Start with this idea:
- An activity A is φly required of x if and only if (and because) x's not performing A constitutes a failure of x's φ faculty.
But there is a problem: how do we identify the epistemic, practical and cardiovascular faculties? We could try to pick them out in some plausible way: the epistemic faculty is our faculty of belief formation, the moral faculty is the will, and the cardiovascular faculties are the heart and blood vessels.
However, I think things aren't that easy. The account of a cardiovascular faculty doesn't work: not every failure of heart function need be a cardiovascular failure, since a heart (if not in us, then in some other species) may have non-cardiovascular function (e.g., of providing an internal clock). On the other hand, a representation is a belief at least in part because it is an output of the epistemic faculties. A faculty is a will because it aims at the production of actions (i.e., one sense of praxeis). The epistemic and practical cases, thus, end up defining epistemic and practical requirement in terms of the proper functioning of the faculties of epistemic and practical production.
But what are epistemic and practical productions? (And by analogy, what's a cardiovascular production?) We could try to identify them by ostension. My believing that I have two legs is an epistemic production, while my writing this post is a practical production.
There are two problems with the ostensive approach. The first is the problem of aliens. Aliens can have epistemic and practical faculties, i.e., intellects and wills, but they might not have exactly productions that fit in the same natural kind as our believings and doings. This problem is similar to the problem of multiple realizability for token identity physicalism in the theory of mind.
The second is the "So what?" problem. If I simply ostend to two of my faculties, whether through their productions or otherwise, that leaves it mysterious why the normativity that they generate is particularly important. There is something deeper and more important about epistemic and practical requirements than about cardiovascular ones. A person who is an epistemic or practical disaster but who has a well-functioning cardiovascular system is much worse off than one who is an epistemic and practical success but has a disaster of a cardiovascular system (maybe is surviving on life support). The obvious natural law solution to the "So what?" problem is to say that our human nature makes epistemic and practical flourishing more non-instrumentally important to us. But that raises the question of whether there couldn't be beings who are very much like us, yet whose natures elevate the cardiovascular over the rational and practical. It seems to be because the epistemic and practical faculties are what they are that they are more non-instrumentally important to our flourishing than our cardiovascular faculties, rather than because of our human nature.
I don't know how serious the two problems are. Maybe one can and should bite the bullet on them.