The Interaction of Philosophy and Theology in Aquinas's Christology

Abstract

Aquinas accepts the harmony of faith and reason, but he does not think that such harmony is always easily arrived at. After making some background points about his views on faith, reason, philosophy, and theology, I explore two cases drawn from his Christology. In the first, philosophical thinking influences how we understand revelation; in the second, theological thinking influences how we understand a topic normally thought of as part of philosophy. In both cases, harmony is not pre-given but instead arrived at only through a process of adjustment.

Introduction

At a time when a good number of theologians were suspicious of non-Christian philosophers, Thomas Aquinas studied Aristotle and others carefully, making extensive use of their ideas. Aquinas's insight was that since reason and faith are both divine gifts, one need not fear that they will turn out to be incompatible.¹ In this context, one often hears that Aquinas believed in ‘the harmony of faith and reason’.

Speaking about Aquinas like this is not wrong, but the matter is more complicated than it might seem as first. Aquinas's view is that the true faith is compatible with the products of true reason. In practice, however, our attempts to understand the faith can go astray, and so can our attempts to exercise our reason. When that happens, tensions and even contradictions can arise. It is sometimes necessary to make serious adjustments in our thinking before we arrive at that happy place where faith and reason are in harmony.

In this paper, I will spell out some ways in which Aquinas makes adjustments of this sort in his Christology. Because Aquinas does not provide us with extensive methodological remarks, I will do this mostly by examining actual cases of how he proceeds. But first I need to address preliminary points at some length.

Background remarks on faith, reason, philosophy, and theology

Aquinas distinguishes various kinds of truth, and various grounds on which truths can be held. Let us start with a rather down-to-earth truth like the truth that tin is a metal. This truth is not revealed. It is nowhere to be found in Scripture or the official teachings of the Church. It is not proposed to us as something to be accepted by the virtue of faith. Instead, it can be held only on the basis of unaided human reason.

It might seem that things that can be held by reason but not by faith have no relevance to theology, but the issue is not quite so simple as that. Scripture tell us that God created animals, but it takes for granted that we already know, more or less, what animals are in the first place. Scripture tells us that Joseph was taken to Egypt, but it takes for granted that we already know, more or less, where Egypt is located. Scripture addresses itself to our minds, then, but it does not assume that our minds are empty; on the contrary, it presupposes that we know certain things already.

Such unrevealed truths are not wholly irrelevant, then, but neither are they of central interest in theology. What is of central interest in theology are truths about God and truths about creatures insofar as they are related to God. (For us, of course, the most important of the truths about creatures are the truths about human creatures.) And all of these truths are revealed.

These revealed truths can be divided into two classes. Those in the first class can be held only through faith, in the sense that human reason cannot arrive at them on its own, no matter what. A good example is the truth that there are three divine persons.² By saying that such truths can be held only through faith, and not through reason, Aquinas does not mean that we should not think about them. It is perfectly allowable, and in fact praiseworthy, to reflect on such truths so as to understand them more fully, to infer further truths from them, and even to think of reasons in favor of believing them in the first place (as long as such reasons are not taken to be proofs).³ However, if someone will accept such truths only on condition that he has philosophically adequate arguments in favor of them, then he is exhibiting a deficiency in the virtue of faith: true faith involves willingness to believe something that one cannot prove. What's more, if anyone does in fact hold such truths on the basis of human reasoning, then he is, ironically enough, making an error in reasoning, because such truths cannot be known philosophically. Someone who, for example, holds the Trinity on purely rational grounds is not someone who is impressively philosophical, but someone who has fallen for some bad piece of reasoning.⁴

The other revealed truths are the ones that human reason can arrive at on its own. Examples include the goodness and the providence of God. Traditionally these are called ‘preambles of faith’, in contrast with ‘articles of faith’. Aquinas has a number of interesting points to make about them.

