Aquinas’s Proof for God in Plain Language: The De Ente Argument

The aim of this article is to present Aquinas’s De Ente argument in plain language. For the uninitiated, this is really Aquinas’s fundamental “proof” for the existence of God.

By proof, we mean something like a metaphysical demonstration. And by metaphysical demonstration, we mean something like this: given a certain system of metaphysical thought, we can construct a sound argument whose conclusion is the existence of God—or, more precisely, the existence of something satisfying a description that classical theists take to identify God.1

This isn’t going to be super technical—you can read my published work or my book The Best Argument for God for more rigorous developments—but I do hope it serves as more than merely a basic introduction. Really, I hope it shows that, despite all the scholastic machinery behind the argument itself, Aquinas’s way to God is actually quite intuitive. For that reason, it should be taken seriously.

So, basically—to use Ethan Muse’s language2—we begin with the seemingly undeniable observation that there are lots of things about which nothing in what they are demands that they exist. Take my cats Twizzler and Dot or my dog Pumpkin.

None of them, just in virtue of what they are—whether a cat or a dog—needs to be included in reality. Their natures—call that their whatness—seem really different and distinct from their existence—call that their isness.

After all, we can easily conceive of these things as never having existed, and we have routine familiarity with things like them actually coming into existence and then expiring from existence. They are, to use a technical term, contingent beings: they need not have been, and they could have been otherwise.

Aquinas thinks this suggests that such beings have a complex—or, rather, composite—makeup, metaphysically speaking. There is a principle of what these things are, and then there is another principle that makes them actual. This is his famous thesis of the real distinction between essence and existence. In every contingent being, essence must be made actual by an act of existence. To be clear, Aquinas has other, more sophisticated arguments for establishing the real-distinction thesis, but we do not need to explore those here.3

Nor do we need—and, in fact, we won’t—wade into all the complicated metaphysics surrounding this thesis, such as whether existence is itself a first-order property and, if so, how that is to be understood. Again, I’ve treated these matters in many places. For now, I simply want to emphasize how intuitive the basic idea is, at least initially. There are things—lots and lots of things—that do not seem capable of accounting, on their own, for why they are included in reality. And, for that reason, we naturally look to something else to account for them.

We’re already onto the next step. If it is right that these things do not and cannot, by themselves, account for their inclusion in reality, then it makes sense to seek out something that does. The basic idea here is that if something cannot explain some feature of itself from within itself—that is, by its own nature—then we look for something else to provide that explanation.

What we are not considering a live option, at this point, is that:

1. Something cannot explain some feature of itself; and

2. Nothing else explains it either.

In other words, the feature or phenomenon under investigation is simply a brute fact. Brute facts, I take it, are bad news, and we should not declare something a brute fact when it is possible that we could instead find a genuine explanation.

As it happens, Aquinas thinks we can find such an explanation for why things that do not themselves demand to be included in reality nevertheless are. He reasons something like this. Since their existence does not follow from what they are, they must be caused to exist: existence is imparted to their essences by something extrinsic.

However, it will not do simply to invoke a cause that is itself the same sort of thing—namely, something whose nature likewise does not demand that it exist.

While such a cause might be involved in explaining how this or that particular thing came about, it would be only an intermediate cause, not the ultimate explanation, because we could simply raise the same question about it that we raised about the original thing. So, what Aquinas thinks is that, if we are going to arrive at an ultimate explanation for why anything exists that does not have to exist, we must eventually reach something that exists because it DOES have to exist.

According to Aquinas, this means reaching something whose nature is not really different or distinct from its existence, but is actually identical to it. Because this being is really the same as its existence, it would indeed have to be included in reality. It could not not exist. It would be the necessary being.4

Aquinas is not terribly bothered by infinite chains or collections. In fact, in some cases, he allows for them. Rather, he is content to stick with the simple logic that merely adding together a whole bunch of things that do not possess the relevant feature or quality in virtue of what they are is never going to magically generate it.

Just as piling up a bunch of white LEGO blocks—even infinitely—will never generate a purple tower, Aquinas takes it that an infinite chain of contingent beings is never going to produce a necessary entity or structure. Taking a bunch of things that are existentially zero by themselves and stacking them—even infinitely—is never going to amount to anything more than a whole lotta’ nothing.

Aquinas thus thinks there must be something, either within or outside the collection or series, whether finite or infinite, that is categorically different and can account for why anything that does not exist in virtue of what it is nevertheless actually exists.5 And that being is something whose nature or essence is really the same as its existence. In the simplest terms, if there were not something that exists in virtue of what it is, then there could not be anything that exists in virtue of something else. But there are things that exist in virtue of something else. Therefore, there must be something—at least one thing, perhaps more—that exists in virtue of what it is: something whose whatness is not really different or distinct from its isness.

At this point, I think Aquinas’s argument is really just drawing upon two very basic—perhaps even Moorean, or very nearly so—intuitions. Namely, there is our intuition of contingency, according to which some things exist but need not have existed, on the one hand; and our intuition of well-foundedness, according to which it is not turtles all the way down, on the other.

I say “Moorean,” or very nearly so, because these intuitions seem not only phenomenologically forceful but also commonsensical—that is, very widespread, deeply entrenched, stable, and so on. Of course, people can and sometimes do give up these intuitions. But, in my experience, this happens only after a whole lot of—if I may—wrongheaded metaphysical theorizing. It is definitely not the default starting point.

