See the first post in this series for an explanation and guide of our tour of Summa Contra Gentiles.
The First Way is in the bag. And now we continue, fresh as it were, with the Second Way. But since this introduces what are unfamiliar concepts to some, we again slow down. But have no fear: after this chapter, we can again speed up, because most of the terms will by then be familiar.
Chapter 13: Arguments in proof of God’s existence
…16 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.viii
17 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.i Therefore it is a contingent proposition that no mover is moved.ii 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, namely, that at any time there be no movement.iii 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.iv
iMost things you know are contingently true. It is true (supposing) that your car is in your driveway, but only because you bought a car and parked it there. The universe did not require you to buy this car. Not everybody has a car (Yours Truly does not). Propositions which are contingently true can be imagined false. I can easily imagine you too do not have car. No rules of logic would be violated were that to be so.
A necessary truth is that which cannot be other than true. A necessary truth cannot be imagined false. Here is a necessary truth: “A thing cannot both exist and not exist simultaneously.” It is impossible to imagine, or more properly to think of, a thing existing and not existing at the same time. Logic cannot be violated.
When speaking of contingency, some speak of “possible worlds”, as if the world in which you do not own a car is, somehow, brought into existence by your imagining you do not have a car; a sort of milky idealism. Or that this “possible world”, simply because it can exist, actually exists; perhaps part of a multiverse or one of the “many worlds” where all events occur.
But this thinking mixes up the existence of a thing with the knowledge of the thing. Ontology is not epistemology. Things can exist without your, or anybody’s, knowledge. And you can have knowledge (or rather can think) of things that do not exist, like conservative sociologists or the Starship Enterprise. The latter does not come into existence just because it can be imagined.
Now some say of necessity that nothing occurs “at random” or “randomly”. But this is to make the same mistake. Randomness isn’t a thing. It therefore cannot be a cause. Best way to think of this is to substitute “ignorance” every time you see random. If the sentence is still sensible, then the author has a proper understanding; if not, then not.
iiIf it is only contingently true that “no mover is moved”, it can be imagined that “it is false that no mover is moved.” And you have to love the next two sentences.
iiiSome highlights from Aristotle (following the footnote):
…Now the existence of motion is asserted by all who have anything to say about nature, because they all concern themselves with the construction of the world and study the question of becoming and perishing, which processes could not come about without the existence of motion.
It is not always asserted by some readers here, but let that pass. Continuing, we see that talk of “infinite possible worlds” is not a new invention. (I added extra paragraphs for readability.)
But those who say that there is an infinite number of worlds, some of which are in process of becoming while others are in process of perishing, assert that there is always motion (for these processes of becoming and perishing of the worlds necessarily involve motion), whereas those who hold that there is only one world, whether everlasting or not, make corresponding assumptions in regard to motion. If then it is possible that at any time nothing should be in motion, this must come about in one of two ways…
…Motion, we say, is the fulfilment of the movable in so far as it is movable. Each kind of motion, therefore, necessarily involves the presence of the things that are capable of that motion. In fact, even apart from the definition of motion, every one would admit that in each kind of motion it is that which is capable of that motion that is in motion: thus it is that which is capable of alteration that is altered, and that which is capable of local change that is in locomotion: and so there must be something capable of being burned before there can be a process of being burned, and something capable of burning before there can be a process of burning.
Moreover, these things also must either have a beginning before which they had no being, or they must be eternal. Now if there was a becoming of every movable thing, it follows that before the motion in question another change or motion must have taken place in which that which was capable of being moved or of causing motion had its becoming.
To suppose, on the other hand, that these things were in being throughout all previous time without there being any motion appears unreasonable on a moment’s thought, and still more unreasonable, we shall find, on further consideration. For if we are to say that, while there are on the one hand things that are movable, and on the other hand things that are motive, there is a time when there is a first movent and a first moved, and another time when there is no such thing but only something that is at rest, then this thing that is at rest must previously have been in process of change: for there must have been some cause of its rest, rest being the privation of motion…
We went as as this for two reasons: (1) Aquinas was not pulling his assertion about Aristotle out of thin air, and that Aristotle’s assertion is true, and (2) for the phrase “rest being the privation of motion”. This is a tease, but the concept of privation will become important for us. For instance, evil, as it will turn out, is privation of the good. But let’s not get distracted by this today.
ivSince we’ve already done a lot, without really have done much, we leave this phrase hanging in the air, as it were. We’ll pick up with it again next week. Very little was proved this week, except Aristotle’s small proposition.
 [8 Phys. v.]
 8 Phys. i.