So, chapter 4 of Feser's book gave me flashbacks to Hubert Schwyzer's Kant seminars. I wonder if Feser ever took those when he was at UCSB; if not, it's unfortunate, as they might have helped him. "Knowing is not like eating," as Schwyzer would say as he tried to explain Kant's struggles with the representational aspect of knowing. Knowing something does not involve taking the objects of knowledge into our minds in the way that eating something involves taking the objects of eating into our bodies. Rather, our minds somehow contain representations of the objects (of course, we may know things about our minds, but even in those cases the knowledge is not the same as what is known). The very name "representation" may be tendentious, but I intend for it to be as content-free as possible; whatever it is in the mind that determines which objects are being thought about is the representation of those objects. The Churchlands may think there are still hidden assumptions buried in this, and perhaps they are right, but leave those worries aside for the moment.
Considerable progress has been made on the nature of representation since Kant's time. In particular, Ruth Millikan provides an extremely interesting account of how thoughts can represent the world in her Language, Thought, and Other Biological Categories. It would take considerable time to go through all of the details, but it is an evolutionary story. Words and thoughts have functions in roughly the sense that biologists speak of things as having functions; the functions are whatever they do that leads to their being favored by natural selection. Millikan argues that a detailed account of these functions can enable us to pick out the objects of thought (in the case of representational thought), because of the way the functions relate to those objects (again, details are lengthy; I highly recommend her book).
The functional story of how my knowledge of the glass of water on my desk is related to the glass of water involves causal relationships between the glass of water and my thought (the functional/evolutionary story picks out which causal relationships are relevant), so the case of knowledge of abstract objects seems to be more difficult, since it will lack such causal relationships. However, while certainly nobody has filled in all the details, I would say this much; a priori knowledge seems to contain a strong conventional component. David Lewis provides a thorough account of conventions in his book Convention, one which connects very naturally to Millikan's theories about thought (Millikan harshly criticizes Lewis, but only on one issue, and I think she misinterprets his theory as being more different from hers than it is; I discuss this in my dissertation). Millikan also has some things of her own to say about a priori knowledge; again, not all the details are filled in, but it all looks very promising to me.
I mention all of this because Feser's argument requires the assumption that there is one and only one way representation could work, and it is nothing like what I've sketched above. Rather, Feser insists on what is surely one of Plato's biggest mistakes, repeated in Aristotle. A Platonic form serves a dual role; it both makes things in the world the way they are, and makes thoughts the way they are, and so one can represent things in the world as being a certain way because the things in the world are made that way by forms that are also present in our thoughts about the world. And so for Feser knowing is like eating after all, in a sense; the object of knowledge, the form, is actually in the mind.
Now, it is a great mystery how something could play such a dual role, especially on Aristotle's version; how can the same thing make a stone heavy and make my thought a thought about heaviness? A thought being about heaviness seems very different from a thing being heavy. The Aristotelian tradition has no explanation beyond that forms are just like that, and have to be like that to play the role that they do. And what about knowledge of the forms? Does that involve forms of forms, or is it some completely different process than ordinary knowledge and representation? Plato already worried about these issues and many others, and scholars of Plato and Aristotle debate them to this day, but Feser sweeps all these controversies and questions under the rug. Really, even if his story wasn't so problematic, it would still be fatal to his argument that it doesn't seem to be the only explanation possible. It shouldn't be necessary to provide an alternative to his account to note that he hasn't proven his is the only possible account, but conveniently we even do have an alternative; the Millikan/Lewis story I allude to above.
Feser "proves" that the mind must be immaterial because the way that, say, the form of the dog works on material things is by making them dogs. It is the form of the dog, on his account, which makes my thought about a dog be about a dog, and if my mind were a material thing, the way it would act on my mind would be by making my mind (well, my thought) a dog. So my mind must be something else. This is not actually valid even given Feser's other assumptions; there doesn't seem to be any logical reason why forms might not operate differently on different kinds of matter, so even given the implausible assumption that knowing is like eating, that the same form must be present in the mind as in the things, it doesn't follow that the mind must be immaterial in contrast to the material things. Some other difference could explain it. And, of course, a more plausible account of representation would completely undermine this argument.
