See the first post in this series for an explanation and guide of our tour of Summa Contra Gentiles.
Recall that the First Way has already been done. Meaning we already have one proof for God’s existence. This is the second, and the good news is that most of our terminology has already been introduced. A lot to do today, but the good news is that this is the last of Chapter 13! But these proofs are all we have. We don’t yet know anything about God except that He’s the Unmoved Mover, the Unchanging Changer. What can that mean? That’s coming.
Chapter 13: Arguments in proof of God’s existence
18 Again, if any two things are found accidentallyi 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.
If, however, the aforesaid proposition is true in itself, again there follows something impossible or 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.ii
And if it be moved by another kind of movement, so that, to wit, that which 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.iii 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 immediately indeed but mediately.iv It remains therefore that we must needs postulate some first mover that is not moved by anything outside itself.
19 Since however, given that there is a first mover that is not moved by anything outside itself,v it does not follow that it 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 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 by another.
20 But, if this be granted, the same consequence follows. 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.vi 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.
21 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.
22 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.vii
23 For in those things which among us move themselves, namely animals, since they are corruptible, 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.
24 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 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…viii
30 The Philosopher proceeds in a different way in 2 Metaph. 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. 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.ix
31 Another reason can be drawn from the words of Aristotle. For in 2 Metaph. he shows that those things which excel as true excel as beings: and in 4 Metaph. he shows that there is something supremely true, from the fact that we see that of two false things one is falser than the other, wherefore it follows that one also is truer than the other. 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.x
32 Another argument in support of this conclusion is adduced by Damascene from the government of things: and the same reasoning is indicated by the Commentator in 2 Phys. 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 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 God.xi
iA circle can be accidentally red or yellow and still be a circle, etc. A thing is not defined by its accidents.
iiAquinas created this argument in ignorance of our modern educational system, of course. But he’s still right.
iiiYou can change quantity, mass, color, place, and the like, but there are not an infinite number of changeable qualities.
ivDon’t loose sight of the main example, the stone moved by the stick moved by the arm moved by the muscles, etc., all in the here and now. This movement cannot be circularly caused.
vWe’re starting to hone in on (but have not reached) the idea that there can be only one Unmoved Mover, and not an assemblage of them. Not gods, but God.
viYou cannot potentially and actually be in Cleveland simultaneously. Aquinas is speaking ontologically, not epistemologically. And this reminds me that we have to, sometime soon, introduce Jaki’s criticisms of complementarity.
viiWe’ll also do this more later. But, for now, the soul of the animal is its form. Things are composed of forms and substances or matter. Clay, a substance, can be in the form of an ashtray (eek!) or (say) a model car. Speaking loosely, and later correctly, the soul animates the substance. The souls of plants are lesser than the souls of animals, which in turn are lesser than the souls of men. Like I said, this is only a crude introduction. Let’s stick here to the argument of movement, else we will be arguing that of which we have incomplete information.
viiiYou may feel buried after these last two paragraphs. Slow reading is in order. Again, Aquinas is leading us to why there can be only one Unmoved Mover. There’s no getting around that this last proof is rather dry—I even leave out some discussion about movement of heavenly bodies—but it dots all the Js and crosses all the Xs.
ixHere it is in a compact form! This is the elevator proof. Pretty, too.
xI don’t think Aquinas expected this snippet to be convincing in itself, but included it as an augment to main argument. Here’s a good article from Swinburne on the argument from the beauty of design, and another by Williams on beauty.
xiAnd this sketch is cruder still. It’s doubtful that it, in this telegraphic form, would convince any moderns. I’m not expert enough in Medieval history to know why Aquinas included it, which audience he had in mind.
Hooray! We are finally done with Chapter 13! But not entirely done with motion.
 8 Phys., l.c.
 8 Phys. vi.
 D. 1a. i. 5.
 D. 3. iv. 27, 28.
 De Fide Orth. i. 3.
 Text 75.