Early in his Summa Theologiae, Aquinas writes: ‘We cannot know what God is, only what he is not. We must therefore consider the ways in which God does not exist rather than the ways in which he does’.Footnote ¹ You may think that this seems an odd thing for a Christian philosopher to assert. So, you might suppose that Aquinas does not here mean what he appears to be saying. But I shall now try to explain why this supposition would be wrong and that Aquinas's philosophy of God is shot through with the conviction that, in a truly serious sense, those who speak about God do not know what they are talking about, regardless of whether they say ‘God exists’ or ‘There is no God’.

Some theologians and philosophers seem to suppose that we have a fairly clear idea what it means to say that God exists. Take, for example, Professor Richard Swinburne, one of the best known of contemporary philosophers of religion.Footnote ²

He tells us that someone ‘who believes that there is a God’ is a theist. By a ‘God’, Swinburne goes on to say, theists understand ‘something like a “person without a body (i.e. a spirit) who is eternal, free, able to do anything, knows everything, is perfectly good, is the proper object of human worship and obedience, the creator and sustainer of the universe”’.Footnote ³ According to Swinburne, ‘theology uses only ordinary words in their ordinary senses’. When talking about God, he says, ‘the theologian uses ordinary words to denote ordinary properties’ but ‘claims that the properties cited are manifested in unusual combinations and circumstances and to unusual degrees’.Footnote ⁴ On this account, God, though different from us, is also rather like us since God is a person who knows and acts.

Some theologians have held that God is immutable and outside time, but Swinburne thinks that such theologians are both philosophically mistaken and unfaithful to biblical teaching.Footnote ⁵ Persons are mutable and temporal, he says; so if God is a person, God is mutable and temporal. Indeed, suggests Swinburne, we can, in a serious sense, understand what it is like to act as God does in the light of his ‘basic powers’. ‘We can imagine finding ourselves having a basic power not merely to move objects, but to create them instantaneously.’Footnote ⁶ Swinburne takes it as obvious that, if God exists, then God resembles what Descartes took himself to be when claiming that he was essentially a thinking thing. Of course, Swinburne holds that God is much more powerful than Descartes understood himself to be. He also thinks of God as more knowing than we are and as much better behaved. On Swinburne's account, however, God is different from us in a quantitative way. God is not radically different from us.

I refer to Swinburne at this stage since I want, at the outset, to provide some context for understanding Aquinas's approach to God. And I want to stress that Aquinas is as far away from Swinburne as it is possible to get when it comes to the word ‘God’. Aquinas would not have agreed with anything that I have just quoted Swinburne as saying. One can begin to see how this is so by looking at the way in which he presents arguments for God's existence in several of his writings.

Aquinas holds that we can come to know that the proposition ‘God exists’ is true. Indeed, he thinks that we can demonstrate the truth of ‘God exists’. Following Aristotle, Aquinas takes a demonstration to be an argument with true premises that entail its conclusion, and he thinks that knowledge that God exists can only be gained from demonstration. More precisely, Aquinas takes a demonstrative argument to have the form: (1) ‘All X is Y’ (e.g., ‘All human beings are mammals’); (2) ‘All Y is Z’ (e.g., ‘All mammals breath air’); (3) ‘Therefore, All X is Z’ (e.g., ‘Therefore, all human beings breath air’).

Aquinas is aware that some people have claimed that ‘God exists’ is somehow self-evident to us, but he rejects that position largely because of what he thinks concerning our ignorance when it comes to God. For example, in response to the suggestion that God has to exist given that the word ‘God’ means ‘that than which nothing greater can be signified’, he says ‘because we do not know what God is, the proposition is not self-evident to us and needs to be demonstrated by things more known to us … that is, by God's effects’.Footnote ⁷ Here Aquinas is arguing that we can know, as opposed to truly believe, that God exists only because we can reasonably claim that God produces effects from which we can reason to God as one who brings things about.Footnote ⁸

Yet Aquinas recognizes two kinds of causal demonstration: (a) from cause to effect, and (b) from effect to cause.

Suppose we reason thus:

1. (1) Hydrogen is the element with atomic number one.

2. (2) The element with atomic number one is the lightest gas.

3. (3) So, hydrogen is the lightest gas.

