Having shown, then, that endeavoring to prove the existence of God is not futile, we may proceed to set forth the reasons by which both philosophers and Catholic doctors have proved that there is a God.

In the first place, we shall give the arguments by which Aristotle sets out to prove God’s existence. He aims at proving this from movement in two ways.

The first way is as follows (7 Physics 1). Whatever is in motion is moved by another. Now, it is clear to the sense that something is in motion, e.g., the sun. Therefore, it is in motion by something else moving it. Now that which moves it is itself either moved or not. If it is not moved, then the point is proved that we must postulate an immovable mover, and this we call God. If, however, it be moved, it is moved by another mover. Either, therefore, we must proceed to infinity, or we must come to an immovable mover. But it is not possible to proceed to infinity. Therefore, it is necessary to postulate an immovable mover.

This argument contains two propositions that need to be proved, namely, that whatever is in motion is moved by another, and that it is not possible to proceed to infinity in movers and things moved.

The first of these is proved by the Philosopher in three ways. First, thus. If a thing moves itself, it must have the principle of its movement in itself; otherwise, it would clearly be moved by another. It also must be moved primarily, that is, it must be moved by reason of itself and not by reason of its part (as an animal is moved by the movement of its foot). For in the latter way not the whole but the part would be moved by itself, and one part by another. It also must be divisible and have parts, since whatever is moved is divisible, as is proved in 6 Physics 4.

These things being supposed, he argues as follows. That which is stated to be moved by itself is moved primarily. Therefore, if one of its parts is at rest, it follows that the whole is at rest. For if, while one part is at rest, another of its parts were in motion, the whole itself would not be moved primarily, but its part which is in motion while another is at rest. Now nothing that is at rest while another is at rest is moved by itself: for that which is at rest as a result of another thing being at rest must be in motion as a result of the other’s motion. And hence it is not moved by itself. Hence that which was stated to be moved by itself is not moved by itself. Therefore, whatever is in motion must be moved by another.

Nor is this argument negated by what might be said: supposing a thing moves itself, it is impossible for a part of it to be at rest. Nor again by the statement that to be at rest or in motion does not belong to a part except accidentally, as Avicenna quibbles (2 Suffic. 1). For the force of the argument lies in this, that if a thing moves itself primarily and of itself, not by reason of its parts, it follows that its being moved does not depend on anything; whereas with a divisible thing, being moved depends on its parts, as does its existence, so that it cannot move itself primarily and of itself. Therefore, the truth of the conclusion drawn does not require that we suppose as an absolute truth that a part of that which moves itself is at rest, but that this conditional statement be true: if a part were at rest, the whole would be at rest. Which statement can be true even if the antecedent be false, even as this conditional proposition is true: if a man is an ass, then he is irrational.

Second, he proves it by induction in this way (8 Physics 4). A thing is not moved by itself if it is moved accidentally, since its motion is occasioned by the motion of something else. Nor again if it is moved by force, as is manifest. Nor if it is moved by its nature, like those things whose movement proceeds from themselves, such as animals, which clearly are moved by their souls. Nor if it is moved by nature, as heavy and light things are, since these are moved by their generating cause and by that which removes the obstacle to their movement. Now, all things that are in motion are moved either in themselves or accidentally; and if in themselves, either by force or by nature; and if the latter, either by something in them, as in the case of animals, or not by something in them, as in the case of heavy and light bodies. Therefore, whatever is in motion is moved by another.

Third (8 Physics 5, 8), he proves his point thus. Nothing is at the same time in act and in potency in respect of the same thing. Now whatever is in motion, as such, is in potency, because motion is the act of that which is in potency, as such (3 Physics 1, 6). In contrast, whatever moves, as such, is in act, for nothing acts except insofar as it is in act. Therefore, nothing is both mover and moved in respect of the same movement. Hence nothing moves itself.

We must observe, however, that Plato (Phoedrus 24). who asserted that every mover is moved, employed the term ‘movement’ in a more general sense than Aristotle. For Aristotle took movement in its strict sense, for the act of a thing that is in potency as such, in which sense it applies only to divisible things and bodies, as is proved in 6 Physics. But, according to Plato, that which moves itself is not a body: for he took movement for any operation, so that to understand or to think is a kind of movement, to which manner of speaking Aristotle alludes in 3 De Anima 7. In this sense, then, he said that the first mover moves itself, inasmuch as it understands, desires, and loves itself. This, in a certain respect, is not in contradiction with the arguments of Aristotle; for it makes no difference whether with Plato we come to a first mover that moves itself, or with Aristotle to something first which is altogether immovable.

He proves the other proposition (namely, that it is impossible to proceed to infinity in movers and things moved) by three arguments.

The first (7 Physics) of these is as follows. If one were to proceed to infinity in movers and things moved, all this infinite number of things would necessarily be bodies, since whatever is moved is divisible and corporeal, as is proved in 6 Physics. Now every body that moves through being moved is moved at the same time as it moves. Therefore, all this infinite number of things are moved at the same time as one of them is moved. But one of them, since it is finite, is moved in a finite time. Therefore, all this infinite number of things are moved in a finite time. But this is impossible. Therefore, it is impossible to proceed to infinity in movers and things moved.