The first point has to do with Aquinas's explanation of why God reveals them. Although they can be known apart from revelation, humans are not, in practice, likely to discover them through reason alone; and even if they do, their discovery is likely to be mixed up with error. The human mind is weak, and time for thinking is short. Knowledge of these truths is necessary for salvation, however, so a divine ‘zeal for souls’ drives God to reveal them to us, lest our salvation be endangered through busyness or stupidity.⁵

Another point is that each of these truths—each, that is, of the truths that God proposes for faith but that can also in principle be known by human reason, without faith—is held by each individual person either by faith or by reason, but never by both. If one holds that God is omnipotent by faith, then one does not hold it by reason; if one holds it by reason, then one does not hold it by faith.⁶

This might seem inconsistent with Aquinas's claim, in ST II-II, q. 2, art. 4, that it is necessary for human beings to believe, i.e., hold by faith, even things that are knowable by natural reason. The inconsistency is only apparent, however, as is clear from the fact that in that very same article, in his reply to the second objection, Aquinas repeats that no one can simultaneously hold one truth both by reason and by faith. His point in q. 2, art. 4 is not that every person needs to hold every revealed truth by faith, but only that generally speaking, given the human condition, (almost all) humans need to hold such truths by faith; this is perfectly consistent with there being a few exceptions, in the form of philosophers who can figure certain truths out on their own.

Here a terminological remark might be in place. In ordinary English, we might say that some people know certain things by faith, while others know them by reason—we might, that is, use the word ‘know’ in two ways, either with ‘by faith’ or with ‘by reason’. But this can be misleading in a Thomistic context. Aquinas typically uses the word scientia specifically to indicate knowledge by demonstrative reason; and scientia is often rendered into English as ‘knowledge’. If ‘knowledge’ means scientia, and scientia is incompatible with faith, then the phrase ‘knowledge by faith’ might suggest something self-contradictory. As long as we are careful, however, this can be treated as a merely verbal issue. In what follows, I will speak not of what is ‘known’ by either reason or faith, but of what is ‘held’ by either reason or faith; ‘holding something by reason’ means having demonstrative proof of it, i.e., scientia, while ‘holding something by faith’ means accepting it as revealed.

What all this means is that there are two ways to divide truths. The first way of dividing them considers truths with regard to how they can be held, and here we have a three-way distinction: there are truths that are provable only (e.g., that tin is a metal), truths that are believable only (e.g, that there are three divine persons), and truths that are both provable and believable (e.g., that God is omniscient). Provable-only truths can be can be held by reason, but they cannot be held by faith, because God does not reveal them. Believable-only truths can be held by faith, but they cannot be held by reason, because they exceed our reasoning powers. Believable-but-also-provable truths can be held by reason, or they can be held by faith.

A second way of dividing truths considers them with regard to how they are actually held by some particular person at some particular time. Here we have a two-way distinction between proved truths, truths that a given person holds on the basis of unaided reason, and believed truths, i.e., truths that a given person holds on the basis of faith. Importantly, where the line between proved truths and believed truths falls varies from person to person. Believable-only truths can only be believed truths, and provable-only truths can only be proved truths, but whether believable-or-provable truths are proved or believed by this or that person depends on whether the person in question has figured out philosophical demonstrations for them. If he has, then they are, for him, proved truths; if not, then they are, for him, believed truths. (Or, to be sure, they might be neither proved nor believed, if he has never thought of them, or is in doubt about them.) But, as noted above, a truth that is proved for one person could be believed by another, and vice versa. Indeed, it seems clear that a believable-or-provable truth could, for a given person, pass from being believed to being proved: he could find a proof for something that previously he had no proof for. Alternatively, it seems that such a truth could, for a given person, pass from being proved to being believed: he could forget a proof he had previously worked out, but then accept that truth on the basis of faith.

In the next section of this paper, I will show how, in Aquinas's Christology, believable-or-provable truths can influence how we understand believable-only truths; I will suggest further that when a believable-or-provable truth is, for a given theologian, a proved truth, we will have, in a rather qualified way, a case of philosophy putting intellectual pressure on how we understand the faith. In the section after that, I will argue that believable-only truths can put pressure on our understanding of provable-only truths. I will end with some brief summary remarks.

Philosophical influence on theology

The picture of God presented to us in Sacred Scripture is not clear, at least at first glance. For example, Jn 4.24 says that ‘God is spirit’, from which it is reasonable to infer that God is not bodily; but on the other hand, various scriptural passages make reference to God's holy arm, his footstool, and so on.⁷

Christians do say that God is an immaterial spirit and that descriptions of God in corporeal terms are metaphorical. This is undoubtedly correct. At the same time, however, it makes sense to ask why we resolve the Bible's internal tension in the way we do. Why not say that God is bodily, and explain the remark about God's being spirit as a metaphorical way of indicating, say, that God is rational?

