Feser’s Aquinas can be adequately summarized as “Aquinas does not believe the obviously wrong things you think he does! He believes different, more complicated, and more subtly wrong things.”
This is, of course, salutary. There is a tendency to assume that people who believe things that seem wrong to us are stupid and have failed to notice the obvious flaws in their reasoning. However, St. Thomas Aquinas was much much smarter than I am; his ideas are complicated and nuanced and the only reason he is wrong is that he didn’t have science yet.
Feser argues that final causes, or teleoi, obviously exist. He points out that it is quite impossible to talk about biology without talking about final causes: if you can’t say “the purpose of the heart is to pump blood” or “the purpose of the penis is urination, sensation, penetration and ejaculation”, you are going to have a really hard time talking about human bodies.
The problem– and this is entirely not Aquinas’s fault, and only a little Feser’s– is that biology does have teleoi. Human bodies are in fact designed by a tremendously powerful optimizer for a purpose.
The problem is that the tremendously powerful optimizer is “evolution” and the purpose is “to have the maximum possible number of grandchildren.”
This is, to put it lightly, not a very satisfying ground of morality.
That gets into the second problem of teleoi– why should I pay attention to the teleoi at all? Feser doesn’t really address this, presumably because his moral sense says that he shouldn’t murder people, should pray regularly, should donate to charity, etc., and his interpretation of the teleoi says the same thing, and so he doesn’t have a problem. But my moral sense says “you should be vegan and give lots of money to charity”, and my interpretation of the teleoi says “are you NUTS? You’re taking money from your children and giving it to STRANGERS? Who can’t even repay you? And what if you NEED those calories you’re turning down?”
And I’m kind of inclined to say fuck the teleoi.
But if I can say “fuck the teleoi, I’m doing what’s right”, then the teleoi cannot possibly be the objective, rational ground of morality.
Another example is Aquinas’s distinction between causal series ordered per accidens and causal series ordered per se. Causal series ordered per accidens work the way that having children works: I exist because my mother gave birth to me, but if my mother died, I would continue to exist. Causal series per se work the way that pushing a cart works. If I stopped pushing the cart, then it would stop moving. Per accidens causal series can continue as far back as you like: it is theoretically possible that I could have an infinity-times-great grandmother. Per se causal series have to end: the cart is moving because my hands are pushing it, my hands are pushing it because of muscles in my arm, the muscles in my arm are moving because of signals from my nerves, and so on. Aquinas believes, therefore, that there has to be an uncaused causer; this being is God.
However, I think this mistake is a simple failure of reductionism. It certainly looks to me like I have to push the cart for the cart to move, but that’s not what’s really happening. What’s actually happening is that my atoms are bouncing against the atoms in the cart, transferring force from my hands to the cart, in accordance with the laws of classical mechanics. (Well, what’s actually happening is quantum mechanics, but I don’t understand quantum mechanics and am not going to venture a guess about how it works.) In reality, every causal series is per accidens and none of them are per se and there is no need to ground anything in God.
The larger issue here is the fallibility of pure reason. Feser says that his philosophy is like math: you start from uncontroversial premises that everyone agrees on, and then you build a larger system that says things you’d never imagine came from the premises. The problem is that humans are regularly wrong about uncontroversial things that everyone agrees on. Our intuitions evolved to deal with things sized somewhere between a tenth of an inch and a few dozen miles, but the universe stubbornly insists on consisting of things lightyears apart or yoctometers wide. Quantum mechanics makes no intuitive sense; astrophysics makes no intuitive sense.
And we are expecting Feser’s uncontroversial premises that make intuitive sense to be true… why, exactly?
Math is simpler, because its premises have been carefully refined to involve as little extraneous detail as possible. Even a statement like “carts stop when you stop pushing them” is a lot less specific and less well-defined than a statement like “if a line segment intersects two straight lines forming two interior angles on the same side that sum to less than two right angles, then the two lines, if extended indefinitely, meet on that side on which the angles sum to less than two right angles.” You can ask what a straight line is and get a real answer– extends infinitely in two directions, has no width or breadth– whereas what a cart is, as we saw, presents innumerable difficulties. (And, of course, you can come up with consistent non-Euclidean geometries, many of which are useful for describing the universe despite our strong intuitions about parallel lines and the crossing thereof. Feser does not seem to have considered developing non-Aquinasian ethical systems.)