4. (4) Any balloon filled with the lightest gas rises in air.

5. (5) So, any balloon filled with hydrogen rises in air.

Here we start from a definition of what something is, and we end up arguing syllogistically for the occurrence of a visible effect. Our procedure here is an example of what Aquinas takes to be an argument from cause to effect starting with a grasp of the nature of the cause.

However, suppose we reason:

1. (1) This balloon is rising in the air.

2. (2) Every rising balloon is full of something that makes it rise.

3. (3) So, this balloon is full of something that makes it rise.

4. (4) Everything that fills a balloon and makes it rise must be lighter than air.

5. (5) So, this balloon is full of something lighter than air.

Here we make no appeal to what it is precisely that fills this balloon rising in the air; we do not claim to know what exactly that is. Is it hydrogen, helium, or just hot air? But we do try to account for what we perceive in terms of a cause of some sort. We reason from effect to cause.

Now, according to Aquinas, if we know that ‘God exists’ is true, that can only be by virtue of a demonstration from effect to cause. He thinks that demonstration from cause to effect is impossible when it comes to ‘God exists’ since we lack an understanding of what God is, since we do not know God's nature or essence. This means that Aquinas's various arguments for ‘God exists’ presuppose that we do not know what God is. They positively rely on the idea that we do not have a grasp of God's essence or nature.

In that case, however, why does Aquinas present them as arguments for it being true that God exists? Does not the question ‘Does God exist?’ already depend on an understanding of what God is? In turning to this question Aquinas distinguishes between two kinds of definition, which I shall refer to as ‘real’ and ‘nominal’.

According to Aquinas a real definition tells us what some existing thing is by nature or essence. So, in his view we can reasonably aim at a real definition of what, say, a cat or a rose is. What, though, of wizards or hobbits? Can these be defined? Since they do not exist, Aquinas denies that we can arrive at a real definition of them. Yet ‘wizard’ and ‘hobbit’ are genuine words, and it is possible to explain what they mean. So, as well as acknowledging that there are real definitions, Aquinas also agrees that there are nominal ones; and his arguments for the truth of ‘God exists’ are grounded only in a series of nominal definitions.

Aquinas, of course, does not think that the word ‘God’ signifies something unreal. But, when formally arguing that God exists, he employs only nominal definitions. This is what accounts for the fairly minimalistic understandings of the word ‘God’ in his famous Five Ways to demonstrate that God exists as found in his Summa Theologiae. When presenting these arguments, all Aquinas claims to have shown is that there is (1) ‘a first cause of change that is not itself changed by anything’, (2) ‘a first cause that is not itself caused in any way ’, (3) ‘something that owes its necessity to nothing else, something which is the cause that other things must be’, (4) ‘something that causes in all other beings their existence, their goodness and whatever other perfection they have’, and (5) ‘something with intelligence that directs all natural things to ends’.Footnote ⁹ Aquinas is here only employing what he takes to be nominal definitions, ones that he derives from what he believes to be traditional talk about God coming from Jews, Muslims, and Christians.

Critics of Aquinas have often said that people mean a lot more by the word ‘God’ than Aquinas does in his Five Ways. Yet Aquinas, Christian theologian that he was, did not need to be told this. However, in his ‘from effects to cause’ arguments for ‘God exists’, he sticks to what he takes to be non-controversial nominal definitions of ‘God’, ones that do not rely on an understanding of God's actually existing nature or essence, this being something that Aquinas takes us to lack.

Why, though, does he say that we do not know what God is? Here we need to note the drift of Summa Theologiae, Part 1, Question 3, and comparable texts, in which Aquinas notes ways in which God cannot be thought of as composite. Like Aristotle, Aquinas thinks that there are various ways in which something can be composite. On the other hand, however, Aquinas has a notion of composition that goes beyond anything that Aristotle presented.