One possible answer to that question is ‘philosophical reason’. On this way of thinking, it is because of philosophical inquiry that we know that ‘God is spirit’ is literal, and that ‘thou didst scatter thy enemies with thy mighty arm’ is metaphorical. That philosophical inquiry could yield such results is undoubtedly Thomistic. Here are some philosophical arguments gathered from ST I, q. 3, art. 1: (a) nothing corporeal can be an unmoved mover, but God is an unmoved mover, so therefore God is incorporeal; (b) anything corporal involves potentiality, but God is the first being, and the first being must be entirely actual and in no way potential, so therefore God is incorporeal; (c) God is the noblest being, and no corporeal being is the noblest being, so therefore God is incorporeal. In the face of philosophical considerations like these, one might say, it simply makes more sense to interpret the bodily descriptions of God as metaphorical than it does to interpret the claim that God is spirit as metaphorical. And so, one might say, that is how we know which way to resolve the apparent tensions in Scripture.

Implicit in this way of thinking is not merely the point mentioned earlier, namely, that revelation presupposes that its recipients already know certain things, but more strongly, that the presupposed knowledge in question includes a fairly sophisticated understanding of the divine nature. But Aquinas would not accept this stronger view. After all, one reason that revelation is there, on his understanding, is to make up for the fact that a philosophical understanding of God is too hard for most people to arrive at. If revelation is meant to make knowledge of God available to people who do not have the opportunity or the ability to understand Aristotle, it cannot be the case that revelation can be understood only by people who already understand Aristotle.

Coming at this another way, if Aquinas held that a philosophical understanding of God is required for understanding revelation, that would seem to imply that the virtue of faith merely enables us to accept what God reveals, whatever that turns out to be. But that is not Aquinas's view. He thinks that faith also involves the ability to grasp what it is that God reveals.⁸ Someone with little or no philosophical training will, by divine faith, be able to grasp which Biblical assertions are literal and which are metaphorical.

We must grant, then, that on Aquinas's way of thinking, holding believable-or-provable truths as proved is not required. Even so, Aquinas does think it is possible to hold such truths as proved, and that seems to imply the following: some people, in some cases, arrive at a correct understanding of revelation on the basis (in part) of a philosophical understanding of the divine nature or of some other important believable-or-provable truth. The understanding that such people have of the faith is informed by non-theological, philosophical knowledge.

One might worry, however, that it is not good for this to happen. Monica, let us suppose, is not a Christian, although she is a theist. She comes, by philosophical reasoning, to the view not only that God exists, but that God is omniscient. If she later becomes a Christian believer, holding many things by faith, the fact that she already holds divine omniscience by reason will, on Aquinas's way of thinking, prevent her from holding that particular truth by faith. Ought she to wish she had never arrived, in her pre-Christian days, at a proof of divine omniscience, because doing so limited the scope of the faith she would later acquire? Alternatively, let us suppose she begins as a believer, holding the truth of divine omniscience by faith. If she comes to discover proofs for it by studying philosophy, she will, on Aquinas's understanding, cease to hold it by faith. Should she avoid this, on the grounds that it is more virtuous for a Christian to hold something by faith, rather than by reason? If she does end up with a proof, does that mean she is now less of a Christian?

The fact that Aquinas so often offers philosophical arguments for believable-or-provable truths suggests very strongly that he does not see it as problematic for Christians to be in possession of such arguments. We find a confirmation of this, with an important qualification, in ST II-II, q. 2, art. 10, where he asks whether having reasons for what we believe diminishes the merit of faith. Focusing initially on his response, and on his reply to the first objection, we can understand him as follows. In the case of a believable-or-provable truth, where philosophical proof is possible, being in possession of a proof for that truth does not diminish one's faith. However—this is the qualification—one must still be willing to accept that truth on the basis of revelation alone. The virtue of faith involves a kind of counterfactual willingness to accept on faith even those truths for which one has proof.

