Jeremy Taylor wrote:

​I think the focus on human nature as the end of morality is important - I have a hard time seeing how any serious objective account of morality, that wasn't a moral sentimentalist one, could take a different approach. And the concept of final causes does add to our understanding of that nature in the world.

I agree. I think all those are important and valid concepts. I think what I'm looking for in an ethics is a larger and more comprehensive positive account of the realization of human nature in the world rather than mere proscriptions and tight analytic arguments for these in terms of the failure to adhere to certain narrowly given aspects of our natures. I want a philosophical ethical vision of life, in other words.

I mean, if, for instance, it tured out that one can act contrary to ones nature but not much happened of any importance in one's life, then it seems to me that natural law would be a valid but absurd enterprise- like chess on a sinking ship.

Jeremy Taylor wrote:

I think natural law is not an exhaustive moral account, but as far as it goes it seems sound to me. You're certainly correct that one of the most important supports for it is the perverted faculty theory.

I'm actually not too impressed with Feser on natural law. His writings seem generally unaware of the big debates and where others would dispute him.

For instance, in setting up the perverted faculty argument, he writes:

[What has been said so far] suffices to give us a sense of the grounds of moral obligation, that which makes it the case that moral imperatives have categorical rather than merely hypothetical force. The hypothetical imperative (1) If I want what is good for me then I ought to pursue what realizes my natural ends and avoid what frustrates them is something whose truth follows from the metaphysical analysis sketched above. By itself, it does not give us a categorical imperative because the consequent will have force only for someone who accepts the antecedent. But that (2) I do want what is good for me is something true of all of us by virtue of our nature as human beings, and is in any case self-evident, being just a variation on Aquinas's fundamental principle of natural law. These premises yield the conclusion (3) I ought to pursue what realizes my natural ends and what frustrates them. It does have categorical force because (2) has categorical force, and (2) has categorical force because it cannot be otherwise given our nature. Not only the content of our moral obligations but their obligatory force are thus determined by natural teleology. (Scholastic Metaphysics, 386)

There is a large exegetical difficulty with this analysis, pointed out by Germain Grisez in his famous 1965 article and acknowledged even by Grisez's critics. Aquinas's synderesis principle is self-evident, yes, but in ST Ia-IIae, 94, 2, Aquinas asks "Whether the natural law contains several precepts, or only one?" The objections argue that the natural law has only one precept; Aquinas says that it has many, and all are self-evident.

Synderesis is the first principle of the natural law, indeed, and the others are "based on" it. Aquinas writes:

Now as "being" is the first thing that falls under the apprehension simply, so "good" is the first thing that falls under the apprehension of the practical reason, which is directed to action: since every agent acts for an end under the aspect of good. Consequently the first principle of practical reason is one founded on the notion of good, viz. that "good is that which all things seek after." Hence this is the first precept of law, that "good is to be done and pursued, and evil is to be avoided." All other precepts of the natural law are based upon this: so that whatever the practical reason naturally apprehends as man's good (or evil) belongs to the precepts of the natural law as something to be done or avoided.

But on Feser's analysis, only synderesis is self-evident. The other principles are derived from it in the way he lays out, so that they are neither first principles nor per se nota. His treatment also breaks down the analogy St. Thomas is drawing with the first principles of speculative reason. The first principles of speculative reason are based on the apprehension of being not as propositions derived from it but (I think) as principles which in some way (though self-evident themselves) owe their intelligibility to the very first principle. The same is the case, I think, with ethics.

Practical reason "naturally apprehends" what is good for man and those generate several first principles of natural law, "based upon" the first, which confers intelligibility but does not figure as a premise in an argument like Feser's. The debate between new and old natural lawyers (to the extent that the latter is even a determinate group) has to do with which "natural inclinations" Aquinas means and what sort of speculative prerequisites there are for seeing the truth of the various first principles of the natural law. Feser gives his own answer to the first question on p. 383 without really acknowledging that it's a point of contention, and the second question he does not recognize because he does not appreciate the role that synderesis really plays.

He does provide a defense of the guise of the good thesis on p. 385 and in the lengthier essay on pp. 297-320, but I think he misapplies it when it comes time to produce his syllogism. For the guise of the good only tells one that I do want what [I take to be] good for me, but it doesn't (obviously) follow from the metaphysics that If I do want what [I take to be] good for me then I ought to pursue what realizes my natural ends and avoid what frustrates them. In other words, the use to which Feser wants to put synderesis equivocates on "good".

This may be part of what accounts for the sense expressed here that natural law theory is incomplete and focuses too much on negatives, as well as the sense that it requires too much formal metaphysical knowledge. Here is Aquinas elsewhere:

Wherefore, taking nature in this sense["that is said to be natural to a thing which befits it in respect of its substance"], it is necessary that the principle of whatever belongs to a thing, be a natural principle. This is evident in regard to the intellect: for the principles of intellectual knowledge are naturally known. In like manner the principle of voluntary movements must be something naturally willed.
 
Now this is good in general, to which the will tends naturally, as does each power to its object; and again it is the last end, which stands in the same relation to things appetible, as the first principles of demonstrations to things intelligible: and, speaking generally, it is all those things which belong to the willer according to his nature. For it is not only things pertaining to the will that the will desires, but also that which pertains to each power, and to the entire man. Wherefore man wills naturally not only the object of the will, but also other things that are appropriate to the other powers; such as the knowledge of truth, which befits the intellect; and to be and to live and other like things which regard the natural well-being; all of which are included in the object of the will, as so many particular goods. (emphasis added)

Contra the new natural lawyers, teleology enters into this picture. But the relationship between synderesis and teleology is more subtle than Feser realizes.