Three Interpretations of Aquinas's Third Way

One of the more notorious issues with Aquinas’s Five Ways is the disproportionate attention they’ve garnered over the years—likely more than Aquinas himself ever intended, especially since they were meant as brief summaries for beginner theology students. For those familiar with Aquinas, they know he offers far more extensive arguments for the existence of God elsewhere, such as in De Ente et Essentia.

Because of their brevity and the amount of metaphysical groundwork they assume, interpretations of the five ways vary widely, even among Thomists. Today, let’s take a look at Aquinas’s Third Way—broadly called the argument from contingency—and see if we can figure out what he’s up to.

Here is the original text (it’s short):

The third way is taken from possibility and necessity, and runs as follows. We find in nature things that are possible both to exist and not to exist, as they are found to be generated and corrupted, and consequently, they can both exist and cease to exist.
It is impossible for everything to be of this nature, because what is possible not to exist will, at some point, not exist. Therefore, if everything could possibly not exist, there must have been a time when nothing existed. But if this were true, then even now nothing would exist, because that which does not exist cannot begin to exist except through something that already exists. Therefore, if at one point nothing existed, it would have been impossible for anything to begin to exist, and thus nothing would exist now, which is clearly false. Therefore, not all beings are merely possible; there must exist something whose existence is necessary.
Now, every necessary being either has its necessity caused by another or it does not. It is impossible to have an infinite regress of necessary things that have their necessity caused by something else, just as there cannot be an infinite regress of efficient causes.
Therefore, we must postulate the existence of some being that is necessary in itself, not deriving its necessity from another, but rather causing the necessity in others. This is what all people call God.

So, what’s going on with this argument? Let’s look at three ways Thomists (followers of St. Thomas) have understood it. Keep in mind that just because Thomists interpret it a certain way doesn’t mean they necessarily agree with it. Some Thomists may support a particular interpretation as Thomas’s but reject that interpretation as unsound. Others may accept both the interpretation and its soundness. Some may even stand by an interpretation that probably isn’t Aquinas’s but still find it sound. In other words, there’s a lot of diversity among Thomists on these matters.

Furthermore, it’s important to realize that being a Thomist doesn’t mean uncritically accepting everything Aquinas says—or at least, it shouldn’t. In fact, I know Thomists who reject nearly all of the five ways! Of course, I also know others who accept all of them. What a party!

Finally, some interpretations may not be obvious from the text itself. The reason for this is simple: they rely on understanding Aquinas’s broader metaphysical commitments, which aren’t made explicit in this text but are supported elsewhere. If one of these interpretations makes you think, “That just doesn’t seem to be what he’s saying,” the argument might be, “Yes, but that’s only because you haven’t seen what he says elsewhere, which necessarily relates to this.” For example, Aquinas holds that all material things have an internal principle of corruption—this isn’t explicitly stated in the Third Way text, but it’s a commitment of Aquinas concerning material beings. He also holds that there are necessary beings that aren’t God—not in the sense that they’re ontologically independent, but because they have no natural principle pushing them toward corruption, unlike most material things, such as plants or humans.

In fact, keep at least this point in mind as we discuss interpretations: when Aquinas speaks of “possible not to be” he isn’t talking in terms of contemporary possible worlds semantics—rather, he is referencing classes of things which we observed to be generated and corrupted; Aquinas takes it that certain things—material beings, namely—have an inherent tendency to corrupt (due to a loss of their parcel of matter).

OK, let’s dive into the interpretations:

Option 1: The Modal Interpretation

So, one interpretation is modal. If everything were contingent, there would be an empty possible world—a world where nothing existed—through a plausible subtraction principle. From here, there are a few ways someone could proceed.

The first, simpler approach is this: Given that there is a possible world where none of these contingent beings exist, it seems reasonable to ask why they do exist in the actual world. What determines them to a state of actuality rather than non-existence? This is where the notion of a necessary being comes in—something that must exist no matter what, in all possible worlds.

A second, perhaps more aggressive approach could be set forth roughly as follows: Assume possibilities are grounded in concrete objects or their powers and that possibilities are the same across all possible worlds (i.e., if something is contingent, it is necessarily contingent). It would then follow that if there is any possible world where nothing existed—where no possibilities at all exist—we would be “stuck” there. Nothing would be possible in any world (in fact, necessarily, nothing would be possible). But this is clearly false. Therefore, there must be a necessary being, and there is no possible world where nothing existed. (This approach is essentially about avoiding the collapse of all possibility; again, it requires two controversial, though defensible, assumptions: that possibilities are grounded in concrete objects or their powers, and that possibilities are uniform across worlds, i.e., a system like S5 is in play).

While I think a modal interpretation can be formulated into a strong argument, I don’t think this is what Aquinas was up to. It strikes me as anachronistic. Recall that when Aquinas talks about possible beings, he’s referring to material entities that have an inherent tendency to corrupt or decay. It seems a bit strained to suggest he was thinking along the same lines as contemporary philosophers of religion who use possible worlds semantics.

Option 2: Temporal Future “System Failure” Interpretation (Focusing on Corruption)

This interpretation, in a sense, looks more toward the future than the past. The idea is that if it’s possible for all contingent things to cease to exist, and each one does so at some point, then, given infinite time, there would eventually come a moment when all contingent beings cease to exist. In other words, over an infinite duration, systemic failure is inevitable, where all contingent things vanish. If this were to happen, nothing would exist now, because from nothing, nothing comes. But since things do exist now, this system-wide failure could not have occurred—which, if infinite time had passed, it already would have. Therefore, there must be something that cannot fail in its existence: a necessary being.

