So, the theory of Forms has been one of the main topics of my discussion, as Aristotle's version of it is central to Feser's account; I have of course consistently criticized Feser's attempts to insist that it is rationally compulsory. Feser knows, of course, that Aristotle's metaphysics have been rejected in the modern era, and in chapter 5 he tries to explain why this is, attempting, naturally, to do so in such a way as to avoid the conclusion that the reasons why it has been rejected are any good.
One of his crucial moves is his attempt to sharply separate Aristotle's scientific theories from his metaphysical theories. Aristotle himself would have recognized no such sharp separation, and Feser's appeals to Aristotle often bleed over into scientific matters (as with his insistence that there are forms corresponding to biological species, which is incompatible with biological facts). But it is true that the new science didn't refute everything in Aristotle. What the new science did incontrovertibly show is that Aristotle must have been in many cases wrong about which forms exist, and what their natures are. The new mathematical physics did not show that the motion of bodies doesn't involve forms at all, but if forms are involved, they aren't the ones Aristotle thought existed. And Darwin didn't show that there are no forms involved in life, but he did show that there are no forms of biological species.
But that result is already enough to have serious consequences. The form of a human being is one of the things that science has discarded, and it is of course central to Feser's project. Furthermore, the mere fact that Aristotle was so badly wrong about the forms casts serious doubt on the idea that the forms are somehow built into us. One of the more puzzling aspects of Plato for a modern reader is that in a number of dialogues he spends a very long time wrestling with the problem of how it's possible for anybody to be wrong about anything. That's actually a serious problem on Plato's theory; if we have access to the forms, and the forms make the world the way they are, why don't they give us infallible knowledge of the world? Aristotle, as usual shallower than Plato, doesn't think through the consequences of his theory far enough to see why this is such a huge problem, and Feser naturally follows Aristotle in sweeping this problem under the rug of "common sense;" contradictions are fine, apparently, as long as common sense endorses both halves of the contradiction.
One natural way to keep forms and avoid the problem is to abandon the dual role of the forms; say they don't both control our thoughts and the world, in different ways, but that they only control the world. If you insist on keeping some kind of forms, but both grant that Aristotle was largely wrong about which forms there are, and also draw the conclusion that knowledge of the forms can't be acquired by pure reason but must depend on scientific inquiry, you're David Armstrong (one of the modern naturalists that Feser says never actually thinks about the really serious issues). I think it's better to dispense with the forms altogether, but Armstrong's view has a huge advantage over Feser's in that it is at least is not contradicted by firmly established scientific knowledge.
Feser's opium example may help illustrate the issue. Feser criticizes the naturalists for inconsistency in claiming both that the explanation of opium in terms of a "dormitive power" is empty, and claiming that it's false. But one of the central problems with scholastic forms is that they are so vague; interpreted one way, claims about scholastic forms are empty, interpreted another way they are false.
The false doctrine is that there is some single thing, a "dormitive power", which is fairly simple and works the same way in all the various things that cause sleep, and which doesn't do anything else and which is very different from the power of anything else to do anything else. The empty doctrine is that there's something about opium that causes sleep. It is true that the full account of the powers of opium can be described with or without forms; it involves the chemical properties of the compounds that make up opium and the chemical properties of animal life, which of course are themselves determined by the physics of the particles that make up those compounds, and one could go along with Armstrong and say that the properties of the physics particles are forms, or one could say that the story about forms isn't really adding anything useful (as I would) and do without it. And the details of this underlying story make the false doctrine false; the properties of opium are very complicated, and there are things very similar to opium at the chemical level in very many ways which don't cause sleep, and things which cause sleep which are in very many ways very different at the chemical level.
Feser claims that these details, which he admits would have to be settled empirically, make no difference to the metpahysics. Unfortunately, Feser's claims about the form of humanity are much more like the false version of the dormitive power story than they are like the empty version. If he were to grant that we could be very wrong about exactly which forms are involved in human life and how they were connected to other forms, and insist only that forms are involved somehow, that would completely undermine his efforts to derive detailed moral conclusions from the form of humanity.
