The fourth way of demonstrating the existence of God, says St. Thomas,
is taken from the gradation to be found in things. Among beings there are some more and some less good, true, noble and the like.
But "more" and "less" are predicated of different things, according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest;
so that there is something which is truest, something best, something noblest and, consequently, something which is uttermost being; for those things that are greatest in truth are greatest in being...
Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum heat, is the cause of all hot things. Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God. (ST, I, 2, 3)
Note that there are two claims here: (a) that the greatest or perfect or infinite thing exists and (b) that it is the cause of all imperfect and finite things.
Objecting to this, Richard Dawkins writes,
That's an argument? You might as well say, people vary in smelliness but we can make the comparison only by reference to a perfect maximum of conceivable smelliness. Therefore there must exist a preeminently peerless stinker, and we call him God. Or substitute any dimension of comparison you like and derive an equivalently fatuous conclusion.
1. The number line has lesser and greater integers, but there is no highest integer, nor is ∞ the cause of any number. Perhaps we need to restrict our inquiry to "perfections."
2. How about sharpness of a knife? A knife may be more or less sharp, but there is no perfect sharpness (an edge one atom in width? I doubt such a blade could work), and even if there were, the perfect sharpness would not cause the sharpness of my own kitchen knives. Now sharpness is a perfection of a means to an end (of cutting things); so perhaps we need to consider only perfections of living things that are ends in themselves.
(I.e., whose final cause of their own happiness is within them.)
3. Dawkins' smelliness example suggests that there must exist a perfectly noxious skunk. The ability to spray stinky fluid is useful to the skunk but hardly defines it. Very well, let's focus on intrinsic or essential perfections of 2nd-grade things.
4. Swiftness is an essential perfection of a horse. A slow or lame horse is unlikely to survive and reproduce in the wild. Yet there is no self-subsisting swiftness; nor can any horse run at the speed of light; nor does the speed of light cause horses to run fast. I guess we must deal only with the most general essential perfections of life.
We are left with things like power, understanding, love, happiness, relation to time, unity, and so on. But at this point, the fourth way collapses, because all the analogies with (1)-(4), including "fire," have been eliminated.
Searching James Chastek's blog suggests that the principle "whenever something is more or less great by being more perfect, there is something most perfect" may be an "axiom." But I do not find it self-evident. So, I don't know how to salvage the Fourth Way.
I mean, it is conceivable that there is something most perfect, but conceivability does not entail possibility, and possibility does not entail actuality. The Fourth Way thus gives us only an idea of God as a perfect being without even spelling out the meaning of perfection, not a proof that this God exists or that He is even possible.
Perhaps if we assumed (b), that creaturely perfections are caused, then (a) would follow straightforwardly. Just as we deduced that God is pure act by considering creatures that are mixtures of act of potency, so we can argue that there is a perfect being by extrapolating from creatures' falling short of perfection though not completely.
It may be a non sequitur, though, since proving that God is an Unmoved Mover on the physical level is much easier than proving that He is a Happy / Perfect Attractor on the spiritual level.