Must God Exist?

A Modern Socratic Look at St Thomas Aquinas’ Proofs of the Existence of God

Rev Fr Jonathan Patrick Steele

A word of explanation seems necessary: In his teachings, Socrates, one of the great Greek philosophers, would ask his students tough questions to encourage them to think about why a certain truth is the way it is. He would let them make intellectual mistakes and then show them how absurd their conclusions were in such a way as to lead them to discover the truth. We have tried to do the same with a different topic, using two characters: one, a teacher whose name, Sapius, is a derivative of the Latin sapiens and means ‘the wise one’, and two, an impetuous young man who is seeking truth and whose name, Aretos, signifies, ‘goodness, excellence, or virtue’.

In his many years of teaching philosophy, Sapius had rarely encountered students who presented him with a real challenge. Most of the lessons were delivered to ears which were attached to more, rather than less, empty heads; however, this student was different. His thoughtful, precise reasoning was on an entirely unprecedented level. The young man's name was Aretos, and he and his magister had been walking through the public square earlier that morning discussing a particularly fascinating topic, one which is both at the beginning as well as the end of philosophy, namely the existence of God. Recalling the conversation, Sapius chuckled at the intensity of his gifted student. Aretos had argued quite vehemently, not that no God existed, since this he firmly believed; rather, he had struggled to wrap his mind around whether that very existence of a deity could, in fact, be proven.

The argument had begun when Aretos had asked what appeared to be a simple question. They had been discussing the beauty of the mountains which towered above their beloved little city of T—. Sapius could remember the conversation quite well. The dialogue had run along these lines:

ARETOS: But surely that beauty is not something that belongs to the mountain in itself, since some mountains are more or less beautiful than others. There is no way the mountain can make itself beautiful.

SAPIUS: Of course not. That beauty is received from something or someone outside. We call this God.

ARETOS: But how do we know that it is God? Of course, I agree that there is a God; everyone who is pious does. But what I cannot understand is how we can know that God is there giving beauty to the mountain? Do we merely have to believe He exists? The existence of such a mighty and generous Being would seem to be impossible to prove directly. He is so far above our power of understanding.

SAPIUS: While He is, of course, beyond our complete comprehension, insofar as we are incapable of understanding Him as well as He understands Himself, yet this does not render it impossible for us to prove that He exists — and that, beyond the shadow of any doubt. This is, moreover, not a matter of mere belief in some mysterious Being, whose existence is entirely beyond our grasp; rather, His existence is clearly and definitely demonstrable using reason and reason alone. 

At this point in the conversation, another student had approached the master with what he considered an urgent question, this one regarding the notion of truth and its correspondence to reality. Consequently, Sapius’ discussion with Aretos had ground to a premature halt. Then, to his joy, a short hour after the mid-morning break in studies, the philosopher saw his young charge striding toward him once more with a puzzled expression in his steely blue eyes.

ARETOS: I have thought carefully about what you said, but still do not see how we can prove the existence of God. Necessarily by the fact that He is God, it is impossible for us to understand Him completely. This is essential to the very notion of who God is. Yet, seemingly, to prove His existence, we would need a full argument starting with a real truth that we know about Him and can use as a universal principle — in other words, a kind of definition. This would appear to be impossible when we are dealing with such profound and transcendent realities. A definition imposes limits, and we are speaking of a limitless Being. Is this not so?

SAPIUS: Yes, and no. Let me, in turn, ask you a question. If I were to ask you to prove to me that this rock exists, how would you go about it? Would you not be forced to rely upon some kind of physical evidence rather than upon a universal principle or deep understanding of ‘rockness’? You would most likely say that the object in question has such and such a shape, a certain weight, a particular colour, demonstrates extreme hardness exteriorly, and so on, would you not? This would indicate, based on the evidence of the senses, that the object which we examine in the street is a rock.

Now, what about the other forms of being — plants, animals, and men? How can we prove they exist? Is it not in much the same way, in fact? We must rely on the evidence of our senses, i.e. the primary source of our knowledge. This is how we come to know everything.

ARETOS: That is all well and good, but you have only raised another problem in addition to the original question, because we cannot attain the knowledge of God directly by our senses, at least in this life. Perhaps in the next, if there is an afterlife, but certainly not here. So, first, how can we come to any knowledge of God if all our knowledge comes from the senses? And second, how can we prove that He exists without this sense knowledge?

SAPIUS: These are deep questions, Aretos, but not impossible to answer. In the first place, you are right. We have added another difficulty to the original question, both of which pose legitimate problems for the mind to consider. Yet, we can proceed to a fuller examination of the truth now that the extent of the difficulties is apparent.

So, let us address the two questions in order. First, would you agree that we can know things in many different ways? For example, we come to knowledge as infants through touching, tasting, hearing, and the other senses, whereas as adults we may take further steps and understand matters in more abstract ways? Of course, you must see that is true. Would you further agree that humans come to the knowledge of various kinds of things? For example, we come to know of rocks, plants, animals, as well as things which do not really exist, unicorns, leprechauns, and the like?

ARETOS: Naturally, this is the case. 

SAPIUS: Correct! So far, so good.

