Aquinas's "Third Way": One Interpolation

Traditional theists believe, among other things, that there exists a “necessary being”—a being that could not have failed to exist—and that this necessary being is the fundamental or ultimate reality which explains the existence of all other things. But in the world of ordinary experience, the world we encounter through our senses, we only encounter beings which appear to be contingent—that is, beings which could have failed to exist. While there are necessary truths about the world we encounter (for example, that given an initial state of affairs and certain causal laws, a subsequent state of affairs must embody certain features), these truths are consistent with nothing actually existing, and so do not imply a necessary being.


Nothing in our ordinary experience fits with such a “necessary” thing. As Hume famously noted, everything that I can imagine existing is something I can imagine not existing. And for Hume, “imagination” is a technical term that refers to our capacity to reorganize the ideas derived from empirical experience in endless ways. As such, imagination refers to a possible way that the building-blocks of empirical experience might be arranged. Hume’s point, then, is that there is nothing in empirical experience from which we can derive the idea of necessary existence.

As such, to believe in a necessary being is to believe in the existence of something that falls entirely outside the world of ordinary experience—and furthermore, to hold that this “transcendent” reality is more fundamental and, in a sense, of greater ultimate significance than the world we encounter through our senses. This transcendent, necessary reality is the source or “creator” of the world we know. In short, to believe in a necessary being is to adopt a kind of religious stance towards the world (at least in one sense of the term "religious").

But can anything justify such a stance, or is it pure fabrication (as some critics of religion are prone to assert with great confidence)? Historically, a number of philosophers have offered arguments in favor of the existence of a transcendent reality characterized by necessary existence. Probably the most famous is St. Thomas Aquinas, the philosopher and theologian whose body of work is typically regarded as the towering intellectual achievement of the Middle Ages.

Aquinas explicitly argues for the existence of a necessary being in the third of his “Five Way” for proving the existence of God. Let me say that, although Aquinas introduces these five arguments as ways that God’s existence “can be proved,” it is pretty clear that the phrase here is intended in a very loose sense. What we get are five arguments, each for a different conclusion. The first concludes that there must exist an unmoved mover, the second an uncaused cause, the third a necessary being, the fourth something that embodies all the “perfections” that other things only approximate, the fifth an intelligence that guides the unintelligent things that exhibit goal-directed behavior. He then tacks on to the end of each argument a phrase such as “and this we name God” or “everyone understands this to be God.”

So, Aquinas is clearly speaking here to an audience of believers who have a robust idea of God—one for which all of these appellations (unmoved mover, uncaused cause, necessary being, etc.) fit admirably. Aquinas is here showing that there are several arguments which support the existence of something which has one or another of the distinctive properties typically attributed to God. But it needs to be remembered that these “Five Ways” are Aquinas’s starting point in a much more expansive case for the existence of the Christian God in which Aquinas believes.

So, what about the “Third Way”? For me, this is the most interesting of Aquinas’ arguments, but it is also the most perplexing. In fact, on an initial reading it seems as if Aquinas is guilty of a logical error. Specifically, it seems as if he moves, without justification, from asserting one proposition (which I will call PN, for “possible nothingness”) to assuming a related one (which I will call AN, for “actual nothingness”). Here are the propositions in question:

PN: “If all existing things are contingent, then it is possible that at some time nothing existed.”

AN: “If all existing things are contingent, then at some time nothing existed.”

While PN seems to be a justified assertion—and while it is the assertion that Aquinas explicitly makes—his argument’s validity depends on asserting AN. Did Aquinas inadvertently slip from one to the other without noticing? Or did Aquinas see (and assume that we would see) that PN, perhaps in conjunction with certain unstated but self-evident premises, implied AN?

Philosophically speaking, the more interesting question is not whether Aquinas in fact saw a sound inference from PN to AN, but whether there actually is such an inference. Is there? If there is, then the discipline of philosophy requires that we interpret Aquinas’ argument accordingly. That is, philosophy calls us to pursue what is called The Principle of Charity—that is, the principle of formulation the arguments of others in their strongest possible form.

Along with many other philosophers interested in Aquinas, I have reflected on alternative ways of justifying the inference from PN to AN in Aquinas’s Third Way. The formulation of the argument that I presented to my philosophy of religion class on Wednesday was the outcome of such reflection combined with an invocation of the Principle of Charity. What follows is a slight modification (for the sake of greater clarity) of what I presented to my class. I have also identified what is a core premise and what is an inference from prior premises.

1. There exist contingent things (core premise)

2. For each contingent thing, it is possible that it not have existed and, hence, that it not have existed at a given time T (core premise)

3. If, for each contingent thing, it is possible that it not exist at a given time T, then it is possible that no contingent things exist at time T (core premise)

4. Therefore, it is possible that at one time no contingent things existed (from 2 & 3)

5. Given infinite time, every possibility occurs (that is, as the timeline moves towards infinity, the probability that some event with a real possibility of occurring does occur at least once along that timeline approaches 1) (core premise)

6. Either the succession of contingent states extends infinitely into the past, or it has a starting point. (core premise)

7. If the succession of contingent states has a starting point, then at one time no contingent things existed. (core premise)

8. If the succession into the past is infinite, then at one time no contingent things existed (from 4 & 5)

9. Therefore, at one time no contingent things existed (from 6-8)

10. Therefore, if all existing things are contingent, then at one time nothing existed (from 9)

11. If at one time nothing existed, then nothing could ever have come into existence and nothing would exist now (core premise)

12. Therefore, it is not the case that at one time nothing existed (from 1 & 11)

13. Therefore, it is not the case that all existing things are contingent (from 9 & 12)

14. Everything that exists is either contingent or necessary (core premise)

15. Therefore, there exists something necessary (from 13 & 14)

The argument as Aquinas presents it actually goes on to distinguish between two ways in which something could be said to exist necessarily (what might be called dependent vs. independent necessity), and then argue for the existence of something with independent necessity. But the part of the argument presented here is enough to digest and reflect on by itself.

Is this the argument that Aquinas intended, or is it merely one charitable interpolation of his words? I can’t say for sure. But either way, it’s an interesting argument for the existence of a necessary being—and it’s hardly silly. Since the argument is formally valid, anyone who wants to deny the conclusion needs to deny one of the core premises (by which I mean premises that are not conclusions derived from subarguments within the broader argument).

So, for those who want to deny the conclusion, which premise are you going to challenge, and on what basis?