St. Thomas begins his Third Way by saying that "we find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be." (ST, I, 2, 3)
Now if the universe began, then "at some point," to use this phrase loosely, there was nothing. If, on the other hand, the universe never began, then things in it must have existed forever, for an actual infinity of, say, years. (This isn't 100% intelligible in its own right, but let's suppose this for the sake of argument.) But if it is possible for an existing thing not to be, then the probability of its corrupting within some finite span of time is non-zero. But in infinite time, all probable events will occur, and an infinite number of times, too. Hence he goes on that "if everything is possible not to be, then at one time there could have been nothing in existence." Either way, this is a problem, because "if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence -- which is absurd."
St. Thomas correctly concludes that "not all beings are merely possible, but there must exist something the existence of which is necessary." Here is where he and I part ways.
First, let's purge the argument of ambiguity: "necessary" here does not mean modal logical necessity but simply "imperishable." An imperishable thing is such that, once it exists, it will always exist and by extension, perhaps it always has existed.
Second, let's enumerate some of those imperishable things. They are:
Matter, and as its limiting variant, prime or formless matter, ghostly pure potentiality; something which can become anything. Matter can change from one form to another and even to energy, but according to the law of conservation, it cannot die.
We can understand prime matter as what remains if we take any real object and strip away all its distinctive characteristics. Prime matter has no essence; but neither is it non-existence. To say that prime matter exists is to say that it is potentially, given the appropriate agent, all possible things, but no actual thing.
Certain forms may well be imperishable. For example, perhaps electrons can never corrupt. (Technically, they, too, can collide with positrons and be annihilated, but it may well be that some actual things are imperishable.)
Note that an electron is considered to be an elementary particle, but it has a definite form; it is not a fully inert and property-less point of prime matter. An electron behaves in highly precise and unique ways. It knows what it is full well.
The universe as a whole seems to be imperishable. For example, there are no "predators" outside it that may kill it. The universe does not seem to have the potential for disappearing. It is not beyond the pale to suggest that it will exist forever.
There are a couple of other possibilities that we can list.
Perhaps a certain object may change forms on a regular basis: say, one year on March 12, A changes into B, and then next year on June 5, B changes back into A, and so on forever. However, we can reduce this situation to one object P whose single essence includes in its definition the potential for all such changes.
Thus, liquid water turns into snow and ice in winter, melts in spring, and evaporates into gas in summer. But it's still water.
Finally, perhaps forms may change into novel and never-before-seen forms forever: A-∞ → ... → A-1 → A0 → A1 → ... → A∞.
But we don't seem to have such forms in our universe.
Third, let's see what the first 3 things tell us, if anything, about God. St. Thomas outlines the procedure by concluding: "Every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another... Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God."
1. Prime matter makes all possible things potential. But what is possible and what is potential are two separate and distinct things that are united within prime matter. For example, it is easy for us to contemplate possible worlds and possible things. The fat man in the doorway has a partially specified essence, but he does not exist in reality. I form an idea of a lamp by abstracting the "essence" of the lamp from the singular object on my desk. This idea exists in the understanding as an ideal abstractum, but it may or may not exist really. Possibilities on their own are causally inefficacious and have no potential to exist.
Even prime matter, and therefore the union in it of the possible with the potential, then is contingent. It must be explained which means be reduced to a cause. Let a certain X cause prime matter. What can we assert about this X? Well, it causes possible things to become potential. If these are separated, then we are back to prime matter which cannot be a cause of itself. The inevitable conclusion is that in X, possibilities and potentialities are numerically identical with each other, self-same. They are not 2 separate things that need to be united by a 3rd object (as, e.g., (1) a man is united with (2) his life by (3) his self-love), because they are literally one and the same thing. That is what we mean by God: a thing in which everything possible is also potential, i.e., can be actualized. But that means that God possesses absolute and unlimited creative power.
This seems like an important first discovery in our investigation.