The interpretation of ST II-II, q. 2, art. 10 is not, however, entirely straightforward. The second objection says that faith presupposes lack of vision—we have faith in what is unseen and unproven—but that reasoning in advance of assent leads to vision of the truth, with the result that such reasoning takes away from the nature of faith (ratio humana videtur diminuere rationem virtutis ipsius fidei).⁹ In his reply, Aquinas grants that demonstrative reasons in favor of ‘preambles’ of faith—i.e., the truths which I have been calling believable-or-provable truths—make such truths evident, with the result that the nature of faith is indeed diminished, although charity is not.

The reference to charity here is connected to a point already mentioned: as Aquinas goes on to explain, it is charity that makes us willing to believe, in the absence of proof, even things that we do, as a matter of fact, have proof for. But there still seems to be some tension between this second reply and things said elsewhere in the same article. In the reply to the first objection, he had said that having a proof for a believable-or-provable truth does not eliminate or diminish the merit of faith (non propter hoc tollitur vel diminuitur meritum fidei). In his reply to the second objection, he says that proofs of believable-or-provable truths diminish the nature of faith (rationem fidei) but not the nature of charity, and this is why the nature of merit remains undiminished.

It is perhaps less than obvious how all this fits together. Here is a possible interpretation. Since proofs give us vision—we see for ourselves that proved truths are true—believable-or-provable truths, when held as proved, are indeed not held by faith. Our cognitive state in such a case does not conform to the nature of faith. However, what is taken away in such a case is not ipso facto the willingness to believe without vision, a willingness based on charity. For this reason, the virtue itself is not removed, but only a particular instance of its exercise. The first reply, I suggest, says that proving a believable-or-provable truth does not take away the virtue of faith, so long as one retains the willingness to believe whatever God reveals, while the second reply says that proving a believable-or-provable truth reduces the number of cases in which one exercises the virtue of faith. Whether one has a virtue, and the merit that comes with it, does not depend on how often one exercises that virtue.

We have seen so far, then, not only that some people can and do hold believable-or-provable truths as proved, but also that this need not be problematic for Christians. But what role do such philosophically-held truths play in such a person's theologizing? One thing Aquinas says is that a theologian can use philosophical ideas to make theological ideas better understood; here he cites Augustine as giving similitudes for the Trinity.¹⁰ Thus, philosophical ideas can serve, for Aquinas, as aids for explaining doctrines understood apart from them. As I will now try to show, however, philosophical ideas for Aquinas can sometimes do more than that, influencing how the theologian himself understands doctrine.

Consider the following difficulty. We can know, from philosophy, that God is immutable and impassible. God cannot receive anything or be modified by anything, cannot undergo any actualizations of potentialities, and so on.¹¹ But it is far from obvious how this fits with the Incarnation. It might seem impossible for an immutable God to become human, and it might seem impossible for an impassible God to be mortal in virtue of his human nature.

It may be tempting to conclude that God does in fact change, that God does receive actualization from created beings. One might even claim this as one of those ways in which the Gospel overcomes human thinking: Greek philosophers may think that God is pure act, but Christians know that God grows and changes in a metaphysical dance with his creation. But Aquinas does not take this approach, and so he seeks to understand the Incarnation in a way that preserves divine impassibility and immutability.

Aquinas's strategy involves re-purposing and modifying an idea he had already used to explain the more general relationship between God and creatures. God always stays the same, but the relationship between God and creatures varies: sometimes we are being led through the desert, sometimes we are being punished, sometimes we are being fed. For Aquinas, this is possible because being led, being punished, and being fed are all relations, and relations can change with only one of the parties to the relation changing. If someone passes from not thinking of the Eiffel Tower to thinking of it, then the relationship between him and the Eiffel Tower has changed; but the Eiffel Tower itself has not changed, only the thinker has. Aquinas applies this sort of analysis to explain how things stand between God and the world. When relations between God and creatures change, only the creatures are changing, without God himself changing.

Aquinas applies this basic idea to the Incarnation, but with a twist. To start with the basic idea, he holds that being related to the Word of God is a modification or enhancement of the assumed human nature, and that this is sufficient for the hypostatic union. There is no need for the Word itself to be modified or enhanced by a complementary relation connecting it back to the humanity it assumes. Something happens to the human nature without anything happening to the Word.