That it is impossible for the aforesaid infinite number of things to be moved in a finite time, he proves thus (7 Physics 1, 2). Mover and moved must be simultaneous, as he proves by induction from each species of movement. But bodies cannot be simultaneous except by continuity or contact. Therefore, since all the aforesaid movers and things moved are bodies, as proved, they must be as one movable thing through their continuity or contact. And thus one infinite thing would be moved in a finite time, which is shown to be impossible in 6 Physics 7.

The second argument (8 Physics 5) in proof of the same statement is as follows. In an ordinate series of movers and things moved—namely, where throughout the series one is moved by the other—we must find that if the first mover be taken away or cease to move, none of the others will move or be moved, because the first is the cause of movement in all the others. Now if an ordinate series of movers and things moved proceed to infinity, there will be no first mover, but all will be like intermediate movers. Therefore, it will be impossible for any of them to be moved: and thus nothing in the world will be moved.

The third argument (8 Physics 5) amounts to the same, except that it proceeds in the reverse order, namely, by beginning from above. It is as follows. That which moves instrumentally cannot move unless there be something that moves principally. But if we proceed to infinity in movers and things moved, they will all be like instrumental movers, because they will be alleged to be moved movers, and there will be nothing by way of principal mover. Therefore, nothing will be moved.

We have thus clearly proved both statements which were supposed in the first process of demonstration by which Aristotle proved the existence of a first immovable mover.

The second way (8 Physics 5) is as follows. If every mover is moved, this statement is true either in itself or accidentally. If accidentally, it follows that it is not necessary: for that which is accidentally true is not necessary. Therefore, it is a contingent proposition that no mover is moved. But if a mover be not moved, it does not move, as the opponent asserts. Therefore, it is contingent that nothing is moved, since, if nothing moves, nothing is moved. Now Aristotle holds this to be impossible (8 Physics 1), namely, that at any time there be no movement. Therefore, the first proposition was not contingent, because a false impossibility does not follow from a false contingency. And therefore this proposition, ‘every mover is moved by another,’ was not accidentally true.

Again, if any two things are found accidentally united in a certain subject, and one of them is to be found without the other, it is probable that the latter can be found without the former: thus if ‘white’ and ‘musical’ are found in Socrates, and ‘musical’ without ‘white’ is found in Plato, it is probable that ‘white’ could be found without ‘musical’ in some subject. Accordingly, if mover and moved be united together in some subject accidentally, and it be found that a certain thing is moved without its being a mover, it is probable that a mover is to be found that is not moved. Nor can one urge against this the case of two things, one of which depends on the other; because those in question are united not per se but accidentally.

If, however, the aforesaid proposition is true in itself, again there follows something impossible or unfitting. For the mover must be moved either by the same kind of movement or by another kind. If by the same kind, it follows that whatever causes alteration must itself be altered, and furthermore that the healer must be healed, that the teacher must be taught, and in respect of the same science. But this is impossible: for the teacher must have science, while the learner must not have it, and thus one thing will be both possessed and not possessed by the same person, which is impossible. And if it be moved by another kind of movement—namely, so that what causes alteration be moved in respect of place, and that which moves in respect of place be increased, and so on—it will follow that we cannot go on indefinitely, since the genera and species of movement are finite in number. And thus there will be some first mover that is not moved by another. Unless someone happens to say that a recurrence takes place in this way: when all the genera and species of movement have been exhausted, a return must be made to the first. For instance, if that which moves in respect of place be altered, and that which causes alteration be increased, then again that which is increased be moved in respect of place. But the consequence of this will be the same as before, namely, that which moves by one kind of movement is itself moved by the same kind, although mediately and not immediately.

It remains, therefore, that we must postulate some first mover that is not moved by anything outside itself.

Since, however, it does not follow that a first mover that is not moved by anything outside itself is absolutely immovable, Aristotle proceeds further, saying that this may happen in two ways. First, so that this first mover is absolutely immovable. And if this is granted, our point is established, namely, that there is a first immovable mover. Second, that this first mover is moved by itself. And this seems probable, because what is of itself is always prior to what is of another. Hence it is likewise logical in things moved that what is moved first is moved by itself and not by another.

But, if this be granted, the same consequence follows (8 Physics). For it cannot be said that the whole of what moves itself is moved by its whole self, because then the absurd consequences mentioned above would follow, namely, that a person might teach and be taught at the same time, and in like manner as to other kinds of movement. And again, a thing would be at the same time in act and in potency, since a mover, as such, is in act, while that which is moved is in potency. It remains, therefore, that one part thereof is mover only, and the other part moved. And thus we have the same conclusion as before, namely, that there is something that moves and is itself immovable.