As it begins, Summa Theologiae, 1,3 remains on familiar Aristotelian ground since Aquinas employs the idea that some things are composite by having both form and matter.Footnote ¹⁰ Among other things, Aquinas argues that being corporeal is being something with potentiality or ability for change, a potentiality that must be lacking in God. He writes: ‘The first being must of necessity be actual and in no way potential’. He continues:

Armed with this conclusion, Aquinas thinks that he has no problem in going on to assert that God cannot be ‘composed’ of matter and form. He says, ‘Since having dimensions is one of the primary properties of matter, anything composed of matter and form must be a body. As I have shown, however, God is not a body. So, he is not composed of matter and form’.Footnote ¹²

In Summa Theologiae, 1,3,3 however, Aquinas goes on to argue that in God there is no composition of suppositum and nature, thereby digging deeper into what he takes God not to be.

By the word suppositum Aquinas means ‘this individual thing as opposed to another one’. So, he would say, for example, that Socrates is a suppositum and is, as such, an individual subject. He is Socrates and not Plato or any other human being. Aquinas would also say that beings such as Socrates, like all material substances, are composites of suppositum and nature (natura) or essence (essentia) – meaning that they display or have a nature or essence that is not simply to be identified with the individual subjects that they are. Socrates is a human being. Yet, thinks Aquinas, there are many things with the same nature or essence as Socrates. Such things, says Aquinas, are, like Socrates, not to be identified with the nature or essence that they have. Rather, they are a composition of suppositum and nature. Yet, Aquinas maintains, in God there is no such composition. So, says Aquinas, God (the individual that God is) and God's nature (what God is) are one and the same immaterial thing.

In propounding this teaching, Aquinas is first of all saying that God is not an individual in the sense of ‘individual’ that we have in mind when saying that an individual is one of a kind of which there is or could be more than one member. Now, as I have noted, Professor Swinburne thinks it important to insist that God is a person. Aquinas, however, does not think that God is a thing of a kind. He does not think it possible to classify God. One might suppose that Aquinas would at least concede that God and everything other than God add up to a definite number of beings. Yet Aquinas believes that ‘___ is a being’ does not tell us what something is. He does not think that there is any such class of things as things that simply are. Footnote ¹³ Aquinas is not even willing to say that God is a substance. In Summa Theologiae, 3,6 he observes: ‘The word “substance” does not mean baldly that which exists of itself, for existence is not a genus … Rather, “substance” means “that which is possessed of an essence such that it will exist of itself, even though to exist is not its essence”. So, it is clear that God does not belong to the genus of substance’.Footnote ¹⁴

In Summa Theologiae, 1,3,3 Aquinas is defending the idea that God is a subsisting form, not something exemplifying or possessing a form. This, of course, means that he is trying seriously to distinguish between God and those objects of which we can claim knowledge concerning what they are essentially. He takes these to be objects of sensory experience which allows us to pick them out as individuals and to ascribe natures to them, this implying a distinction in them of individuality and nature. Yet Aquinas takes God to be a single non-corporeal and subsisting nature. He thinks of God as something whose individuality has to be thought of as its nature, something that does not have a nature but is a subsisting nature. So, he argues, ‘God is identical with his own divinity, his own life, and with whatever else is predicated of him’. Aquinas means that, say, God's goodness is not something different from God's power, and that neither of these is different from God.

I think we can put this point by saying that Aquinas thinks that the logic of our language fails us when we try to talk or think about God. Our language, and, therefore, our thinking, is heavily grounded in a distinction between subject and predicate, and our understanding of things in the world reflects this. For Aquinas, however, God does not have properties. Aquinas agrees that, for example, God is living, knowing, good, and powerful. Yet he does not think of God's life, knowledge, goodness, and power as aspects of God or as attributes that God happens to have as you and I happen to have various attributes. Nor does he think of them as being really distinct in God, as they are in us, or as distinct from God. His view is that terms like ‘God’ and ‘the power of God’ both refer to one and the same reality, which is one reason why he says that we do not know what God is. For how can we claim to understand the nature of something that, in reality, is not material and not one of a kind lacking properties or attributes distinct from itself?

