Summa Contra Gentiles Bk. 1, Ch. 13 (Complete)

In chapter 10 of the SCG St Thomas presented the opinion of those who held that the existence of God could not be demonstrated, being self-evident. These opinions were answered by Aquinas in chapter 11. In chapter 12 he presented and responded to those who held that the existence of God could only be known by the light of faith and, as a consequence, could not be demonstrated. Here in chapter 13 he presents his demonstrations in proof of God’s existence. The presentation here is significantly longer and more detailed than that found in the Summa of Theology, Article 2. I originally posted this chapter in three posts but now consolidate them here.

Chapter 13
Part 1
Demonstrations in Proof of the Existence of God

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

II. In the first place we shall give the arguments by which Aristotle sets out to prove God’s existence: and he aims at proving this from the point of view of movement, in two ways.

III. The first way is as follows (Physics VII. 1. Whatever is in motion is moved by another: and it is clear to the sense that something, the sun for instance, is in motion. Therefore it is set in motion by something else moving it. Now that which moves it is itself either moved or not. If it be not moved, then the point is proved that we must needs 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. Aquinas lacked our advanced scientific knowledge and believed that the sun revolved around the earth. As Peter Kreeft rightly notes however, “the invalid illustration does not invalidate the principle” (SUMMA OF THE SUMMA, footnote 28, pg. 69). Incidentally, Aquinas correctly held with most medievals that the earth was round (ST. I. Q. 1, art. 1).

IV. 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.

V. The first of these is proved by the Philosopher in three ways. First, thus. If a thing moves itself, it must needs have the principle of its movement in itself, else it would clearly be moved by another. Again it 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. Again it must be divisible and have parts, since whatever is moved is divisible, as is proved in Physics VI, 4.

VI. These things being supposed, he argues as follows. That which is stated to be moved by itself is move 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 needs be in motion as a result of the other’s motion, and hence it is not moved by itself. Therefore whatever is in motion needs to be moved by another.

VII. Nor it this argument traversed by the statement that might be made, that supposing a thing moves itself, it is impossible for a part thereof to be at rest, or again by the statement that to be at rest or in motion does not belong to a part except accidentally, as Avicenna quibbles (Sufficientia II, 1). Because 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 some thing; whereas with a divisible thing, being moved, like being, depends on its parts, 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 are rest, but that this conditional statement be true that if a part were at rest, the whole would be at rest. Whcih statement can be true even if the antecedent be false, even as this conditional proposition is true: If a man is an ass he is irrational.

VIII. Secondly, he proves it by induction, thus Physics VIII, 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, as heavy and light things are, since these are moved by their generating cause and by that which removes obstacles the to the movement. Now whatever things are in motion are moved either per se or accidentally; and if per se, 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.

IX. Thirdly, he proves his point thus (Physics VIII, 5). Nothing is at the same time in act and in potentiality in respect of the same thing. Now whatever is in motion, as such, is in potentiality, because motion is the act of that which is in potentiality, as such (Physics III, 1). Whereas whatever moves, as such, is in act, for nothing acts except in so far as it is in act. Therefore nothing is both mover and moved in respect of the same movement. Hence nothing moves itself.

X. We must observe, however, that Plato, who asserted that every mover is moved (Phaedrus p. 247c), employed the term movement in a more general sense that Aristotle. For Aristotle took movement in its strict sense, for the act of a thing that is in potentiality as such, in which sense it applies only to divisible things and bodies, as is proved in Physics VI, 4. Whereas according to Plato that which moves itself is not a body; fo rhe 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 De Anima III, 7. In this sense, then, he said that the first mover moves itself, in as much 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.

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

XII. The first of these is as follows (Physics VII, 1). 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 Physics VI, 4. 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 in a finite time. But this is impossible. Therefore it is impossible to proceed to infinity in movers and things moved.

XIII. That it is impossible for the aforesaid infinite number of things to be moved in a finite time, he proves thus. Mover and moved must needs be simultaneous; and he proves this by induction from each species of movement. but bodies cannot be simultaneous except by continuity or contact. Wherefore since all the aforesaid movers and things moved are bodies, as proved, they must needs be as one movable thing through their continuity or contact. And thus one infinite thing would be move in a finite time, which is shown to be impossible in Physics VII, 1.

