What Is the Purpose of the Proofs of God's Existence?

Faith and Proof

Almost all of the undergraduates I teach think that it is a proposition bordering on the absurd to say that one might be able to demonstrate God’s existence. This view is held not only, as might be expected, by unbelievers, but by devout believers as well. Typically, when I press them as to why they are so sure of this, most of them will say that we lack physical evidence of God’s existence, and in the absence of physical evidence you cannot prove anything. Some—and this is typically those who are believers—will also say that the whole point is that belief in God cannot be proven—that is why they call it “belief,” after all.

It is no secret that Thomas Aquinas thought differently. Thomas quite famously offered in the Summa theologiae (1.2.3) five ways in which the statement “God exists” might be demonstrated to be true. Each of these argued, based on something that is evident to us (the phenomenon of change or the orderliness by which natural things attained their ends, for example), for the existence of something not evident to us, a transcendent source of what is evident to us that at least some people called “God.” Thomas went to some trouble to argue that, while God’s existence is not self-evident to us (in the way that a whole being greater than a part or a line being the shortest distance between two points is evident to us), it could be made evident to us (in the way that, in a right triangle, the square of the hypotenuse equaling the sum of the square of the two sides can be made evident to us). This would seem to put Thomas in direct opposition to my students, who so fervently believe in the indemonstrability of God’s existence.

Thomas sums up the matter of what reason can know with demonstrable certainty about God in this way:

We know of his relationship with creatures, so that he is the cause of them all; and we know that creatures differ from him, so that he is not any of the things that are caused by him; and we know that this is not attributed to him on account of any defect in him, but because he surpasses all things (Summa theologiae 1.12.12).

This suggests that it is not merely the bare affirmation “God exists” that we can demonstrate; there are in addition other statements that might also be shown to be true, such as “God is eternal” and “God is good” and “God is wise” and (perhaps most mysteriously) “God is simple.” These would seem to add to the affirmation of what people call God many of the divine attributes that people traditionally associate with God. So it would seem that Thomas held for a rather capacious body of rationally demonstrable truth claims concerning God.

Yet, at the same time, Thomas also said that what we could know about God by this means was relatively meager. Were human reason left on its own, “the truth about God such as reason could discover, would only be known by a few, and that after a long time, and with the admixture of many errors” (Summa theologiae I q. 1 a. 1). In numerous places[1] Thomas alludes to John of Damascus’s statement that we do not know what God is but rather only what God is not, and this seems to be the case especially with what human reason can discover on its own. So, Thomas thought, because our whole salvation depends on knowing God, God has not left us to our own devices with regard to knowledge about God, but had given us, through divine revelation, knowledge surpassing what reason could discover on its own.

How does Thomas manage to combine what seems to us a quite astonishing optimism about human reason with a realism-bordering-on-pessimism concerning the extent of what we can know concerning God? Also, how might he help us think about how those things that reason can discover on its own are related to the life of faith and to those truths about God that we hold on the basis of faith? A chief hint Thomas gives us to how he sees that relation is the term he occasionally uses for the truths that reason can discover: he calls them praeambula fidei—literally, “what walks ahead of/before faith”—and he contrasts them with the articles of faith, such as we find in the creed.[2] So we might ask, in what sense do these rationally demonstrable truths about God precede or walk before what we hold on faith?

Building Foundations

One perhaps obvious way to think about this is as a kind of temporal priority. That is, we can apply our minds to the question of God and then, once we have proven what can be proven, we can add to those truths other, non-provable, truths, such as the Trinity and the Incarnation. We might think of this as the building-foundation approach: you must first have the basis for faith laid in reason, and the truths of faith can then rest securely on truths of reason. I do not think you get many people in the tradition who explicitly advocate for this approach, though if you look at some older manuals of apologetics—which seek first to demonstrate the existence and attributes for God, then the possibility of a divine revelation, and then going on to make probable arguments for the Catholic Church being the bearer of this revelation—you can get the distinct impression that such works presume that one needs to be rationally convinced of some truths concerning God before one can move on to embrace the truths of faith.