You might say that there is nothing here to understand since Aquinas is talking nonsense while claiming that God lacks composition. However, and as Peter Geach once noted, there is a comparison to hand which might give us pause when consigning Summa Theologiae, 1,3,3 to the flames as obviously incoherent. What Geach says is worth quoting in full. He writes:

I would add that, even if we reject the comparison to which Geach refers, Aquinas's claim that God is all that God has (that there is no distinction in God of suppositum and nature) does not seem to be manifestly absurd since it is not offered as a description of God. In Summa Theologiae, 3 Aquinas is concerned to deny various things when it comes to God, not to paint a portrait of God. He wants to say is that God is not to be thought of as being like those things the nature of which can be distinguished from the individuals that they are, that God is not one of a kind, that God is not something having distinct properties. You may think that in 1a,3 Aquinas is describing God in positive terms. But he is actually noting what cannot be thought when it comes to God. That is because he is merely trying to flag ways in which God does not exist on the understanding that to say that God is not X, Y, or Z is not to be committed to any account of what God positively is.

So much, then, for Summa Theologiae, 1,3,3. In 1,3,4, however, Aquinas introduces another thought that runs through his writings in general. This is not at all an Aristotelian one, and it amounts to the claim that God is non-composite since in God there is no distinction of essence (essentia) and existence (esse). As he dwells on this notion, Aquinas suggests that our knowledge of God is even more limited than we might take it to be even if we concede that in God there is no composition of suppositum and nature.

Here Aquinas is drawing on his claim that God is the first cause of all creatures from nothing (ex nihilo). By this he means that the existence of everything other than God derives at all times from God directly and without intermediaries. He also means that what God is, God's nature, cannot be caused to exist by anything and can therefore be spoken of as ‘subsisting existence’ lacking any conceivable potentiality. So, says Aquinas, God is not just his own essence. God ‘is also his own existence’.Footnote ¹⁶ Hence, and drawing on the reasoning of Summa Theologiae, 1, 3, 4, Aquinas can say in 1, 13, 11 that the ‘most appropriate name for God’ is ‘He who Is’ (Qui Est). ‘Since the existence of God is his essence’, argues Aquinas, ‘and since this is true of nothing else … It is clear that this name is especially fitting for God, for we name everything by its form’.Footnote ¹⁷ He continues:

The philosophical value of this way of talking has been challenged by a number of authors, especially those anxious to defend the claim that ‘being’ or ‘existence’ cannot be attributed to individuals. According to this claim, ‘X exists’ tells us something about concepts, not objects, and is equivalent to sentences of the form ‘Something or other is F’ where F is equivalent to expressions such as ‘___ is canine’ or ‘___ is hungry’.Footnote ¹⁹ Be that as it may, however, Aquinas certainly wants to speak of God as ‘existence itself’, and his putting that wish into practice contributes to his consistent teaching that we do not know what God is. That is because, as I have said, Aquinas does not think that ‘exists’ can enter into a real definition of something that expresses what we know it to be. When speaking of God as ‘Being itself’ Aquinas is not trying to define God. He is saying that, since God accounts for the existence of everything other than God, God is not something derived from anything. In short, he is saying that God is not created.

At this point I should pay some attention to what Aquinas means when he speaks about God being the Creator, for this is something that does not just lead Aquinas to say that we do not know what God is but is part and parcel of his teaching to that effect.

As I have said, Aquinas argues causally for the truth of ‘God exists’. In doing so, he claims that God accounts for there being change, numerous secondary causes of different kinds, and things which exist but do not themselves account for their actually existing. We might put all this by saying that Aquinas argues that God exists and is the Creator of all things other than God.

The belief that God is the Creator of all things is often taken to mean only that God, so to speak, got the universe going some time ago. So, some people who believe that God exists think it critical to reject any theory that seems to suggest that the universe had no beginning. Yet Aquinas is not of their mind. He maintains that for God to create is for God to make something to exist for as long as it exists and whether or not it had a beginning of existence. As it happens, Aquinas believed that the world did begin to exist, but, like Aristotle, he did not think that this fact can be established philosophically.Footnote ²⁰ For Aquinas, that the universe had a beginning is a matter of faith. Yet Aquinas does not think that we should reject belief in God as Creator should we happen to conclude that the universe had no beginning. For, he holds, to speak of God as Creator is to speak of God as accounting for the sheer existence of things at any time.