XIV. The second argument in proof of the same statement is as follows Physics VIII, 5. In an inordinate series of movers and things moved, where namely throughout the series one is moved by the other, we must needs 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 intermediate movers as it were. Therefore it will be impossible for any of them to be moved: and thus nothing in the world will be moved.

XV. The third argument amount to the same, except that it proceeds in the reverse order, namely by beginning from above: and 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.

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

XVII. The second way 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 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, namely, that at any time there be no movement (Physics VIII, 5). 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.

XVIII. 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 it is possible to find white 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.

XIX. If, however, the aforesaid proposition is true in itself, again there follows something impossible and unfitting. For the mover must needs 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 needs have science, while the learner must needs not have it, and thus the same will be both possessed and not possessed by the same, which is impossible. And if it be moved by another kind of movement, so that, to wit, 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, perchance, someone say that a recurrence takes place, in this way, that 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, not imimmediately indeed but mediately.

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

XXI. Since however, given that there is a first mover that is not moved by anything outside itself, it does not follow that it is absolutely immovable, Aristotle proceeds further, saying that this may happen in two ways (Physics VIII, 5). First, so that this first mover is absolutely immovable. And if this be granted, our point is established, namely that there is a first immovable mover. Secondly, 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: wherefore also in things moved, it is logical that what is moved first is moved by itself and not another.

XXII. But if this be granted, the same consequence follows (Physics VIII, 5). For it cannot be said that the whole of that which 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 that a thing would be at the same time in act and in potentiality, since a mover, as such, is in act, while that which is moved is in potentiality. 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.

XXIII. 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 potentiality and in act, and moreover that the whole would move itself not primarily but by reason of its part. It remains, therefore, that in that which moves itself, one part must be immovable, and must move the other part.

XXIV. Since, however, in those things among us which move themselves, namely animals, the part which moves, namely the soul, though immovable by 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 (Physics VIII, 6).

For in those things which among us move themselves, namely animals, since they are coruuptible, the part which moves is moved accidentally. Now those corruptible things which move themselves must needs 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.

XXV. It is clear that, in accordance with his hypothesis, some self-mover must be everlasting. For it, as he supposes, movement is everlasting, the production of these self-movers that are subjects 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, 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.

XXVI. 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 which movement the mover that moves itself is moved accidentally. Whence we may gather that no self-mover, whose mover is moved per se 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 per se nor accidentally.

XXVII. 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.

XXVIII. Since, however, God is not part of a self-mover, Aristotle goes on in his Metaphysics XII, 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 that 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 appetible is a moved mover, whereas the appetible is a mover altogether unmoved. Therefore there must needs be a first mover separate and altogether immovable, and this is God.

XXIX. 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.

XXX. To this we reply that the most effective way to prove God’s existence is from the supposition of the eternity of the world, which being supposed, it seems less manifest that God exists. 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 it origin from some cause of its becoming, since nothing evolves itself from potentiality to act, or from non-being to being.

XXXI. The second is that the aforesaid arguments suppose that the first moved thing, namely the heavenly body, has its motive principle in itself, whence it follows that it is animated: and by many this is not granted.

XXXII. To this we reply that if the first mover is not supposed to have its motive principle in itself, if 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 (Physics VIII, 5).

XXXIII. The Philosopher proceeds in a different way in Metaphysics to show that it is impossible to proceed to infinity in efficient causes, and that we must come to one first cause, and this we call God (Metaphysics Ia, 2). This is how he proceeds. In all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate cause is the cause of the ultimate, whether the intermediate be one or several. Now if the cause be removed, that which it cases 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.

XXXIV. Another reason can be drawn from the words of Aristotle. For in Metaphysics Ia, 1 he shows that there is something supremely true, from the fact that we see that of two false things one is falser that the other, wherefore it follows that one also is truer than the other (Metaphysics IV, 4). Now this is by reason of approximation to that which is simply and supremely true. Wherefore we may further conclude that there is something that is supremely being. And this we call God.

XXXV. Another argument in support of this conclusion is adduced by Damascene from the government of things (De Fide Orthodoxa I, 3): and the same reasoning is indicated by the Commentator (Averroes) in Physics 2 (i.e., In II Physicorum, t.c. 75). 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, whereby each and all are made to tend to a definite end. Now we see that in the world of 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.