This is not the approach of Thomas Aquinas. We might think it is, however, when confronted with statements like this one in his commentary on Boethius’s De Trinitate, where regarding proposition such as “God exists” and “God is one” he notes that, “such truths about God or about his creatures, subject to philosophical proof, faith presupposes” (Commentary on Boethius De Trinitate q. 2 a. 3). But whatever he means here by “presupposes,” he does not mean that such proofs must precede faith in time, nor does he mean that faith cannot be had without a basis in such proofs. Certainly by the time he writes the Summa theologiae, he does not approach the preambles of faith as a distinct body of knowledge that one must establish before one can embrace the quite different body of knowledge contained in the articles of faith. Thomas writes:

The existence of God and other similar things that can be known about God by natural reason, as Romans says (1:19), are not articles of faith but preambles to the articles. For faith presupposes natural knowledge just as grace presupposes nature and perfections presuppose something that can be perfected. Nevertheless, nothing prevents something that in itself is capable of being demonstrated and known from being accepted as worthy of belief by someone who cannot grasp its demonstration (Summa theologiae I q. 2 a. 2 ad 1).

I will come back to Thomas’s statement that “faith presupposes natural knowledge just as grace presupposes nature,” but for now I want to focus on the last sentence.

Thomas recognizes three things regarding knowledge of God and salvation. First, our salvation depends on our knowledge of God, because we can only move toward God as the goal of our existing if we have at least some knowledge of God. If you walked north from Baltimore you would, as a point of fact, be walking toward Canada; but you could only be engaged in the activity we call “being on a journey to Canada” if you had the purpose of arriving in Canada, which involves having some knowledge that there is a Canada, even if this is only a dim and cloudy perception of the true nature of that mysterious place. Likewise, if our lives are to be understood as a journey toward God, then we who are on that journey must have some knowledge that there is a God, even if it is only a dim and cloudy perception. So, salvation, the culmination of the journey toward God, requires some knowledge of God.

Second, metaphysics is hard. The kind of abstract thinking it calls for, the passage from knowledge of the mundane physical objects that are the natural objects of our human intelligence to the non-material truths that can be deduced from them, is something that not everyone is inclined toward or has time for or is capable of. Moreover, because the natural object of our intelligence is the physical world, metaphysics as a human practice has a certain kind of imprecision to it; it is like trying to do fine wood sculpting with a chain saw. It is pretty impressive what some people can achieve with a chain saw, but, let’s be frank, it’s not exactly fine art. All of this is why Thomas says, “the truth about God such as reason could discover, would only be known by a few, and that after a long time, and with the admixture of many errors” (Summa theologiae I q. 1 a. 1).

Third, God is a gracious and loving God who, as 1 Timothy says, desires that all people be saved and come to knowledge of the truth. This is why God, knowing the difficulty and imprecision of metaphysics, shares with humanity knowledge of himself in divine revelation, so that even those who do not have the inclination or time or ability to engage in metaphysical reflection, as well as those who do but do it in an inadequate way (“I’m sorry, but that sculpture of an owl looks more like a beagle”), might have the knowledge of God needed to journey toward God as the goal of their existence. So, Thomas tells us, the truths about God that can be rationally demonstrated—that God exists and is the cause of all things; that God is not any of the things that are caused by him; that God surpasses all things—are also contained in the deposit of faith. Most people, in fact, accept these truths on the basis of faith and not on the basis of rational demonstration.

All of which is to say that Thomas clearly does not think that faith requires a foundation of rationally demonstrated truths about God. Indeed, for most people, it is faith all the way down, and this is just fine. Not only fine, but in some ways better. The knowledge had through faith is not a second-best substitute for knowledge had through demonstration; it is in fact the sublime knowledge of God and the blessed shared with us wayfarers. Thomas says in one of his sermons (and I do not think he was merely pandering to his lay audience), that an old woman with faith knows more about God than the greatest of philosophers (Exposition of the Apostles’ Creed, prologue).

Her knowledge is, of course, more extensive than the knowledge that the philosopher has wrested from the world via reason—including such mysteries as the Triune nature of God and the Incarnation—but it is also more intensive, because faith involves not only intellectual assent to truth, but also the will being drawn by the beauty of the truths proposed. Truths held on faith engage the volitional aspect of the human person in a way that demonstrated truths do not. This is why faith is meritorious, while acceptance of a demonstrated truth, in and of itself, is not (see Summa theologiae II-II q. 2 a. 10).

Layer Cakes

So for Thomas the preambles are not a foundation that must be laid before faith can be established. But what about a variation on the building-foundation approach, one that we might call the layer-cake approach? Here the demonstrated truths of reason do not need to come before the truths embraced in faith in terms of a temporal sequence—you can bake both layers of your cake at the same time, as it were—but they are clearly distinct, and the faith-layer definitely rests on top of the reason-layer. Presumably, you could also have a cake without layers—an all-faith-cake—and this may be what most people will do, given the difficulty and opportunities for failure involved in baking the reason-layer. But if you do, for whatever reason, opt for the layer cake, your goal (as the judges on The Great British Bake Off would tell you) is to make sure that your layers are distinct and do not bleed into one another.