Feser maintains that the idea of secondary qualities requires dualism, but this is only true of one insists for some reason that, for example, a perception of redness must be red. But that is, of course, absurd. I myself lean toward a dispositional theory of secondary qualities; red is a disposition of surfaces to produce sensations of a certain kind. Feser thinks dispositions require final causes; more on that next chapter, but if we're talking about final causes in the bare logical sense I mentioned at the end of chapter 2, it's not so much that dispositions require final causes as that they are final causes, and so final causes can't explain dispositions because if any explanation is needed, final causes need it to the same extent. Of course, Feser's final causes are supposed to be more than bare logical final causes, but while Feser tries to define them as things which explain more, he actually doesn't have any worthwhile account of how they provide this additional explanatory help. Thus, since explanations can't be conjured up out of nothing by definition, his final causes don't actually have any more explanatory power than the bare logical final causes.
Feser also claims that intentionality cannot be accounted for on materialist grounds; he keeps harping on this throughout his book, but in only one place (in the next chapter) does he make any effort to directly address the best materialist story about intentionality (and his effort is disappointing, to put it mildly). I am greatly annoyed by what appear to be Feser's frequent attempt to appeal to authority, and have tried to avoid such nonsense myself, but I feel I must note that many of the "allies" he cites for his anti-materialist cause think the materialists have won on the intentionality issue; David Chalmers, for example, thinks this, and Frank Jackson not only has long granted that materialism can explain intentionality, he has even recently come around to agreeing that qualia aren't an obstacle to materialism either.
Feser's "solutions" to assorted philosophical problems:
Skepticism: Feser says that the rejection of the Aristotelian view produces skepticism, since without Aristotle's claim that the same forms work on our minds and on the world, there's no guarantee that the world will match up to our thoughts at all. But this is of course nonsense. Skepticism became dominant at Plato's academy shortly after Plato's death, and not because they already anticipated Cartesian views. I suspect it was partly because of Alexander the Great and the return of military autocracy as the default government pattern; if you don't claim to know anything, you are never claiming to know the rulers are wrong about anything, so you're slightly safer from hostile rulers. Still, skepticism seemed to be an available option because all it actually requires is admitting that people can be wrong about anything. Once that obvious fact is granted, it is hard to see how you can completely rule out the possibility that we're wrong about everything
And that's why Aristotle's view doesn't actually solve skeptical problems. After all, we do make mistakes, so even an Aristotelian has to have some account of how such errors are possible. And whatever the account is, it will always be possible to raise the skeptical worry that errors like that could be much more common than we think.
Induction: The problem of induction suffers the same fate; Aristotelianism has a solution to the problem of induction if our knowledge of forms is infallible. But if our knowledge of forms were infallible, we couldn't make a lot of mistakes we quite obviously make. So the Aristotelian view doesn't actually solve the problem of induction, and so arguments like Goodman's can be run against the Aristotelian; how do we know we're not making one of our mistakes about essences in thinking that it's green rather than grue which can genuinely be part of the essences of things? Not that I think an Aristotelian should actually be a skeptic, of course, but Aristotelianism alone is not an answer to the problem; the Aristotelian must join the rest of us in seeking other answers.
Personal identity: Derek Parfit and other contemporary philosophers seem to have been rediscovering Buddha's insight that our concept of ourselves is in many respects deeply misleading, indeed incoherent. Parfit also follows Buddha in thinking that good consequences follow from this insight (as they often do from recognizing the truth); the mistaken notion of the self is deeply implicated in destructive forms of selfishness, so recognizing the flawed notion of the self should help free one from those harmful selfish impulses. Feser will of course have none of this; selfishness is central to the conservative mindset, and must be defended at all costs! Now, of course the objectionable moral implications of Feser's view don't constitute evidence that his view is false (it's false because it involves the same impossible theory of souls that I've criticized all through his discussion), but since he's constantly harping on the supposed immoral consequences of modern naturalism, I couldn't resist giving a nod to the morally appealing consequences of the Buddhist view. Incidentally, Feser insists that skepticism about personal identity originates with Descartes; I find it implausible that Descartes' influence was the source of Buddha's views on this matter.
Free will: A free choice cannot be derived from the world as understood by the chooser; when you freely choose to do something, you don't examine your evidence to determine what choice you are going to make, you make the choice. Hence, it must come from outside of that? Perhaps it must come from outside of everything? Feser says not quite; it must come from outside of everything material, but that's fine, because there's the realm of souls and forms and final causes. But that's actually no help at all, at least so long as we understand the realm of souls and forms and final causes (as Feser insists we do); the problem comes from our understanding of the world, not the material nature of the world. Kant recognized this, and so said free choices comes from an unknowable realm; others since have found the idea of an unknowable realm problematic (how do we know it's a realm? It's unknowable!) and so Heidegger and Sartre and others say free choices comes from nothing. Crazy answers, to be sure (though much depends on how you interpret them), but at least they recognize that there's a serious problem here. Compatibilists also take the problem seriously for the most part. Feser does not take the problem seriously; he gives no explanation of what it is about final causes that is supposed to connect them to freedom, probably because to do so he'd have to say a lot of things that are contrary to the common sense he claims to be respecting.
