- Whatever is in motion is moved by another
- Some things are in motion
- Therefore, some things are moved by other movers
- These movers are either themselves moved, or not moved
- If not moved, they are unmoved movers
- If moved, they must be moved by still another mover
- Either this series of movers will continue to infinity, or it will end at a first mover
- It is not possible to proceed to infinity in such a series
- Therefore, there is a first mover
Premise one is a version of a causal principle. Basically, it states that whatever is in motion has a cause of its motion, which, for most people, probably seems initially intuitive. Even so, a metaphysical defense of the premise can be made.
For Aquinas, motion means “change” broadly. In the Summa Theologiae, he gives the example of fire heating a piece of wood as an instance of motion. Change, metaphysically, is the actualization of some potential. Before a piece of wood is heated by a fire, it has to have the potential to become heated; otherwise it could not. When a change occurs, some potential in an object is actualized. But a potential can only be actualized by something already actualized. This is because something cannot be both potential and actual in the same respect at the sime time. A door that is actually closed is potentially open. A door cannot be both actually closed and potentially closed at the same time, upon pain of contradiction. But if a thing cannot be both potential and actual in the same respect at the same time, then that thing cannot possibly actualize itself. Think of it this way: could the potential hotness of the wood cause itself to become actual? No, because the potential hotness of the wood is only potential, and has no causal efficacy. Only something that is already actually hot can actualize the potential hotness of the wood.
What this means is that when we encounter things in motion, i.e. things changing, we know there must be some cause of their motion. And that cause itself will either be something unmoved, or will itself be moved. If it is the former, it must be an unmoved mover, in which case we have reached the conclusion of the argument. But if (as is more likely) it is the latter, it too must be caused to move by something. Thus we have a series of movers.
In such a series, Saint Thomas goes on to argue, there are two possibilities: either the series will proceed infinitely; or it will end in some first mover. But an infinite regress in a series of movers is impossible. To see why this is so, consider the following image: a moon does not, like a star, give off its own light; it rather reflects light from a star. A star, on the other hand, is its own source of light. Now suppose we had a “chain” of moons. One moon, instead of reflecting light from a star, reflected light from another moon. And that latter moon, likewise, reflected light from a further moon, and so on. No matter how many moons there are in the series, we’d know that at some point, the chain terminates at a star. Why? Because all those moons have to be getting their light from somewhere. Similarly, each mover in the series of movers derives its power to move from its own cause. Now if each mover derives its power to move from another, then they do not have that power to move in themselves. If the series went on infinitely, there could be no source for the power to move and hence no actual motion. So instead we must posit that there is some first mover, which has of itself the power to move others.
It’s important to make clear here what type of causal series we’re referring to. Aquinas distinguishes between what he calls per se and per accidens causal series. A per se causal series is one in which the effect is here and now dependent upon the activity of the cause — as, in our example, the light coming from the moon is derived from and simultaneously dependent upon the light of a star. A per accidens causal series, on the other hand, is one in which the continued activity of the cause is not essential to the activity of the effect. Hence, a father begets a son, but a son does not require the continued existence of the father to exist himself.
This distinction is important because, for Aquinas, there is no problem in thinking that a per accidens series could in principle go back infinitely. A per se series, however, cannot. Since in a per se series the activity of one cause is completely dependent upon the continued activity of previous causes, there must be some first cause which has the power to cause motion from itself, and not from another. This is the first mover, the unmoved mover.
What about Newton’s laws, which state that an object in motion will continue in motion until acted upon by an external force? Doesn’t this mean that an object can be in motion without a continual cause? Not quite. Several philosophers have done a lot of work on this specific question (see specifically the work of Edward Feser). But for now, let’s just assume Newton’s laws do imply this. Newton’s laws apply only to local motion — the motion of an object from one spatial location to another — and not other types of motion, such as qualitative change. The first way works just on the basis of the latter.
There seem to be multiple series of movers in the world. Doesn’t that mean there must then be mutliple first movers? There are indeed multiple series of movers in the world, but all of them must ultimately converge on a single, absolute “first” mover. We can know this based upon the nature of what an ultimate first mover must be. Remember that motion is the actualization of a potential by something already actual. Recall also that, like the star in our previous example, the first mover must be the source of all the power for motion in the whole series. Just like all the moons got their light from the one star, so all the movers get their “actuality” from the first mover. The first mover must be its own source of actuality. It must be, as Saint Thomas puts it, “Pure Act,” a being that is entirely actual and has no unactualized potentials.
Something that is Pure Act is immutable, i.e. cannot change, because change is the actualization of potential and a being of Pure Act has no potentials to be actualized. Furtheremore, there can only possibly be one being that is Pure Act in existence. Why so? Because the only way to distinguish separate entities is by some distinct potentials which they posses. For instance, suppose you draw two triangles on a sheet of paper that are exactly the same in every way except that one is blue in color, the other green. Notice that the two different colors are qualities added onto the triangle itself. Both triangles are the same, but one has the quality of blue added, and the other has the quality of green added. In order to have these added qualities, the triangles must first have had the potential to have them. But if the triangles are exactly the same except for these distinct colors, then they both must have had the same potentials. In other words, each must have had the potential to be either blue or green. If that is the case, then the blue triangle now has an unactualized potential to be green, and the green triangle now has the unactualized potential to be blue. The only way to distinguish entities is by distinct potentials. If there are beings without any potentials at all, there would be no way to distinguish the beings from each other; they would be identical. Hence, there can only be one being of Pure Act, only one ultimate first mover.
Why think that this first mover is anything like the theistic concept of “God”? To answer this question, we must consider the nature of what a first mover must be. We’ve seen already that a being that is Pure Act must be one. What else? A being that is Pure Act cannot ever begin or end to exist, since such would be a change, and change requires potency. A being that is Pure Act must be timeless, because time requires succession/movement, which is change. A being that is Pure Act must be immaterial, because “matter” itself just is potential, potential to interact and enter into composition with other things. A being that is Pure Act cannot be spatial, because spatial extension likewise requires potential; and, furthermore, whatever is spatial must also be temporal and material. Hence we know that the first mover must be an immutable, eternal, timeless, immaterial, incorporeal being which is the the purely actual cause of all motion/change. This is enough to demonstate the falsity of naturalism. It is possible to move from here to other classical attributes of God, such as possessing knowledge and will, but that is more complicated and unnecessary to arrive at the conclusion that at least some supernatural being exists.
Thomas Aquinas. Summa Contra Gentiles. I, 13.
Thomas Aquinas. Summa Theologiae. I, Q. 2, Art. 3.
Ibid. I, Q. 46, Art. 2.