Now, let us proceed further. In considering men, would you say that we can know about them both directly, as we know our friends by direct contact, as well as indirectly by the things they do and the effects they have, as we know politicians by the laws they enact?

ARETOS: Well, of course! We can know others both directly and indirectly, as friends or as politicians. This is basic common sense. I do not see how it has any bearing on the question.

SAPIUS: Have patience, my young friend. We will come to the point shortly. Let us ask a further question. That we come to know someone directly with ease, by direct contact and sense knowledge, is clear; however, can we say that we can come to know a person, in a real way, only by what they do or say, even if we never meet them and never come into direct contact with them?

ARETOS: Perhaps? Yes, it would seem so.

SAPIUS: Very well. In that case, you would agree that we can know God by what He does in a way that is indirect, but real knowledge?

ARETOS: That would seem to make sense.

SAPIUS: So, we can know God at least indirectly by the things He does, would you agree?


SAPIUS: In that case, we can proceed to the second problem, which is that we cannot prove that God exists without having some kind of sense knowledge. In fact, this is true. However, this does not take into account that we can know Him in a way that is not direct, that is rather indirect, but true knowledge all the same.

So, how can we prove that God exists? Clearly, we cannot do this in any way other than by His effects, for although He is utterly above our comprehension and full understanding, yet His work may be understood as it finds expression in the material and physical world, i.e. in a way that we can sense.

There would seem to be at least five ways in which we can prove the existence of God, using His effects. These are:

1. Motion, or change;
2. A first cause (in agents);
3. Possibility and necessity;
4. More and less in things; and 5. Laws/order in nature.

Let us examine each in detail, starting with the first.

The first, a proof taken from motion or change in things, is perhaps the easiest to understand. We notice all around us that things move and change, do we not?

ARETOS: Yes, of course. But this seems very far from proving that God must exist.

SAPIUS: We will come to that in due course. Of those things that move and change, we notice that things which are in motion are moved by someone or something else. For example, this rock which I have just cast away from us was at rest before and then put in motion by my throwing it. This is a change and it must come from outside of the rock in some way. The rock cannot throw itself. Does this make sense so far?

ARETOS: I believe so, but still do not see the connection.

SAPIUS: Let us take what may be an easier example. You have experienced fire, no doubt? Very well. You have also experienced wood in some form, I would expect. Now, fire is hot. Would you agree?

ARETOS: Naturally!

SAPIUS: There you are correct. Now, is wood naturally hot? Or, on the contrary, is it naturally cold?


SAPIUS: But can it be both cold and hot?

ARETOS: Not at the same time, but yes, in theory.

SAPIUS: What is it that changes the wood to be either hot or cold?

ARETOS: I suspect it would be some kind of influence from a hot or cold source, as fire or a bitter wind.

SAPIUS: Very good! Now we are making progress. So, you are saying that the wood, in order to be hot, requires some kind of movement or change acted upon it by something which is actually hot, namely fire?

ARETOS: Yes, that makes sense. There is no way that wood can become hot by itself. But if it comes into contact with fire, then it would undoubtedly become hot and would even be changed completely into ash.

SAPIUS: Exactly! In fact, you have come to the next step of the argument by common sense. The thing that changes or creates a movement must be different in some way from the thing that is changed or moved. Is that not so?

ARETOS: Certainly!

SAPIUS: So, the mover and the thing moved cannot be exactly the same in all respects. In that case, we run into the following problem. How does the motion come to exist in the first place? Let us examine this carefully. If a change or motion, say in the wood of the above example, can only come to exist from something outside or other than itself, this means that the fire itself would have to begin to exist from something outside of it, and what caused the fire would also be moved

by something else ... and so on until there is a beginning motion. Take, for example, a line of dominoes. The last one is moved by the penultimate one, which is in turn compelled to fall by the one before it, and so on until the movement begins. Yet, there must be a beginning, otherwise no movement would occur at all. In other words, if nothing pushed over the first domino, then the last one would not fall. Likewise, if we imagined an infinite ‘beginning’ to the domino train, then the result would simply be that the movement would never begin in the first place. There has to be a starting movement, a Mover who is Himself Unmoved. This we call ‘God’. Do you begin to see?

ARETOS: Maybe, but you said there are other proofs as well. Perhaps these will clarify the confusion in my mind and help to convince me.

SAPIUS: Yes, there are, in fact. The problem is that I have another lesson in a few minutes. So, we will have to carry on our discussion later, perhaps tomorrow.

ARETOS: That sounds good to me. I will look forward to it!


The central content of this work (and the work to come), including several of the examples, is taken from St Thomas Aquinas’ Summa Theologica, Prima Pars, Q. 2, article 3, in the Response, in which the Angelic Doctor gives five ways to demonstrate the existence of God, based in large part upon Aristotle’s proof of the same in his Metaphysics, Book 12, Chapters 6–10. The author of the above work has attempted to simplify what is in

St Thomas and make it digestible and even enjoyable for the modern reader. As a result, some of the finer points of philosophy have been lost. For those wishing for a fuller treatment of the matter, whether of a philosophical background or who merely enjoy a challenging intellectual read, St Thomas’ discussion is second to none in terms of clarity and precision.

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