Feser's discussion of mathematical abstractions is if anything even worse, perhaps because he makes the common mistake of identifying the thoughts with the objects of thought in mathematical cases. But even in mathematical cases they are not the same things, and so the triviality that mathematical objects are universal and determinate while nothing material is either universal or determinate is simply irrelevant to whether thoughts of mathematical objects are material; thoughts are not universal either, and even if I didn't already think they were material, I wouldn't have suspected them of being determinate. Fortunately, for thoughts to be about objects it is not required for the thoughts and objects to be identical. This is true even for Feser, of course; the same form must be involved in the thought and the object, but that falls far short of saying that the thought must be the object (the thought of a dog is not a dog, as Feser himself insists). Presumably this is obscured for Feser because he thinks the object of thought is the form in mathematical cases, but that is another unargued assumption, and in any event that would not entail that the thought itself is the form in mathematical cases, as would be needed for his argument to be valid.
Souls and forms:
In other words, Feser's supposed "rational proof" that souls must be a certain kind of thing is anything but. There are further problems with his idea that the soul is the form of a human being. Really, there are endless mysteries and confusions here. The soul is both a special kind of thing which the forms act on to produce knowledge, and it is itself one of the forms; how does it play both of those functions? Feser thought he needed radical metaphysics to explain the dual role of forms in affecting thought and affecting the world, but the dual role of the form of a human being passes without comment. But, more importantly for Feser's subsequent discussion, post-Darwin, biological species are known to be poor candidates for forms.
A Darwinian species is a population united primarily by a certain history; it has related capacities for inter-breeding and similarities in appearance and features, but the history of how selection favored the ancestors of the current members of the population is the decisive element. There is thus no such thing as a "perfect" or "ideal" member of a species, no exact template that members of the species are supposed to match. We may speak of a member of the species as being defective if it lacks one of the traits that selection favored in its ancestors, but this is a somewhat loose way of speaking. A deviation from what is most common among members of the species may be a disadvantage in some sense, or it may be an advantage of some kind (if it's a survival/reproduction advantage, selection will make it more common over time, of course), or it may be of no significance at all; species always display many kinds of variation. This variation makes it impossible to even construct a bare logical form for a species; there may not be any traits which are universal among members of the species while absent from all non-members of the species (unless you include the having of a certain history among those traits, which would produce a very different kind of form than Feser intends).
Feser argues that human life begins at conception because that is when someone acquires the form of humanity (and so the soul). Now, species-membership does plausibly begin with conception, but being a member of a Darwinian species is not like having a soul (as Feser himself would admit). But one does not aquire the form of humanity at conception because there is no such form; "human" is a biological species, and biological species don't work like that. And since the soul is not the form of humanity, it remains an open question when (or perhaps whether, depending on one's theory of souls) it is acquired. So Feser's metaphysical anti-abortion argument fails.
Feser's natural law morality generally relies on their being a form of humanity, so that deviations from the form can be classified as defects, and as immoral if they are under conscious control. Absent such forms, one must have some other standard for identifying moral deficiencies. However, in practice natural law morality tends to (somewhat ironically) identify traits as defects on grounds rather similar to those which lead something to be identified as an evolutionary disadvantage. Notably both natural law theories and evolution see reproduction as central human purposes.
And so Feser classifies homosexual sex as unnatural because of the reproductive function of sex, and so immoral because people can consciously choose whether to engage in it or not. However, one difference between at least most evolutionary biologists and most natural law moralists is that evolutionary biologists recognize that it can be difficult to identify all of the functions and purposes involved in the traits and behaviors of a living thing, while natural law moralists seem to think identifying functions is easy (Feser thinks our bones are somehow involved in our mysterious ability to identify such fuctions).