I am not proposing that the existence of prime matter entails the existence of the omnipotent shaper of prime matter directly. Oil has existed under the ground for far longer than there have been humans and for longer than humans have known how to transform oil into gasoline. At first glance, there may be formless matter with no one to use it. It is superficially conceivable that there is prime matter without God. But only because of the initial weakness of the human intellect. Such a thing is not possible.
It may be objected that God may have the power to create prime matter but lack any power to create anything further out of prime matter. God can make everything makeable but not necessarily makeable by Himself later on. Moreover, prime matter seems exceedingly simple. Perhaps it is easy for God to create it but impossible to create anything else with its assistance. In reply, I say that prime matter is simple actually but is infinitely complex potentially, since absolutely everything, from ladybugs to angels, can be fashioned out of it. Prime matter is inseparable from the art of using it. There is no way that a weak God could make a substance as infinitely versatile and wondrous as prime matter, unless His power to make reached everything which prime matter's power to be made also reaches. Hence God is ad extra omnipotent.
NB: Prime matter is a technical term for pure potentiality. Hence, even immaterial souls, including human and angelic, can be made out of it, too. There is no need to add "prime spirit."
2. This one is easier to unravel. An imperishable thing may nevertheless be contingent. That this is so can be established by the ease with which I can abstract the electron's essence from the singular thing. I can say that while I am contemplating this essence in my mind, the electron is an ideal abstractum; when instantiated, it becomes real. The essence of the election is a possible thing, and it may or may not be actual.
In the previous argument we considered the union of potentiality (matter) with existence. Now we proceed to the union of actuality (form) with existence.
This union must be effected through some "glue" that unites essence and existence which is not identical with either but is something super-added to both. The glue is probably some form of divine love which is the preeminent unitive force. Again then, even if the electron is imperishable, it is contingent, and the glue must be caused by something else.
But of course the cause must be a really existing object X, also, since ideal abstracta are causally inefficacious. (Nor is the electron's existence a proper accident of the nature of the electron, as the capacity to laugh is a proper accident of humanity. Nor, furthermore, can a thing cause its own existence.) Which raises the question of what united X's essence and existence. We cannot go to infinity, so the first cause of this union must be something in which essence and existence are numerically identical to each other, self-same. Again they are one, showcasing an aspect of divine simplicity. And that is God.
God then is "being itself subsisting," as St. Thomas expresses it.
3. Now that we have analyzed the consequences of the union of (potentiality + existence) and of (actuality + existence) in creatures, what remains is to see what the union of (potency + act) in them reveals about God.
As before, this union is artificial, contingent, and must be caused by something in which potency and act are entirely one. It does not make sense to say that whatever God is or is doing (act), He also can be (potency). But the reverse is quite meaningful: everything that God can be, He already is. And this signifies simply that God has no potentiality at all. Whatever God can extend to or do or enjoy, He already is in full possession of.
The conclusion is that God is pure act unmixed with any potency.
We have seen from argument 1) that in God, everything possible is potential. Argument 3) demonstrates further that in God, everything potential is actual. Hence, whatever is possible is actual in God. Therefore, all possible, and therefore all actually created, things pre-exist in God somehow as in their source of being, essence, and everything else.
Note that this argument does not preclude pantheism. Perhaps we are part of God, and all possible worlds are actual as constituent parts of God, especially if we argue that in God, the concrete and the abstract are self-same. More plausibly, however, creatures pre-exist in God as ideas in His mind, though this precise formulation may need a separate argument to be established. God is concrete as a self to be known as the Son, abstract as the thought knowing it as the Holy Spirit, which come together in a self-aware mind as the Father.
The identity of the abstract and concrete may be understood as that "2 + 2 = 4" is both an ideal proposition contemplated by God and a real component of the structure of the divine mind.
4. The argument from the contingency of the universe as a whole leads to a different conclusion. The question now is: Why is there something rather than nothing?
At first glance, the question seems befuddling. Why should there be nothing rather than what we have around us? Why privilege either "nothing" or "something"? Why cannot this world be everything that has ever been and will be?
