We’re blogging through St. Thomas Aquinas’ Compendium Theologiae, sometimes called his Shorter Summa. Find the previous posts here.
In Chapter 3, Thomas showed that there must be a “First Mover”, something that stands at the head of all per se causal chains, where the word “move” means any kind of change, not just motion from place to place. Here he goes on to show that this First Mover is also an Unmoved Mover, that is, an unchanged changer.
We clearly infer from this that God, who moves all things, must Himself be immovable.
This is what we mean when we say that God is eternal: not that He’s around from the beginning of time to its end, but that He is essentially timeless. Time is a measure of change, and God does not change; He simply is. (On top of that, God is metaphysically prior to time, which is, after all, part of creation. But I’m getting ahead of myself.)
Thomas goes on to prove this by assuming that God changes and reasoning from that assumption to a contradiction; this is called a reduction to absurdity. If we assume that God can change, then there are two possibilities:
If He, being the first mover, were Himself moved, He would have to be moved either by Himself or by another.
I can move myself, as when I choose to get up from my chair. I can also be moved by another, as when my chair collapses under me. Neither of these possibilities is going to work in God’s case. The second is clearly wrong on the face of it:
He cannot be moved by another, for then there would have to be some mover prior to Him, which is against the very idea of a first mover.
If the First Mover is moved by another, then it isn’t the First Mover. So what about the second possibility? Perhaps God changes Himself, as when I change my position (or my mind)? Again there are two possibilities:
If He is moved by Himself, this can be conceived in two ways: either that He is mover and moved according to the same respect, or that He is a mover according to one aspect of Him and is moved according to another aspect.
What does Thomas mean by his talk “according to the same respect” and “one aspect” versus “another aspect”?
Consider two coins, a nickel and a dime. Which is bigger depends on how you look at it. The nickel is physically bigger than the dime: it is thicker, and has a larger diameter. But the dime is monetarily larger: it is worth ten cents and the nickel is only worth five cents. Thus, the nickel is bigger than the dime in one aspect, but smaller than the dime in another aspect.But the nickel cannot be both bigger than and smaller than the dime in the same respect: it cannot be worth both more money and less, it cannot have both a larger diameter and a smaller diameter. Looking at it another way, the nickel is round in one aspect, but flat in another. But it cannot be both round and not-round in the same respect.
Thomas says that God cannot be both mover and moved in the same respect:
The first of these alternatives is ruled out. For everything that is moved is, to that extent, in potency, and whatever moves is in act. Therefore if God is both mover and moved according to the same respect, He has to be in potency and in act according to the same respect, which is impossible.
A nickel is actually round; but it cannot become round, because it’s already round. You cannot become what you already are. For God to be both mover and moved in the same respect would be like the nickel being actually round and also able to become round.
Well then, suppose that He could be the mover (the cause of the change) in one respect and the moved (the effect of the change) in another respect. This may sound obscure but it’s actually quite the normal thing. Suppose I choose to move my hand. I am the mover, in that I am willing the change, and I am the mover, in that my hand is moving. But my hand isn’t willing, and my will isn’t moving physically.
This won’t work for the First Mover either.
The second alternative is likewise out of the question. If one part were moving and another were moved, there would be no first mover Himself as such, but only by reason of that part of Him which moves. But what is per se is prior to that which is not per se. Hence there cannot be a first mover at all, if this perfection is attributed to a being by reason of a part of that being. Accordingly the first mover must be altogether immovable.
In other words, if a part of the First Mover can move the rest of the First Mover, then the First Mover isn’t the First Mover—the part is.
Thus, Thomas has examined the various ways that the First Mover might change, and has found that all of them lead to logical contradictions. And therefore the First Mover is unmoved; and this is why you’ll sometimes hear the terms “First Mover” and the “Unmoved Mover” used interchangeably.
You might worry: if God is immobile, unchanging, how can He do anything? But immobile doesn’t mean having no effect; we’ve already determined that God is the ultimate source of all change. It isn’t that God isn’t doing anything; it’s that He is always doing everything He does.