Having had a chance to delve more deeply into this "Unwritten Universal Moral Principle", I must say that I'm not impressed.
It wasn't meant to be impressive, and it shouldn't be. It's only a beginning point from which to add.
First off, I see no reason why it should only apply to 'human beings who have a certain rational capacity'. Humans are perfectly capable of behaving according to some intuitive moral principles, even if they fail to comprehend the rationale behind those principles. More power to them if they can deduce the rationale behind them, but their behavior isn't governed by knowing why those principles exist, but only by intuiting that they exist.
I think you've misunderstood me. Have you ever studied Kant and his Categorical Imperative? Obviously, intuiontion will play into the eventuating, even stronger notions of Ethics that most of us (MOST but not all) have. We can refer to the late G.E. Moore for more on Intuitionism. He had some interesting things to say.
Logically, even highly social animals should be able to intuit many of the exact same moral principles that we do. After all, those intuitable moral principles are essential to any stable social structure. So I don't accept the argument that an "Unwritten Universal Moral Principle" requires 'human beings who have a certain rational capacity'.
I think that by referring to highly social animals intuiting similar things, you've just inadvertently added to my argument, most particularly the fact that the Principle is UN-written, not written. Sure, some aspects of what we see as behavior which contributes to forms of morality will be (and from the Christian perspective even) expected to be evident througought portions of the animal kingdom, which is partly why I refer to the work of atheist anthropologist, Barbara J. King. ... Like I said, what consider to be but a beginning principle is only that. It's the first step in dozens of other additional steps that come in our human efforts to conceptualize Ethics and put forth action on behalf of moral social integration.
But yeah. There are sociopaths and psychopaths, among other psychological maladies and disorders, which do get in the way of "certain rational capacities" that are needed for ethical recognition and moral functioning.
You and I both know this.
Reference
King, Babara J. (2007/2017).
Evolving God. The University of Chicago Press.
I think that any social animal that has the ability to intuit acceptable social behavior, has morals. And furthermore, I see no reason to assume that human moral principles are any more valid than theirs are.
If you want to take the views of atheist moral philospopher, James Rachels, then be my guest. As a Christian who thinks that we have just a bit more than "other animals" where morals are concerned, I'll have to disagree with you and lean toward my own sources like rather than yours.
Reference
Rachels, James. (1990). C
reated From Animals: The moral implications of Darwinism. Oxford.
Secondly, how are you differentiating this "Unwritten Universal Moral Principle" from "Natural Law"?
Natural Law Theory in Ethics (rather than Natural theology) proffers a fuller account of all of the naturally interconnected moral inclinations that we and animals may have. But it's a bit teleological too in that it posits the general idea that "good humans are those who fulfill their true [natural] nature; bad human beings are those who don't...actions that are right just are because they are natural, and wrong just because they are unnatural. And people are good to the extent that they fulfill their true nature, bad insofar as they flout it" (Landau, p. 72). More can be said about Natrual Law Theory, and I don't intend to get into that here.
By contrast, my "unwritten principle" is a little tiny idea about rational moral human
cognizance and it simply implies that human beings everywhere should have the ability to recognize moral solidarity; it's human in that, too, we can't realize in an a posteriori fashion that we shouldn't feel like we could just kill our neighbors for food or convenience or anger, and we don't eat our young when the going gets tough. If we're rational. Of course, because the world is situationally complex, we need more than simply this principle alone in order to build, support and assert a robust Ethical Framework, even if it's one of a dozen we can choose from.
So, nowhere am I saying that this one principle alone is enough by which to 'be moral'; it's not an Ethical framework. Just a little ol' principle. It's only a very, very, very basic principle at that, like one dot on a connect the dot drawing that you and I could then color in after we've ... connected all of the many other dots that need to be connected.
One dot is not the whole picture; one principle is not a system or frame for Ethics. Of course, one dot isn't impressive. It shouldn't be. But it's also necessary.
Reference
Landau, Russ Shafer. (2010).
The Fundamentals of Ethics. Oxford.