The belief that there is something called "Natural Law" has been around for quite some time. Since at least the time of the Stoics, it has been claimed as an unquestionable guide to conduct. In its great days, Roman jurisprudence was informed by the belief. One can see its influence in medieval thinkers such as Aquinas, and in the thinkers of the Enlightenment. It's influence on those we call the Founders of this great republic was profound.
Like all claims that morality may have an objective basis, or worse yet to some a divine basis, it has been the subject of attack if not derision in intellectual circles for quite a number of years, but my recollection is that even in the remote time when I was receiving a philosophical education there were hints that it might be having a kind of renewal. Aquinas's name was being whispered by some back then. "Should old Aquinas be forgot and never brought to mind?" I remember a visiting professor singing during a meeting of our university philosophy club--not very well, and to some nervous titters by us undergrads, and stern looks from the members of our very modern (back then in any case) philosophy department.
I confess I have a certain fondness for the concept of Natural Law, which I suppose is to be expected in an admirer of the Stoics. and Cicero, that hybrid Stoic-Academic. And it seems to me that some, at least, of the objections made to it may be addressed.
First, I think it's quite possible to recognize Natural Law without believing it to be of divine origin. The universe exists, as do we humans, whether we are creatures of a divine creator or not. Created or uncreated, the universe, and human beings, have characteristics which can be observed and regarding which intelligent inferences can be made. Certain characteristics can be said to lead to certain conduct. Certain characteristics and desires seem to be shared.
Second, it's not necessary to believe in absolute laws, or laws founded in some kind of unchangeable certainty, applicable in all circumstances. Warranted conclusions may be drawn based on available evidence, which are not absolute but are recognized as being subject to modification as necessary--just as secular laws are understood to be subject to amendment or repeal. Even as the "laws" of science are understood to be subject to modification.
It may be that Natural Law understood in this fashion won't satisfy those who deny that morality can have any objective basis apart from humanity. However, it doesn't seem to me that their satisfaction is of much importance. For my part, though, the fact that morality is essentially human is neither surprising nor a sign that it must necessarily be defective or inadequate in some sense. We are as much a part of nature as anything else. Our thoughts and desires are influenced by the universe of which we are a part, and they in turn influence our conduct in the universe, which influence other creatures and things in the universe. To speak of them as not being "objective" in the sense other constituents of the universe may be objective seems, to me, to be dependent on the view that they are not part of the universe in some sense.
If they're not part of the universe, though, just what are they supposed to be? Is it possible that the critics of Natural Law themselves assume there are things outside or beyond nature? Or are they as much victims of the "quest for certainty" as those they criticize, and have come to believe that without absolute certainty all is subjective, relative?