The major issue with this interpretation involves a composition fallacy, specifically a quantifier shift fallacy. Just because every human being has a mother, it doesn’t follow that there is one individual who is the mother of all humans. Similarly, just because everything has some time at which it doesn’t exist, it doesn’t follow that there is a single moment when everything ceases to exist. The concern arises from shifting the quantifying expression between the first and second statements, which can lead to obviously false conclusions, as shown in the mother example.

However, while this interpretation isn’t my preferred one, I don’t think it’s unsalvageable. A case could be made for a relevant difference between the nature of contingent system failure and the mother example. One could re-motivate the argument with a more fitting analogy, like a symphony.¹ Plausibly, because no single member of a symphony can hold their melody forever, it follows that given infinite time, there would come a point when the entire symphony fails—when all members fail to hold their melody. In this way, system failure, while not logically inevitable, becomes probabilistically inevitable, and thus bound to happen eventually. Understood along these lines, I think this is a halfway decent argument—perhaps not a “demonstration” but suitable for reasonable belief.

Option 3: Atemporal Interpretation (Focusing on Per Se Causality)

The final interpretation deals with the nature of possibilia or possible beings as such as Aquinas understands them. This is Gaven Kerr’s preferred interpretation, so for convenience, I’ll quote him:

According to this view, Aquinas is not speaking of coming to be and ceasing to be in the past or future, but he is addressing the nature of possibility itself—namely, the very idea of coming to be and ceasing to be as such. Given that every possible thing's nature is such that it is generable and corruptible, not everything can share this nature. If everything were contingent, we would have only dependent beings with nothing to ground their existence. The point here is akin to the argument against an infinite regress in a per se ordered series. In such a series—for example, where the mind moves the hand, which moves the stick, which moves the stone—each subsequent member is dependent in some significant respect, like motion. Multiplying these dependent things infinitely won’t help, since you’d still just have an infinite series of dependent beings. There still needs to be something independent on which these dependent things can rely. So, multiplying dependent entities doesn’t explain their dependence; only something independent can. In the Third Way, Aquinas is arguing that not everything can be a possible thing because all such things are inherently dependent by being generated and corruptible. Thus, within the created order, there must be something necessary.²

This interpretation aligns somewhat more with contemporary contingency arguments: dependent (because generable) things exist, and no matter how many of these dependent entities you stack or arrange—whether finite or infinite—doing so doesn’t magically turn them, or the collection as a whole, into something independent.³ To claim otherwise would be like reversing the composition fallacy; a "construction fallacy,” as it were. In other words, multiplying contingent, dependent things doesn’t get us any closer to explaining why any of them exist in the first place. Without some independent thing to anchor it all, there couldn’t be any dependent things at all. So, the existence of these dependent beings can only be explained if there is something that isn’t dependent—something that’s not contingent or merely possible, but necessary.

I think this interpretation is quite strong (unsurprisingly, since I generally find contingency arguments strong) and the argument becomes especially forceful once one properly grasps and incorporates Aquinas’s other commitments—particularly his real distinction thesis and the logic of per se ordered causality. (To be clear, I’m not defending Aquinas’s 3rd Way in this article—just offering a sketch of the various interpretations along with some commentary. There’s plenty of published material on these issues for those interested in more technical developments.)

The initial hesitation with this reading, however, is that it seems to push against the literal sense of the text. After all, Aquinas does seem to be using temporal language, at least in some way. Kerr addresses this by pointing out other cases where Aquinas uses temporal terms in a tenseless or atemporal sense. He suggests that, to some degree, it's inevitable in philosophical discussions like this that we stretch ordinary language to capture ideas that are otherwise tough to express. As Kerr explains, “Linguistic strains are not a reason to reject an interpretation of a piece of philosophical reasoning unless that interpretation can be shown to be inconsistent with something else in the philosopher’s thought. But we’ve seen that this interpretation is far from inconsistent with Aquinas’s wider metaphysical views; in fact, it avoids the inconsistencies that arise from the past and future-tense interpretations.”

One final note: the idea that a necessary being might receive its necessity from another might strike some readers as odd, but Aquinas held that certain beings could be necessary without being truly ontologically independent or ultimate. For Aquinas, a necessary being is simply anything without an internal principle of corruption—for example, angels. So, the idea that something could be necessary and yet still caused isn’t all that strange. Just imagine God creating an immaterial being from all eternity, willing that this being would always exist. Nothing could remove it from existence. Such a being would be necessary, but its existence would still be owed to God.

1. This interpretation is motivated by Fr. Thomas Joseph White.

2. All quotes from his Collected Articles volume.

3. For Aquinas, these contingent beings wouldn’t just be modally-contingent but dependently-contingent given that he is operating from their nature as generable and corruptible. This should be obvious since the “necessary” being Aquinas initially appeals to isn’t necessarily God, but might well just be a being that is still modally contingent (occurs in some but not all possible worlds) but just not a material entity that is generated and corrupted—for example, an angel. Hence why Aquinas needs to argue further for that which is necessary through itself.

Would Aquinas need something like PSR to run this argument? Yes, I think he would.