And it cannot be said that both parts are moved, so that one is moved by the other; nor that one part moves both itself and the other; nor that the whole moves a part; nor that part moves the whole, since the above absurdities would follow—namely, that something would both move and be moved by the same kind of movement, and that it would be at the same time in potency and in act, and (finally) that the whole would move itself not primarily, but by reason of its part. It remains, therefore, that in whatever moves itself, one part must be immovable, and must move the other part.

Since, however, in those things among us which move themselves (namely, animals), the part which moves (namely, the soul), though immovable of itself, is nevertheless moved accidentally, he goes on to show that in the first mover, the part which moves is not moved neither of itself nor accidentally (8 Physics 6).

For in those things which among us move themselves (namely animals), the part which moves is moved accidentally since they are corruptible. Now those corruptible things which move themselves must be reducible to some first self-mover that is everlasting. Therefore, that which moves itself must have a mover, which is moved neither of itself nor accidentally.

It is clear that, in accordance with his hypothesis, some self-mover must be everlasting. For if, as he supposes, movement is everlasting, the production of these self-movers that are subject to generation and corruption must be everlasting. But no one of these self-movers, since it does not always exist, can be the cause of this everlastingness. Nor can all of them together, both because they would be infinite and because they do not exist all together. It follows, therefore, that there must be an everlasting self-mover, that causes the everlastingness of generation in these lower self-movers. And thus its mover is not moved, neither of itself nor accidentally.

Again, we observe that in self-movers some begin to be moved anew on account of some movement whereby the animal is not moved by itself, for instance, by the digestion of food or a change in the atmosphere. By this movement, the mover that moves itself is moved accidentally. From this, we may gather that no self-mover whose mover is moved essentially or accidentally is always moved. But the first self-mover is always in motion, else movement could not be everlasting, since every other movement is caused by the movement of the first self-mover. It follows, therefore, that the first self-mover is moved by a mover who is not moved, neither essentially nor accidentally.

Nor is this argument rebutted by the fact that the movers of the lower spheres cause an everlasting movement, and yet are said to be moved accidentally. For they are said to be moved accidentally not by reason of themselves, but by reason of the things subject to their motion, which follow the motion of the higher sphere.

Since, however, God is not part of a self-mover, Aristotle goes on in his Metaphysics 11, 7, to trace from this motor that is part of a self-mover, another mover altogether separate, which is God. For, since every self-mover is moved through its appetite, it follows that the motor which is part of a self-mover moves on account of the appetite for some appetible object. And this object is above the motor in moving, because the one desiring is a moved mover, whereas the appetible is a mover altogether unmoved. Therefore, there must be a first mover separate and altogether immovable, and this is God.

Now two things would seem to weaken the above arguments. The first of these is that they proceed from the supposition of the eternity of movement, and among Catholics this is supposed to be false.

To this we reply that the most effective way to prove God’s existence is from the supposition of the eternity of the world: when this is supposed, God’s existence seems less manifest. For if the world and movement had a beginning, it is clear that we must suppose some cause to have produced the world and movement, because whatever becomes anew must take its origin from some cause of its becoming, since nothing evolves itself from potency to act, or from non-being to being.

The second is that the aforesaid arguments suppose that the first moved thing, namely, the heavenly body, has its motive principle in itself, from which it follows that it is animated. Many do not grant this.

To this we reply that if the first mover is not supposed to have its motive principle in itself, it follows that it is immediately moved by something altogether immovable. Hence also Aristotle draws this conclusion with an alternative, namely, that either we must come at once to a first mover immovable and separate, or to a self-mover from which again we come to a first mover immovable and separate (8 Physics 5, 12).

The Philosopher proceeds in a different way in 2 Metaphysics to show that it is impossible to proceed to infinity in efficient causes, but that we must come to one first cause: and this we call God. This is how he proceeds. In all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate, whether the intermediate be one or several. Now if the cause be removed, that which it causes is removed. Therefore, if we remove the first, the intermediate cannot be a cause. But if we go on to infinity in efficient causes, no cause will be first. Therefore, all the others which are intermediate will be removed. Now this is clearly false. Therefore, we must suppose the existence of a first efficient cause: and this is God.

Another reason can be drawn from the words of Aristotle. For in 2 Metaphysics 1, 5, he shows that those things which excel as true excel as beings, and in 4 Metaphysics 3, 4, 27-28, he shows that there is something supremely true because we see that of two false things one is falser than the other, from which it follows that one is also truer than the other. Now this is by its approximation to that which is simply and supremely true. Therefore, we may further conclude that there is something that is supremely being. And this we call God.

Another argument in support of this conclusion is adduced by Damascene (On the Orthodox Faith 1, 3) from the government of things, and the same reasoning is indicated by the Commentator in 2 Physics. It runs as follows. It is impossible for contrary and discordant things to accord in one order always or frequently except by someone’s governance, through which each and all are made to tend to a definite end. Now we see that in the world things of different natures accord in one order, not seldom and fortuitously, but always or for the most part. Therefore, it follows that there is someone by whose providence the world is governed. And this we call God.

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Summa Contra Gentiles bk. I, c. 13 | St. Thomas Aquinas