This might seem closer to Thomas’s view, but still does not quite get it. And to see why it does not quite get it, we can return to Thomas’s statements that “faith presupposes natural knowledge just as grace presupposes nature and perfections presuppose something that can be perfected” (Summa theologiae I q. 2 a. 2 ad 1). This is one of a handful of similar statements we find in Thomas about grace and nature that have excited intense interest among theologians, particularly in the mid-twentieth century. For my purposes, I would like to focus on the analogy Thomas draws:

natural knowledge is to faith        as         the perfectible is to perfection

It seems to me that the key here is to remember that for Thomas the model of what it means for something to be perfected is the perfection of matter by form. That is to say, a thing’s potential to become something in the fullest sense involves the complete transformation of that thing through the reception of a new form. The potential of a tree stump to become, through the agency of the artisan and the instrumentality of the chainsaw, a sculpture of an owl, involves the tree ceasing to be a tree and becoming wholly an owl sculpture. There does not remain a “tree-layer” over which is laid an “owl-sculpture-layer.” Metaphysically speaking, it is owl sculpture all the way down. So too with natural knowledge and faith. Even for the philosopher who has attained some natural knowledge of God, through grace that natural knowledge becomes wholly transformed into the meritorious, charity-infused assent we call faith.

But this cannot be quite right. After all, Thomas famously says that “grace does not take away nature but perfects it” (Summa theologiae I q.1 a. 8 ad 2), and what I am saying here might seem to suggest that the knowledge of faith takes away natural knowledge. Here again, I think the analogy of the perfectible and the perfect, thought in terms of matter and form, is helpful. Because in the real world we never encounter prime matter (which exists only in the realm of thought), but only matter existing under some form; so it seems to me that things often, if not always, retain some legacy of the previous forms under which their matter existed, and that legacy gives a distinctive quality to those perfected things. That is to say, an owl sculpture that is a perfection of a tree stump is going to have certain distinctive qualities not possessed by an owl sculpture that is the perfection of, say, a lump of plasticine. The matter lends to the perfected object something of its prior history as a tree stump: the grain and color of the wood, the inherent possibilities and limitations of the medium, etc. The wooden owl sculpture is going to being an owl sculpture differently than a plasticine owl sculpture.

In like manner, I think that the metaphysically-minded believer is going to believe differently than the non-metaphysically-minded believer. Not, of course, in terms of the content of faith. Both will be believers—and, as it were, believers all the way down—but the believer who has, whether before or after having come to faith, labored at demonstrating those truths concerning God that can be demonstrated, will possess his or her faith in a distinctive way; it will have a particular quality to it. Being metaphysically-minded is perfected by faith, not taken away. And, being perfected, this inclination to abstraction and rational argument will also serve the faith in ways that go beyond demonstrating things like God’s existence, unity, goodness, etc.

For Thomas says that demonstrating such preambles is only one way in which reason serves faith. Reason also serves faith in providing “a clearer notion, by certain similitudes, of the truths of faith” (Commentary on Boethius De Trinitate q. 2 a. 3). This often works by means of what Thomas calls arguments from “fittingness”—which use the tools of reason not to demonstrate truths, but to see the patterned coherence of the truths of faith in relation to one another. Reason also serves faith by “resist[ing] those who speak against the faith, either by showing that their statements are false, or by showing that they are not necessarily true” (Ibid.). The mind that is skilled in demonstration ought to be equally skilled in debunking fallacious demonstrations, and so serve faith in this way as well.

In some ways, it maybe be unfortunate that Thomas chose on occasion to use the term praeambula fidei to speak of those truths about God that are capable of rational demonstration, since it can suggest something that is a necessary prelude to faith, without which faith has no foundation. I hope I have shown that this not what he meant. But the word “preamble” does still have some value. It has the value of asserting that some truths that most of us hold on faith are in fact capable of rational demonstration, such that faith is the perfection and not the destruction of the reason one possessed before receiving the gift of faith. This is certainly useful in dealing with students, both believers and unbelievers, who think that faith involves embracing the irrational. It is also useful for suggesting that those who labor to grasp the highest reaches of human understanding—whether the great thinkers of antiquity or the honest seekers of today—can be understood as people who are, though perhaps only dimly realizing it, wayfarers walking toward faith, the faith that can give them a truer sense of that toward which they fare.