The people who, today, seem to me to be making “natural law” type arguments of the sort Aristotle himself would recognize are the evolutionary psychology folks – the people who are trying (we can debate with what success) to develop a genuinely scientific genealogy of morals, to know our natures by understanding, scientifically, how they got that way. But this isn’t at all what people who call themselves supporters of a “natural law” approach to law and ethics do.
It seems to me that this is the reason that natural law arguments fail in practice. It’s not that we can’t accept that we have natures, or that those natures might be constraining in one fashion or another – outside of certain politically touchy topics, we entertain the idea that our natures constrain us, and how we can pursue (and achieve) happiness, all the time. It’s that the advocates of a natural law approach cannot explain adequately how they know what they claim to know about our natures, and expose that purported knowledge to scientific criticism of the kind that we would recognize if the question were, say, “do dogs feel pain?” And the suspicion grows, over time, that this question isn’t opened not because it cannot be opened but because it must not be opened, because it is really the conclusions that are “known” absolutely, and not the premises.
Pascal-Emmanuel Gobry takes the argument in a slightly different direction, defending the Enlightenment notion of natural rights while holding that this “is not a religious concept, and what’s more, it’s much more useful as a secular concept”:
Without appealing to God, the only way to ground the idea of universal human rights is if there is such a thing as human nature, which is shared by human beings, because they are human beings, and which includes the endowment of rights. This is the classic formulation of secular Enlightenment morality. Because human beings are beings “of a rational nature”, they have rights, the Enlightenment tells us—the key word here being nature. The insane still have human rights, the Enlightenment tells us, because even though they may not individually be rational, they share human nature, which itself is rational, and thereby endowed of rights. This seems absolutely crucial to me. No human nature, no natural law, no human rights, no secular Enlightenment morality (as we have thus far been able to understand these things).