See the first post in this series for an explanation and guide of our tour of Summa Contra Gentiles.
Last time we only proved one premise, that some things move. Review this first or else you will be lost. This was an observational premise. Of course, some, sensing the coming inescapable conclusion to this argument, suggested with Parmenides that things don’t actually move or change because they only move “relatively”, or whatever. None of these counters are in the least convincing, as we’ll further see. Settle in, we have a lot to do today.
Chapter 13: Arguments in proof of God’s existence
4 This argument contains two propositions that need to be proved: namely that whatever is in motion is moved by another,i and that it is not possible to proceed to infinity in movers and things moved.ii
5 The first of these is proved by the Philosopher in three ways.iii 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.iv 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 6 Phys.v
6 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 needs 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 needs be moved by another.vi
7 Nor is 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. 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.vii 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 that 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 he is irrational.viii
8 Secondly, he proves it by induction, thus. 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.ix 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.x 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.xi Now whatsoever things are in motion are moved either per sexii 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.
9 Thirdly, he proves his point thus. 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. 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.xiii
10 We must observe, however, that Plato, 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 potentiality as such, in which sense it applies only to divisible things and bodies, as is proved in 6 Phys. Whereas 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. 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.xiv
iYou, dear reader, are potentially in Cleveland. Unless you’re not actually there, you are not actually there, but you potentially could be. Right? Now what could bring you there? Well, many things. A car, your feet, a plane. Unless you are dead, there is no logically necessary reason you couldn’t (sometime) be there. Point is this: if you are now here and later in Cleveland, some actual thing or things caused you to be there. It could not have been the potential of you being in Cleveland that caused you to be there.
In front of us is a blank canvas. It is potentially a painting of a horse. If it becomes such, some actual thing caused it to be. That it was potentially a picture of a horse could not be the cause. That is, the potential is not and cannot be a cause. Some actual thing or things was the cause.
A block of wood that is here may potentially be there, but to get there some actual thing must cause it to move. The potential that is there cannot be a cause. And so on.
We need these observations for background.
iiOnly the first premise today. This premise is handled later. As promised, slow going.
iiiWe only do this first proof today.
ivAn animal can move itself, as the next premise demonstrates. But pay attention to what it means when an animal moves itself. A rock, for instance, or a molecule of oxygen in the atmosphere must be moved by something actual.
vHere is Aristotle on this crucial subject (from Aquinas’s footnote):
Further, everything that changes must be divisible. For since every change is from something to something, and when a thing is at the goal of its change it is no longer changing, and when both it itself and all its parts are at the starting-point of its change it is not changing (for that which is in whole and in part in an unvarying condition is not in a state of change); it follows, therefore, that part of that which is changing must be at the starting-point and part at the goal: for as a whole it cannot be in both or in neither. (Here by ‘goal of change’ I mean that which comes first in the process of change: e.g. in a process of change from white the goal in question will be grey, not black: for it is not necessary that that that which is changing should be at either of the extremes.) It is evident, therefore, that everything that changes must be divisible.
Thus divisible also means extended in space. Even photons, which haven’t any mass, are extended waves. They are not particles of no, or absolutely zero, size.
viIn other words, in those creatures capable of self-locomotion parts of them are being moved by other parts, as when you walk. Do not forget we are interested in the parts which are moving themselves, and how and why they move. Re-read the example given in the previous post. This is absolutely necessary to understand. We are at stick-moving-the-stone example, where your arm is moved by muscles are moved by cells are moved by chemicals are moved by individual atoms are moved by electrons and protons are moved by quarks are moved by strings (or whatever) are moved, ultimately, by an unmoved (and unmovable) mover all now, at this instant.
viiWhat might “a thing moves itself primarily and of itself, not by reason of its parts” mean? If a thing could move itself not by its parts, but primarily, it would be like—and this is a weak and fictional analogy—a psychic remaining still and concentrating his mental “powers” so that his body flies through the air. But of course, that would still involve the movement many physical things (physical forces must come into play to interact with the body to shift it) and of the mental powers themselves, from a quiescent to active state, so it doesn’t ultimately work (see below). The point Aquinas is saying is that nothing can move primarily. Something must set off (and sustain) every motion or change. And, as above, this something must be something actual and not potential.
viiiZing! Ha ha ha! The great man himself with a hilarious pun.
ixThese are self-evident.
xThomas does not mean to invoke the psychic example, but by soul he means, if you like, by consciousness. Anyway, if the soul moves its object, then another has moved that object.
xiThat this premise is outdated doesn’t change the proof. A light thing is moved by (say) the wind, and not primarily; or it is moved accidentally or by a force, etc.
xiiPer se, i.e. by virtue of itself; accidentally, here by one part pushing another, or the whole being pushed by something exterior, as in you in a TSA groping line at the airport.
xiiiIf you are moving from A to B, it is only because you are potentially in or at B that you can move. If it is impossible—and not just unlikely, however unlikely—then you are not potentially in or at B. And since potentiality cannot be a cause, it must be act, or something actual, which is driving the movement. The key to this argument is “Therefore nothing is both mover and moved in respect of the same movement.” Understand that, and you have it made. All three parts are necessary: the mover, that which is moved, and this here and now movement.
xivIsn’t that pretty? Exciting, too, because our task is not only to prove God’s existence, but, having done so, to show what we can about God’s nature. This is a kind of first step in that direction. Here, don’t forget that Aristotle and Plato do not mean just physical movement from A to B, but change of any kind; Plato includes changes in, if you like, thinking, or willing; Aristotle does not. The psychic analogy might be welcomed by Plato but not Aristotle. Our intellects are not material, and therefore not divisible or extended.
Again, it is an act which must change something in A but which is potentially in B to B. It will turn out that God is, in Aristotle’s and Thomas’s terms, pure act or actuality itself, with no potentiality (this was not proved today). This is another reason to thoroughly grasp the difference between act and potential. We’ve only done a bare outline. For the terrific introduction in modern language, see Ed Feser’s new Scholastic Metaphysics: A Contemporary Introduction. Note: I’ll be reviewing this book soon.
All that was proved today was whatever is in motion is moved by another. Every scientist accepts this premise. In this sense, today’s arguments were scientific proof. This premise is something upon which all science relies. Its implications are, of course, deeper than science, but it encompasses the motivating force behind science. No scientist says, “This went/changed from A to B for no reason whatsoever.”
Don’t forget we have one more premise in the first way (of proof of God’s existence). Then we have a whole second way.
 [Physics] Ch. iv.
 2 Suffic. i.
 8 Phys. iv
 8 Phys. v. 8.
 3 Phys. i. 6.
 Phoedrus xxiv. (D.).
 Ch. vii.