St. Thomas Aquinas is famous for seeing the most mundane things in our human experience as starting points for reasoning to God’s existence. For example, in the first of his famous “Five Ways” he starts with motion (Summa Theologiae I:2:3). In the second, he starts with efficient causes.
The Third Way draws our attention to the fact that things come into being through generation and go out of being through corruption. Last year’s flowers from our garden at one time did not exist before their parent flowers produced them in seed form, and they don’t exist now because they died and became compost.
So, as Aquinas puts it, that which comes into being and goes out of being “at some time is not.” Aquinas calls this kind of being a “possible being.”
Now, an interesting question to ask is, “Can all of reality consist only of possible beings?” In other words, does reality consist only of beings for which nonbeing both preceded and followed their limited duration of existence? Aquinas answers in the negative:
If everything is possible not to be, then at one time there could have been nothing in existence. Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing. Therefore, 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. Therefore, not all beings are merely possible, but there must exist something the existence of which of necessary (ST I:2:3).
Notice Aquinas concludes that there must exist a necessary being whose existence has no beginning or end.
Now, to reach that conclusion Aquinas says that if all reality (every single thing that currently exists or has ever existed) consisted of possible beings (beings that begin to exist and go out of existence), then nothing would exist now, and since things obviously exist now this cannot be true.
But why does Aquinas think nothing would exist now? I take Aquinas to be focusing in his Third Way on the nonexistence that precedes each possible being. Remember, it’s a fact of sensible experience that before my flowers were generated in seed form they were nothing. Nonbeing preceded their existence: “at some time they were not.” (Note that this doesn’t entail that there was absolutely nothing before the flowers were produced in seed form. Only that they were nothing before they began to be).
Now, suppose reality consists only of possible beings, represented by possible beings A, B, and C. At some time in the past, possible being C didn’t exist. Likewise, at some time in the past possible being B didn’t exist because it too began to exist, before which it was not. If we were to suppose that reality only consists of possible beings, and for our purposes all that exists are possible beings A, B, and C, the nonexistence preceding the beginning of possible being A (or a host of possible beings that came into existence at the same time) would be absolute nonexistence (complete nonbeing), since there would be nothing else in existence.
Aquinas is not trying to prove an absolute beginning of time. His point is that if we say only possible beings exists, then necessarily there would be a first possible “before” whose coming to be there would be absolutely nothing (“before” is used to represent imaginary time, since there would be no real time before a first possible being).
Since for Aquinas it’s obvious that from nothing only nothing comes, on this line of reasoning, nothing would exist now. Since things do currently exist, Aquinas concludes that not all reality can consist of possible beings. There must exist at least one necessary being, a being such that it has no beginning or end.
At this point Aquinas is not concerned about any sort of causal relation that a necessary being has to possible beings. He’s only trying to show the absurdity of the view that all reality consists only of possible beings.
Some might think Aquinas has proven God’s existence at this point, since he’s shown that a being whose existence is without beginning and without end (a necessary being) must exist. But that’s not the case.
For Aquinas, it’s possible to have a being such that it exists without beginning and without end (a necessary being) and still be within the created order, having its beginningless and endless existence received from a cause outside itself (e.g., heavenly bodies in ancient cosmology, angels).
This is why Aquinas goes on to reason, “[E]very necessary thing either has its necessity [its beginningless and endless existence] caused by another, or not.”
If such a necessary being has its existence in virtue of its own essence, and thus doesn’t receive it from a cause outside itself, then we’ve arrived at what we can call God—a being such that it doesn’t receive existence but is existence itself (ipsum esse subsistens—subsistent being itself).
If, on the other hand, this necessary being (necessary being one) has its existence in virtue of some other necessary being outside itself (necessary being two), then at every moment necessary being one exists it would depend on necessary being two causing it to exist.
Now, Aquinas argues “it is impossible to go on to infinity in necessary things which have their necessity caused by another.” Here, Aquinas is not thinking of a causal series like my grandfather, who begot my father, who begot me (a causal series that extends temporally into the past). Rather, he’s thinking of a causal series like the hand that moves the stick that moves the stone (what philosophers call an essentially ordered series). At every moment the stick is moving the stone it requires the moving action of prior causes; the action of the prior causes is essential to the stick acting as a mover.
For Aquinas, such a causal series of necessary beings can’t be infinite because on such a scenario, there would be no source from which the necessary beings could receive their existence, in which case no necessary being would exist.
Consider that any being such that it receives its existence from another doesn’t have existence in virtue of its own essence. A thing can’t receive from a cause outside itself that which it already has by essence (e.g., a triangle doesn’t receive three straight sides from a cause because that’s just what it is).
So, if no necessary being in the series had existence in virtue of its own essence, then no necessary being would have anything to contribute when it comes to existence (e.g., necessary being two couldn’t contribute to existence because it’s existence-neutral, necessary being three wouldn’t have existence because it’s existence-neutral, ad infinitum).
But if no necessary being had anything to contribute when it comes to existence, then there would be no source from which the necessary beings in the series could receive their existence. And if no source, then no necessary being would have existence.
But Aquinas has already shown that a necessary being (a being that exists without beginning or end) must exist. Therefore, there must exist at least one necessary being that has its necessary existence in virtue of its own essence, “not receiving [its necessity] from another, but rather causing in others their necessity.” As mentioned above, such a being would be existence itself or subsistent being itself. In the words of Aquinas, “This all men speak of as God.”