to His power of understanding, and both are identical to God Himself.
 
The act of a power is the perfection of the power. A power that isn’t exercised is imperfect. So, if God’s act were distinct from His power of understanding, then that act would perfect His power. Furthermore, the perfecting of this power would be the perfecting of God’s essence. Hence, God’s perfection would depend on something other than God, which would contradict the fact that God is infinitely perfect. God’s act must be identical to God, so that it is God who perfects Himself.
 
An act stands to a power as actuality to potentiality. So, if God were not identical to His act, then His power of understanding would have a potentiality that is actualized by His act. But God has no passive potentiality. He doesn’t stand as potential to anything else. So, He must be identical to His own act.
 
The Modal Collapse Argument
 
Many critics of Thomas argue that he is committed to God’s willing necessarily everything that He wills, because of Thomas’s strong doctrine of divine simplicity. The argument typically goes something like this:
 

1. God exists necessarily (Third Way)
2. God is identical to His act of willing (Divine simplicity – SCG I.45 and II.9)
3. God’s act of willing is identical to His act of willing to create this universe.
4. So, God’s act of willing to create this universe exists necessarily. (From 1, 2, 3 by the law of identity)
5. So, God necessarily wills to create this universe. (From 4?)

 
The argument is guilty of a fallacy of equivocation. Understood in one way, the term “God’s act of willing to create this world” picks out something that exists necessarily, namely, God Himself. Understood in a second way, the term picks out something that exists only if this world exists. On either meaning, conclusion 5 does not follow from sub-conclusion 4. On the second meaning, the phrase “God’s act of willing to create this universe” is actually a kind of quantifier: “There is something that is uniquely an act of willing by God to create this universe and…”
 
To make clear why 5 does not follow from 4 using the first meaning, the inference to 5 on that reading requires an additional assumption:
 
4b. God’s act of willing to create this universe is essentially God’s act of willing to create this universe.
 
On the first reading, assumption 4b is false. God’s actual act of willing (which is in fact an act of willing that He create this world) could have been an act of willing that He create a different world, or no world at all. Consider, for example, a parallel proposition:
 
BD. Ben’s father is essentially Ben’s father.
 
Not true—although I am in fact Ben’s father, there are possible worlds in which I have no children at all.
 
Critics of Thomas will complain that the cases are not parallel. Assumption 4b just must be true, because every act of willing has its own object essentially. My choosing a chocolate cookie for a snack could not have been my choosing an oatmeal cookie. Different objects necessarily imply different acts of will. However, this is wrong, for two reasons. First, because God is different from creatures. God does not have to undergo any kind of process of deliberation in order to make a choice. Hence, He and His act of will are exactly the same in every possible world. They have different objects in different worlds, but this difference is merely a Cambridge difference in God—it doesn’t require any internal modification. The objects of God’s choosing are immediately present to God as chosen by Him—they don’t have to be re-presented within God as chosen.
 
Second, even in the case of human actions, it is possible for the same act of human willing to have different objects in different worlds. I’m thinking of a case of spontaneous but voluntary action—acts taken without any prior deliberation, but which are nonetheless guided by the human will. Consider, for example a musician who is improvising as he plays, or a speaker who is speaking very rapidly. The notes or words that are chosen are chosen by will, and yet there need be no prior mental event guiding the action. The action has “voluntariness”, as Elizabeth Anscombe puts it, without being the product of some internal volitional event. In a different possible world in which the person chooses a different note or different word, there may be no internal difference despite the fact that a different choice was made. Consequently, there is no reason to deny that the very same act of will could exist in both worlds.
 
Let’s return to the argument and consider using the second meaning. On that interpretation, steps 4 and 5 look like this:
 
4c. There is something that is uniquely an act of willing by God to create this universe, and that thing exists necessarily.
5c. There necessarily exists something that is uniquely an act of willing by God to create this universe.
 
Again, 5c does not follow from 4c, unless we assume that anything that is in this world an act of willing by God to create a certain universe must be an act of God to create that same universe in every possible world in which it exists. Again, we have to assume that God’s act of willing is essentially an act of willing to create this universe specifically. And that Thomas will deny.

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.