On the one hand, we humans privilege nothing readily. We apparently come from nothing and go into nothing. In between we live for a little bit, always in danger, such that if we do not struggle with all our might, the nothing will arrive even quicker. All living things are born, thereby beginning to exist, and die, thereby ceasing to exist. But the inference from this human experience to the universe as a whole need not be taken.
Moreover, "something" is also privileged. The moment we are born, we are surrounded with stuff to use, enjoy, and manipulate. Disembodied existence, while not inconceivable, is not part of our human experience, though it may ultimately be natural in "heaven." But "nothingness" is inconceivable; one can't close his eyes and picture nothingness. However, that nothingness is inconceivable does not mean that it is impossible.
Let possible world Empty be defined as follows: ∀(x I can think of) [x does not exist in Empty]. Then
(1) Nothing = ∃[Empty]. We are dealing with "universes," uni = "one"; so, any possible world is a maximally consistent state of affairs. As a result, Empty swallows up every other reality; so, it is not necessary to say "there exists only Empty."
Let our actual world be called Terra. "Terra" is the name of the universe we live in, not of planet Earth. Then
(2) Something = ∃[Terra]. Neither is privileged, but just as before, (2) is only contingently true and demands an explanation.
Terra either has existed forever or was created. In either case, a physical cause of Terra is precluded from consideration, because it is situated before the effect. In the first case, infinite past cannot have a prior cause; and in the second case, time, too, began along with Terra, and it is meaningless to ask what happened "before" Terra began.
Since we are interested in the origin of the universe, neither teleological nor Aristotelian causation is applicable, either.
The cause of Terra joined the essence of Terra with existence by creating it. As a result, (2)'s being true has a "cause," and Terra's existing has a "ground" of its existence. This ground is called God. We have already seen the mode of causality of the first two grades: physical and teleological. An eternal grounding cause is the effecting of the 3rd grade, of goodness. Terra was united with its existence not at any moment in time but as a whole in eternity which "covers" merely everlasting existence (that we allowed for the sake of argument).
This is only half the task. Now we ask: What is this God? It cannot be another real thing, for then it, too, would stand in need of its own ground. It must then be "beyond" being. We conclude that God is not a thing at all but a kind of force, a primal principle that permeates all, that creates this world, so that its inhabitants might enjoy life or try to. That is what we mean when we say "God." God is not a thing but Creator of things. We may call it by the less ambitious and less potentially objectionable name, "creativity," or by the more ambitious one, "goodness," to the extent that one is inclined to consider Terra to be on the whole good and beautiful. But nothing is not a thing, either. So, in the beginning (of our story), we postulate nothing whatsoever (other than the 2nd-level God). It is a kind of clean slate, in which whatever is created (by goodness) can be made into a top-notch project or performance from ground up, with no need for backward compatibility.
Again, if goodness reigned, then in the beginning, there could not be anything, because only goodness creates good things, and nothing can exist whose existence goodness has not authorized. The choice goodness faced was: 2nd-level God + Empty or God + Terra.
To simplify: our options are: (a) goodness + nothing in the beginning and (b) a good thing, i.e., the world, in the beginning. Goodness implies "nothing," and "nothing" implies goodness; and now we see that their combination, i.e., (a), is also implied.
Note the difference between the last two arguments. The first establishes the identity of God's essence and existence. This is possible, because the essence of our electron can be fully comprehended. But it does not establish God's goodness, because there is no reason to hold that it is a "good thing" overall that a particular electron exists.
The second establishes God's goodness, because it is evident to any reasonably sane man that the universe as a whole is a very good thing. At the same time, it cannot prove the identity of the divine essence and His existence, because the universe as a whole is too big for me, not the least because I am part of it, and I cannot know myself fully, since in me the subject knowing is forever distinct from the object known. I cannot know the essence of the created universe. But I see that there may have been in its stead nothing at all.
These proofs relied on what I think is an uncontroversial principle that every contingent thing, including an actually imperishable one, must have a cause or perhaps "sufficient reason" which itself is necessary. We have by uniting in God what is diverse in creatures in the end demonstrated God's maximal creative power, an aspect of His simplicity, His pure actuality, that He is the archetype of all finite things, and His goodness.