In other words, Aquinas's view is that God is the Creator of everything since God accounts for there being something rather than nothing at all. So, at one point he says:

In Aquinas's view, God is the cause of things that have being but do not exist by nature. So, in Summa Theologiae, 1,46 he can say ‘The world exists just as long as God wills it to, since its existence depends on his will as on its cause’.Footnote ²² We need to realize that, in Aquinas's thinking, ‘___ is being created by God’ is truly to be affirmed of any creature at any time. We also need to note the extent to which Aquinas maintains that ‘creates’, in ‘God creates’, takes us beyond causality as he thinks that we understand it. In speaking of God as Creator, Aquinas, while drawing on Aristotle's thinking about causes, is focusing on God as an efficient or agent cause, something that produces one or more effects in something. On the other hand, he also holds that God cannot be an efficient or agent cause in what we might call ‘the usual’ or ‘the Aristotelian’ sense.

Following Aristotle, Aquinas thinks that talk about efficient or agent causes has its roots in our recognition of change in the world and our looking, so to speak, for culprits. If I start running a temperature, I ask ‘What is responsible for this?’. If I return to my apartment to find it robbed, I ask ‘Who did this?’. Aristotle and Aquinas have no inclination to construe talk about efficient causation as deriving from our experience of what David Hume called ‘constant conjunction’. They think that causal explanation aims at understanding why this or that thing is producing a result of a certain kind, regardless of how often it seems to be doing so.

Also, and abstracting from our experience of what tends to follow from what, they think of efficient causes as agents active in their effects, and they conclude that an efficient cause and its effect are, in a sense, one thing. As Aquinas puts it, ‘what the agent does is what the patient undergoes’ so that ‘action and passion are not two changes but one and the same change, called action in so far as it is caused by an agent, and passion in so far as it takes place in a patient’.Footnote ²³ If I cause the drapes on my window to be drawn, that is because they are being drawn because of me, because my act of causing the drapes to be drawn amounts to the drapes being drawn by me.

Yet this account of efficient or agent causation is an account of it as it occurs in the universe, and it diverges from what Aquinas has in mind when it comes to God making the difference between there being something and nothing. That is because Aquinas does not think of God's creating as making any difference to anything and, therefore, as bringing about a change in something.

In short, though Aquinas takes God's creating to be a form of making, and an instance of efficient or agent causation, he also takes it to be making that is not to be understood as we understand making in general. It is making that brings about something from nothing, not making that results in a transformation of some kind. It is making that does not modify anything but makes things to be what they are. It is making by a maker that is not a thing as objects in the universe are things. It is making that is thoroughly mysterious to us.

One might, of course, say that if that is what ‘creating’ as ascribed to God is, then ‘making’ has undergone what Professor Antony Flew once referred to as ‘the death by a thousand qualifications’ and that Aquinas is incoherent in what he says about God as Creator.Footnote ²⁴ Flew's point, intended to be critical of belief in God, is that those who make assertions about God typically allow nothing to count against them and end up saying nothing of significance since the words they use when talking about God are deprived of their original meaning. Unlike Flew, however, Aquinas welcomes this death by a thousand qualifications when it comes to talk about God since he takes it to be a positive consequence of rightly seeking to account for there being something rather than nothing, since he takes it as implied by his basic reason for supposing God to exist in the first place.

As I have said, when trying to say why we should think that ‘God exists’ is true, Aquinas does not start from an understanding of what God is. He starts from questions that he thinks to arise when it comes to what is in the universe, questions that lead him to a question about the very existence of the universe. Wittgenstein once said ‘It is not how things are in the world that is mystical, but that it exists’.Footnote ²⁵ Aquinas seems to agree with Wittgenstein here. However, while Wittgenstein goes on to say that when wondering why the world exists we should resort to silence, Aquinas, tries to keep talking, albeit that he recognizes that he is using language that is being stretched to breaking point, to the point where he is trying to say more than he can mean, to the point where he ends up seriously asserting that we do not know what God is and are better informed when it comes to what God is not.Footnote ²⁶ Aquinas thinks that we have some (albeit limited) knowledge when it comes to what created things in the universe are. But he also holds that God is not a created thing having existence as an effect and is, therefore, not a knowable thing at all. Why not? Because God is not one among the beings in the universe that exist as created. In his Commentary on Aristotle's Peri Hermeneias, Aquinas says that ‘God's will is to be thought of as existing outside the realm of existents, as a cause from which pours forth everything that exists in all its variant forms’.Footnote ²⁷