Natural rights: I of course am a consequentialist, and think doing without natural rights is an improvement. But I would have thought this would be the place to discuss Kant. Since Feser instead uses it as an opportunity to heap scorn on Locke, I have little to say about this part of his discussion; since Feser is unfair to nearly all of his opponents, I expect he is unfair to Locke, but I am not sufficiently a Locke scholar to say how.
Morality in general: I haven't done a lot of quoting of Feser, because he usually buries his points in considerable excess verbage (and ad hominem), but I'll quote this one. He complains that Hume does not "really have anything to say to a group of sociopaths - Nazis, communists, jihadists, pro-choice activists, or whomever - who seek to remake society in their image by social or genetic engineering, say. The Platonist, Aristotelian, or Thomist can say that such people are behaving in an inherently irrational and objectively wicked manner, given human nature. All the Humean can say is 'Gee, I hope they don't succeed.'" Leaving aside the somewhat suspect character of his list of sociopaths, does Feser really think it is of such over-riding importance, or indeed any importance at all, what one says to Nazis? Plato was not so foolish (Socrates himself could not move Callicles, or Meno for that matter, and the conversion of Thrasymachus is presented in such a way as to make its sincerity highly dubious, to say the least), and I hope no Platonist would be. I am less expert on the Aristotelians and Thomists, but I had not heard that they believed in magic spells, for surely it would have to involve magic if a Nazi ever changed his behavior in the slightest upon hearing the incantation "you are behaving in an inherently irrational and objectively wicked manner."
Hume was actually a more practical sort; he favored saying whatever would work best, and employing other means, again whatever accomplishes the most for the least cost, when words fail. I never cease to be astonished at how often people try to argue against consequentialism on the basis that endorses policies with bad consequences; if your argument is that consequentialism is committed to a policy which has bad consequences, that is a sure sign that there's something wrong with your argument, not that there's something wrong with consequentialism. If the policy has bad consequences, consequentialism correctly interpreted will always say do something else.
Now, Feser admits that his opponents may have space for a "pretense" of morality, but he thinks that unless they use the magic spells, that can have no impact. Of course, a consequentialist moral theory is hardly a pretense of morality, but perhaps more to the point, Feser's discussion of this issue suggests that he thinks people behaved more morally in the middle ages than they do at present (elsewhere in the chapter he suggests that they were more devoted to wisdom as well!) I usually don't think much of the incredulous stare as an argument, but I'm not sure where to begin with this aspect of Feser's discussion.
Feser is aware that the modern era has produced non-consequentialist ethics as well; Kant's moral theory is perhaps even more influential than any variety of consequentialism. But Feser's discussion of Kant's moral theory would barely get a passing grade if it came from an undergraduate in an introductory ethics class. Kant's central idea is that ethics is supposed to be both inherently universal, and chosen by humans. So what we can choose is constrained only by the requirement that our choices must be consistent with the requirement of universality. This means I cannot make choices that undermine the rights of others to their choices; I must treat all choices as mattering. I don't want to try to give Kant's full story, but since Feser seems to think the modern worldview means you can't criticize anyone (perhaps the reason he doesn't like it; he loves to criticize!), I should probably at least mention that detail in Kant's approach. One way of putting Kant's version is that we must be tolerant of everything except intolerance; we must treat impartially all perspectives which are not themselves partial. If we respect the choices of others who do not themselves respect the choices of others, we are failing to respect the choices of their victims; if we give equal weight to the biased, we are in part sharing their bias. So we can't fully respect the choices of others like that (of wrongdoers). The closest we can come, Kant thinks, is to respect their choice to be biased by applying their standards to them; being biased against them. Hence proportional retribution as the response to wrongdoers.
Feser only discusses the first form of the categorical imperative, and indeed the first form is very hard to sensibly interpret if one ignores the other two forms. But Feser's criticism of Kant goes from being pathetic to high comedy when he complains that there is no way Kant could possibly derive detailed moral rules from "reason alone," using only his principles. I know that Feser is under the delusion that his own Thomistic principles are clearer and less susceptible to conflicting interpretations than Kant's, but it certainly is nothing more than a delusion.