Now for the twist. In the case of the Incarnation, and only in the case of the Incarnation, the created reality is not merely related to God; it is united with God ‘in person’, i.e., in a way that results in there being only one person. The assumed human nature does not give rise to a newly-existing human person, a person joined to the person of the Word in, let us say, a relationship of friendship; instead, the assumed human nature makes one and the same already-existing divine person to be human as well.¹²

Let me conclude this section in a way that brings out what is important for interaction between philosophy and theology. Aquinas does not infer from the idea that the Word became flesh that God is not, after all, impassible and immutable. Instead, he allows this believable-and-provable (and, for Aquinas himself, proved) idea about God to be a constraint on his theological account of the Incarnation. Under the force of this constraint, he has to re-deploy his understanding of creation in a new way. Something held by reason is serving as a constraint on theology, and the result is theology's coming up with a new and interesting way of thinking about what is possible. Theology does not lose force in this way but, if anything, gains force.

Theological influence on philosophy

Now I want to explore a different kind of case, one in which theology puts pressure on philosophical thinking.

Recall the classic formula for the Incarnation that was arrived at by the fifth century: Jesus is one person who exists in two natures, divinity and humanity.

A person, as classically understood, is a special kind of substance. A substance is an independent, unified individual, and, therefore, since a person is a special kind of substance, a person is a special kind of independent, unified individual. What makes a person different from substances that are not persons is this: persons have reason. A cat and a human are both substances, but humans are persons while cats are not.¹³

To say that persons have reason is to suggest something about their natures, so that leads to the second element of the classic formula, namely ‘nature’. A nature is what something is at the most basic level; a thing's nature constitutes that thing as an independently subsisting thing of a certain kind.

How do these notions of person and nature function in Christology? To say that Christ is one person is to push back against any suspicion that he might actually be a team of persons working closely together. Jesus is not a human person working closely together with a divine person, the Son; rather, he and the Son just are the very same person. To say that Christ has two natures, and that these natures are humanity and divinity, is to say that Christ really is human, and that he really is divine. He is not merely human in outward appearance, but really and truly human; not merely inspired by God, but literally personally divine.

The core points can be put in terms of two principles, the ‘integrity principle’ and the ‘unity principle’. The unity principle states that Christ is really one person. The integrity principle states that each of Christ's natures is there in its fullness and integrity.

And now a difficulty begins to show itself. A substance's nature, unlike any of its accidents, constitutes or establishes it at the most basic level. It explains why that substance is an independently subsisting thing of a particular kind. But suppose we apply this thought to the case of the Incarnation. If Christ has two natures, then it seems he has two principles in virtue of which he is an independently existing thing. That would seem to mean that ‘he’ is two independently existing things. His divinity, one might think, establishes a divine person, and his humanity establishes a human person. This pretty clearly violates the unity principle.

To protect the unity principle, one might argue that Christ is not entirely human; he has the outward appearance of being human, maybe even a body, but ultimately he does not really have a complete human nature. This maneuver would indeed protect the unity principle, but at the cost of abandoning the integrity principle. Another, more subtle way of doing the same thing is by thinking of Christ's human nature as being an accident rather than a substantial nature. Neither of these ideas is really consistent with holding to the idea that Christ has a true human nature.

Faced with all this, someone who was still convinced of the traditional doctrine might be inclined to say: ‘Well, I don't understand it, but I still believe it, so that's that’. Accepting the truth of things we cannot fully understand can be legitimate (consider how few of us really understand how airplanes fly). However, in the case at hand, I think that faith can still make progress as it seeks understanding.

I said that the role of a substantial nature is to make something be an independently subsisting thing of a certain kind. This way of putting it obscures the fact that substantial natures actually play two distinguishable roles or functions: they make something be independently existing, and they make it be of a certain foundational kind. Distinguishing these suggests a way of honoring the unity principle and the integrity principle together. First, we can say that both of Christ's natures perform the second function, i.e., the function of making something be of some fundamental kind: his divine nature performs the function of making him divine, and his human nature performs the function of making him human. But then we can say that things are different with regard to the first function, namely, the function of making something be independently existing: his divine nature does perform that function, but his human nature does not. If his human nature does not perform that function, then Christ will be only one person after all.

It seems that something like this approach is what Aquinas had in mind. He says, at ST III, q. 3, art. 1, ad 3, that ‘the Son of God does not have existence simpliciter from his human nature… but only existence as human’.¹⁴ His human nature does not give him existence simpliciter (i.e., existence in an unqualified sense, the independent subsistence characteristic of a substance). His human nature does not make him be an independently subsisting person. It does, however, give him existence as human. In other words, it performs the second function, but not the first function, and therefore it does not give rise to a second person and thereby cause a violation of the unity principle.