Yet is Aquinas not well known for holding that some of the words we use when talking about creatures can be applied to God in a literal and non-figurative sense? Indeed he is, and in various places he argues that some words can be used both of God and creatures analogically while taking analogical predication to be a literal mode of discourse and not a figurative one. Be that as it may, however, when arguing in this way Aquinas does not go back on anything that I have been ascribing to him. He claims that certain affirmative statements about God can be known to be literally true for quite precise reasons. So, he holds that when talking of God or arguing about God's nature we are not always engaging in pure equivocation since pure equivocation spoils arguments.Footnote ²⁸ Again, Aquinas also holds that God is not composite, so he reasons that when we say that God is such and such, when we say, for example ‘God is good’, we are not speaking of God and of things in the world univocally – since things in the world can be distinguished from the properties or attributes that they have while God cannot be so distinguished. In Summa Theologiae, 1,13 Aquinas explicitly says that in ‘This creature is F’ and ‘God is F’ we cannot construe F as univocal or purely equivocal, though its use in both cases can sometimes be justified with reference to causal reasoning from creatures to God. And this is where Aquinas's notion of analogical predication enters in. When it comes to some words signifying perfections, says Aquinas, we can ‘name’ God from creatures without speaking metaphorically, or by means of simile, since God accounts for the existence of perfections in creatures who can, therefore, be said to resemble God to some extent (on the principle that you cannot give what you do not have).Footnote ²⁹ In spelling out these points, however, Aquinas never casts aside his claim that we do not know what God is. Indeed, he reiterates it by saying:

Aquinas goes on to say:

As I have said, Aquinas thinks that attribution such as this always fails to represent God adequately. When we talk about God affirmatively, says Aquinas, we are always ‘signifying imperfectly’. We may, he thinks, speak literally and positively about God while ascribing certain perfections to God. Indeed, ‘as far as the perfections signified are concerned, we use the words literally of God, and in fact more appropriately than we use them of creatures, for these perfections belong primarily to God and only secondarily to other things’. Yet, Aquinas immediately adds: ‘But so far as the way of signifying these perfections is concerned, we use the words inappropriately, for they have a way of signifying that is appropriate to creatures’.Footnote ³²

It is the purpose of texts like Summa Theologiae, 1,13 ‘to show in what way theological language must differ from ordinary usage’.Footnote ³³ This is because Aquinas is deeply struck by our ignorance concerning what God is, ignorance that leaves Aquinas effectively saying that we do not know what the word ‘God’ really signifies, albeit that we can provide nominal definitions of the word. Or, as my late friend Herbert McCabe once said:

If McCabe is right here, we should conclude that when Aquinas talks about God he is doing something seriously different from what Professor Swinburne seems to be doing when he tells us that God is a person, albeit one better and more powerful and knowledgeable than us.Footnote ³⁵ And I take this fact to mean that Aquinas represents a challenge to what many contemporary philosophers of religion, whether or not they believe in God, have to say when it comes to what ‘God exists’ means. Were he alive today, Aquinas would be appealing to a text like Summa Theologiae, 1,13 as one that, if cogent in its reasoning, ought to lead those who talk about God as if God were some understandable member of a kind to think again. An appeal like this, coming from someone who can reasonably be taken to be one of the greatest of western philosophers and theologians, ought, perhaps, to be taken very seriously, if only to be thought of as setting up an approach to God that atheists might bear in mind when saying that there is no God. Aquinas's treatment of God's unknowability might be full of mistakes. It might be wrong for some philosophical or theological reasons. But I think that it is something that should be borne in mind by anyone who wants to understand what his teaching on God amounts to in general.Footnote ³⁶

Naturally enough, I suppose, people often tend to think of God as the biggest thing around, the Top Person, and so on. Aquinas, however, definitely does not think in this way. He is very much an agnostic, not because he does not believe that God exists, but because, while ‘modern agnosticism says simply “We do not know, and the universe is a mysterious riddle”’, Aquinas says ‘We do not know what the answer is, but we do know that there is a mystery behind it all which we do not know, and if there were not, there would not even be a riddle. This Unknown we call God. And if there were no God, there would be no universe to be mysterious, and nobody to be mystified’.Footnote ³⁷