Another difference is that even a consciously chosen evolutionary disadvantage does not seem to automatically be immoral (nor is an evolutionary advantage always moral), but whether homosexual activity is even an evolutionary disadvantage is unclear. In general, humans have far more sex than they need to for the purposes of reproduction (most mammals have mating seasons to reduce the amount of effort and energy expended on sex). It seems likely that this additional sex serves some purpose or purposes (or there would have been heavy selection pressure against it; the pressures that produce the mating seasons of other mammals), and mostly the evolutionary biologists seem to consider it an unsolved problem what those purposes might be (though they have plenty of guesses).
The natural law moralists also seem to feel the need for such theories, and try to provide them (as I said, they are eerily and ironically similar to the evolutionary theorists in this area); generally the story involves producing bonding in married couples, which are important for raising offspring. Now, this is hardly an absurd theory, but in the context of evolution it would be laughable to pretend that one could rationally prove it was the only possible story. And if that isn't the whole story of the excess human sex (and, in fact, this story doesn't seem to work very well as an evolutionary account; our current evidence doesn't seem to suggest that the family structure implicit here was present in our distant ancestors, so it couldn't have been selected for), the other purposes of sex may include purposes for homosexual sex.
The natural law moralist could claim that this is an area where there theories are different from the evolutionary theories, but in fact they don't seem to have any more evidence for their claims about the purposes of sex than the kind of thing an evolutionary story could give (well, except for evidence gleaned by consulting the bones), so it is unclear how they rule out the possibility of such alternative purposes. Feser mocks Andrew Sullivan for suggesting that the purposes of sex might be more complex than Feser supposes (Feser likes to mock a lot), but he doesn't actually provide any evidence that Sullivan is wrong.
Still, one of my biggest problems with natural law morality is just that its stories sound so much like evolutionary stories, and evolutionary stories don't sound to me like stories of morality. So many horrible things are adaptive in the evolutionary sense. Evolution produced the black death! Admittedly, when something they intuitively judge as bad ("in their bones") seems to serve natural purposes, Feser and his ilk are not likely to call it good; they instead invent convoluted and implausible stories about how it doesn't really serve natural purposes after all. Better to recognize that morality is about purposes humans choose, not purposes given to us by nature; Kant was right to advocate an ethics of autonomy, however inconsistently he may have done so in practice and however many details he might have gotten wrong.
Evil and Miracles:
I mentioned in my discussion of chapter 3 that Feser treats God's attributes as "analogical" when he's in trouble, but most of the time ignores that part of his story. His section on "Faith, reason, and evil" contains particularly egregious examples. The miracles of Jesus are to be considered credible because of the proofs that there's a God who could produce miracles like that. Sorry, Feser, we don't know that about the "analogical" God; miracles seem possible when you're thinking of God as being a person with human-like purposes that could be served by such interventions. As Hume argued, if we think of God as the source of the laws of nature, then God's purposes seem to be clearly given by those, so we should expect God to never produce anything but consequences of those laws of nature.
Similarly, when he discusses the problem of evil, the evil in the world seems unnecessary in the way a parent's punishment can seem like an unnecessary bad thing to a child. Sorry, "analogical" God isn't much like a parent. Really, the problem of evil seems less serious for "analogical" God, but only because we have no way to know what to expect from such a God (and so for the same reason "analogical" God seems a poor basis for any religion, and certainly no help in shoring up Feser's shaky moral conclusions). But here the shuffling back and forth is especially rapid. God might have good reasons for evil which are very much like the good reasons a person might have for causing or enduring some unpleasantness, totally ignoring that for a person causing or tolerating something bad often seems like the best choice precisely because our options are limited, because we're not omnipotent. But Feser hasn't forgotten God's omnipotence; it gives God endless capacity to bring unlimited good out of any evil. And yet Feser's imagination suddenly stops again before the obvious next step, or perhaps God's omnipotence disappears and we're back to non-analogical human-like powerful God, as Feser doesn't even consider that God's omnipotence should surely extend to the capacity to bring all the same good out of no evil. Feser's discussion of the problem of evil is an embarrassment to anybody who has ever seriously examined the issue (from either side).
I've ignored most of his tedious ad hominem attacks in chapter 4, but the chapter is, of course, full of them, like the rest of the book. As usual, I wish he had spent less time on those, and more time trying to fill in a few of the gaps in his arguments.