What Aquinas does not do, however, is to address the following objection. Any substantial nature must, necessarily, establish something as a substance; a cat-nature is not a cat-nature unless it actually establishes something as an independently-existing feline substance, and a human nature is not a human nature unless it actually establishes something as an independently-existing human substance. Therefore, the fact that Christ's human nature does not establish an independently-existing human substance means it is not a true substantial human nature. If this way of thinking is right, then the problem cannot be solved, because Christ's humanity will not be a real human nature unless it actually does establish something as an independently-existing human person, which would violate the unity principle. On this way of thinking, preserving the integrity principle makes it impossible to accept the unity principle.

One can respond by proposing an alternative way of thinking about substantial natures. Instead of saying that a substantial nature is something that must establish an independently-existing substance, we can say that a substantial nature is something that can establish an independently-existing substance. Something could be a substantial nature even if it did not actually establish a substance, as long as it—the nature—was still the sort of thing that could do so. (Compare: something does not have to actually explode in order to be a bomb, it just has to be the sort of thing that could explode. Of course a nature and a bomb are very different; the point is only that capacity is sometimes enough for classification.) On this alternative way of thinking, the assumed nature, in order to be a true human nature, need only be capable of establishing an independently-existing person. It does not matter that Christ's human nature does not in fact establish one, because it still has the intrinsic capacity to do so.

If the human nature has that capacity, why doesn't it exercise it? The following strikes me as the most promising answer: because that human nature is joined to the pre-existing person of the Son. Whatever the Son's human nature does will be something that it does to the Son, or for the Son. (If Socrates's head lacks all hair, that makes him bald, not anyone else.) In a roughly parallel way, the Son's human nature, if it makes anyone human, will make him human, and if it establishes anyone as an independently existing person, then it will establish him as an independently existing person. Now, prior to having a human nature, the Son was not human, so when the human nature was joined to him, it had an opportunity to exercise the function of making something human, and it did so. But prior to having a human nature, the Son was already an independently existing person; so when the human nature was joined to him, it did not get an opportunity to make the Son an independently-existing person. In and of itself, the human nature had the capacity to make something be a person, but its union with the Son made it unable to exercise that capacity. It arrived too late to make an independent person, but not too late to make an existing person human.¹⁵

All this talk of natures having opportunities to do things, such as making supposits exist in a certain way, is rather dangerous. It can give the misleading impression that a nature is a kind of quasi-agent, with powers that it exercises under certain circumstances.¹⁶ To clear all this up would take a very lengthy discussion, but perhaps the following will be sufficiently brief and yet helpful for understanding Aquinas. Aquinas sometimes speaks of a nature or a form ‘constituting’ a substance as F, meaning thereby that the substance exists as F in virtue of having that form.¹⁷ Using this language, my proposal can be put as follows. For Christ's human nature to be a true human nature, it is necessary not that it actually constitute Christ as a subsisting supposit, but only that it be a nature that could so constitute something; alternatively, what is needed is not that Christ be a subsisting supposit in virtue of having that nature, but only that the nature be such that something might have been a subsisting person in virtue of having it.

Aquinas does not say explicitly that we should rethink our notion of substantial nature, but doing so is a way of making sense of what he does say. It preserves, in a way consistent with Aquinas's overall approach, both the unity principle and the integrity principle. But there still might be a problem. Taking this approach might mean giving up on philosophical metaphysics in favor of some kind of theological metaphysics. Putting the worry in a relatively non-aggressive way, we could say, ‘It's a shame that we didn't start with theology; we could have saved a lot of time’. Putting it more aggressively, we could say, ‘It's rather ridiculous to allow one special case, based on revelation, to displace philosophical thinking’.