Indeed, people in the relevant literature sometimes complain that well-foundedness—the idea that chains of dependence must eventually terminate—is rarely rigorously argued for. Karen Bennett makes this point. Instead, it is often treated as obvious simply because people find it intuitively compelling.6

Of course, Aquinas has a lot of scholastic machinery at work in his argument. But, at the end of the day, that machinery is there to support and explain what I think is really the commonsense position: that contingent reality calls for explanation and that chains of causal dependence must ultimately terminate in something relevantly different—something metaphysically special. And I take it that any philosophical system capable of doing that—that is, capable of making the best sense of our strongest intuitions—is, other things being equal, to be favored.

The final challenge concerns identifying this necessary being as God. This is where Aquinas’s scholastic metaphysics does a lot of really important work. For Aquinas, the fundamental being whose essence just is its existence is also pure actuality and (by extension) absolutely simple.

According to Aquinas, this allows for a fairly straightforward conceptual unpacking of the traditional divine attributes, at least given the underlying metaphysics, insofar as a being of pure actuality—or pure existence—is absolutely ontologically simple, immaterial, eternal, omnipotent, omniscient, and perfectly good.

I’m not sure there is a similarly direct and intuitive shot from the bare notion of a non-contingent being all the way to God as understood by classical theists. There is, however, at least this much: it seems rather strange to identify any physical or material being—or even the physical universe as a whole—as the necessary being. People tend to find that suggestion strange, in my experience. Certainly, I find it strange.

I have other reasons for thinking it is definitely false. But even on the face of it—yeah, strange! For my money, I have always found the atheist position that physical reality is just brutely contingent to be, for whatever reason, the more plausible of the two options. I’d rather something be contingently absurd than necessarily absurd, if you catch my drift. In other words, sprinkling necessity onto something that does not seem relevantly different appears to compound the mystery rather than reduce it.

To be clear: I don’t actually find the brute-contingency position plausible—like, at all. But whatever tiny bit of plausibility it has seems greater to me than the suggestion that physical reality, or some segment of it, exists necessarily.

But whatever.

So, here I’ll simply re-emphasize that this is where Aquinas’s background metaphysics does a lot of important work, insofar as it provides extremely useful resources for traversing the so-called gap problem—that is, the problem of making the case that the necessary being is God. (For those wanting a more technical development, in my recent article criticizing William Lane Craig’s model of God, I attempt to explain how classical theism is especially well positioned to make God’s necessary existence intelligible—indeed, to show how God exists necessarily in virtue of what He is—and thereby reduce the number of brute facts to zero.)

Very briefly—and, again, there is far more detail in my book—Aquinas holds that existence is the principle of actuality in things, and that familiar, everyday objects are metaphysical composites of principles of actuality and potency. The essence of a contingent thing stands in potency to an act of existence, insofar as its essence is not, as it were, automatically actual—which seems obvious enough.

Aquinas’s act–potency distinction is, of course, inherited from Aristotle and is used to make sense of change, composition, and the like. Again, it does important work—not merely important work, but necessary work, I would say—in the philosophy of nature and metaphysics generally. But when it comes to the philosophy of God, it has an especially large payoff.

How so? Because if there is a being whose essence just is its existence, that being turns out to be purely actual. It is not a metaphysical composite and contains no intrinsic passive potency. The upshot is that—again, internal to Aquinas’s system of thought—this being possesses the traditional divine attributes listed above.

Because it is purely actual, it is immutable, since change requires the actualization of some potency. And because it is immutable, it is eternal: time is the measure of change, so a being entirely beyond change is also outside time.

It is immaterial because matter is itself a principle of potency for Aquinas and Aristotle, and this fundamental being utterly lacks potency.

It is omnipotent because it is the ultimate necessary condition for the actualization of every possibility of being, and the actual cause of every actual being apart from itself.

Omniscience follows from Aquinas’s metaphysics of mindedness and immateriality—which we are definitely not going to get into now—together with the fact that this being is the intelligent cause of everything that exists. In other words, it knows everything there is to know by, first, knowing itself perfectly, including itself as cause, and, second, knowing all the happenings of the world precisely because it is actively bringing them about.

And it is perfectly good because of Aquinas’s metaphysics of goodness as convertible with being—again, not getting into all that now. A being that is pure existence or pure actuality will therefore also be a being of pure, unrestricted goodness.

God is the paradigmatic good.

Naturally, one might protest against the Thomistic metaphysics used to derive the divine attributes. But what should be clear is that, once the relevant metaphysics is in place, one arrives by a remarkably direct and systematic route at a robust conception of the divine—and at the conclusion that fundamental reality could be nothing other than God.

Again, this result comes by way of fairly direct implication, not by comparing probabilities or theoretical virtues that might cause one to favor—or “slightly incline” toward—some vaguely theistic hypothesis over a more naturalistic one.

To me, that is a significant advantage, especially once one realizes that metaphysics is not something one can ever really avoid doing anyway. It is simply a matter of when you are going to do it—and how well.

I’m ranting. I do apologize.

So, let’s close this thing out. My aim was to provide a simple introduction—though I fear things became a bit more complex than I originally intended, but… ah well—to Aquinas’s core approach to God as found in De Ente. Many, myself included, have suggested that De Ente provides an especially illuminating lens through which his later Five Ways should be interpreted.7 Effectively, the Five Ways can be understood as different angles into this same general kind of “existential proof”—that is, as different entry points into a particularly metaphysical form of cosmological reasoning toward a fundamental cause whose existence is established, and whose nature is conceptually illuminated, through Aquinas’s understanding of existence.