In response, I think we can say that in a very important sense, this theological refinement of the philosophical idea of nature doesn't undercut the philosophical idea. The philosophy-only understanding of essence or nature says that a person's nature makes that person exist independently. The theological refinement of essence or nature says, more cautiously, that a person's nature can make that person exist independently. However, as we developed the point above, the only reason why a nature would not exercise that person-making capacity would be that it was united to an already-existing substance or person, as happens in the Incarnation. This means that even according to the theology-influenced version of the idea of nature, a person's nature will establish that person as an independently-existing person unless that nature is a second nature, a nature added to an already-existing person. In Socrates's case, and in everyone's case but Christ's, human nature is not joined to a pre-existing substance or person. In every case but Christ's, a person's human nature is his or her only substantial nature, with the result that it, the substantial nature, most definitely will exercise its person-making capacity. In other words, a substantial nature will refrain from exercising its substance-making capacity only when it belongs to a substance with multiple substantial natures; but the Incarnation is the only case where this happens; therefore, outside of the Incarnation, the philosophy-only understanding of nature gives us the same result as the theology-influenced version does. And because it would be awkward, or worse, constantly to be saying ‘unless we are dealing with the Incarnation’ every time we wanted to talk about natures, we thus have excellent reason to continue using the philosophical formulation in philosophical contexts. Believable-only truths inform philosophical thinking, but not in a way that undercuts philosophy's integrity.

Someone who accepted what was just said might still consider the move too desperate and costly. If we have to modify one of the basic concepts of metaphysics to avoid conflict with Christianity, one might say, this just shows that Christianity is a problem. (If my bank account shows that I'm overdrawn, no one will allow me to revise the laws of mathematics to prove that really I'm solvent.)

There is no way around admitting that it makes a difference, a huge difference, whether or not one actually believes in Christian revelation. If Christian revelation is not true, then this whole discussion has been, at most, an interesting thought-experiment, fun to play around with but not telling us anything about how the world actually is. But if, on the other hand, Christian revelation is true, then the Incarnation is a fact, and of course there is nothing wrong with adjusting our preconceived philosophical notions to fit the facts.

The worry was that the theological refinement involves modifying a basic concept of metaphysics, and the response to the worry is to admit that this is what is going on, but to deny that it is worrisome. If divine revelation is real, then bringing our thoughts into line with it could hardly be a mistake.

Here are some additional considerations. First, if there really is such a thing as God, then God is surely very different from us. It should not be surprising to find that our ordinary philosophical ideas—ideas we developed from, and for the purpose of, thinking about the created world around us—are not adequate for understanding the transcendent creator.

Second, even though the theological version of the concept of nature is different from the philosophical one, it is hardly devoid of content. Re-thinking the notion of nature is nothing like ‘blind faith’ or ‘just repeating slogans that you don't understand’. The fact that the rethinking is based on revelation does not prevent it from having plenty of intelligible conceptual content.

Further, to repeat a point made earlier, theological refinements to metaphysics do not mean that the metaphysical concepts we were applying to creation have been wrong the whole time. Theology pretty much leaves philosophy as it is, only adding a special consideration that makes it possible to handle a unique case. It enables us to see just a little bit more, while leaving what we used to be able to see fully visible.

I just said that ‘we’ are now able to see a little more. But who is this ‘we’? In particular, do philosophers as philosophers now see things differently? Or is this something that only theologians do?

Under the influence of theology, a philosopher, even a non-believing philosopher, can see this new possibility as a possibility: he can see the possibility of there being a substantial nature that merely can, but in actuality does not, give rise to an independently-existing substance. But a philosopher as such will not affirm that any such nature exists in actuality: only revelation gives us any reason to suppose that this possibility has been realized. The philosopher as such—either the non-believing philosopher, or the believing philosopher abstracting from what he holds by faith—will go no farther than acknowledging it as a possibility. But that is far from nothing. It is still a way for the philosopher to learn something from theology.

To conclude this section, we have been looking at another case of the interaction of philosophy and theology in Aquinas's Christology. The original idea of substantial nature comes from philosophy, of course. When brought into a theological context, it comes to be understood in a broader and deeper way—a way that can, in its turn, be taken on by the philosopher as part of an intensified understanding of how things can be.¹⁸

Concluding remarks

Faith and reason are in harmony in Aquinas's Christology, but as we have seen, the harmony must sometimes be established through some difficult thinking. Sometimes philosophical thinking points to constraints that the theologian works within. Sometimes theology uncovers ways in which purely philosophical analysis falls short of grasping the nature even of created reality. Alas, there is no algorithm for knowing how the interplay between philosophy and theology will work out in any particular situation. One simply has to push forward in one's inquiries, as best one can, and see what comes up.¹⁹