I haven't done a great deal of reading on the subject, but Plato first posited the separation of morality and a divine basis for its existence. The Euthyphro Problem is essentially a chicken and egg type of question, asking if God tells someone to follow a course of action because it is morally right, or if by the very fact that God has given a command the action is made morally right, as God is the fount of morality.
This has actually long been a key question in my mind, and the fact that I haven't put more effort into resolving it isn't exactly a mark in my favor. Although the following arguments are now well over two millennia old, I fall back on them because my experience with any philosophers past the Classical era is sadly deficient.
Plato's moral musings (which Kant, if I understand his work correctly, later expanded upon) were brought about at least in part in response to his dislike of the Sophists. In this case, they held what I now refer to as the "teenager worldview", believing that truth is relative, and thus there can be no absolutes. It's a common refrain among young would-be philosopher kings, and I distinctly remember struggling with the concept myself for a few months in our undergrad days once the idea had finally occurred to me.
I imagine Q can add more insight into this discussion (I really need to actually devote some real time to not only reading The Republic but actually sitting down and studying it), but as I understand it, Plato felt that morality was a function of human rationality and the innate human urge to gather in communities. Ergo, because moral actions promote secure and harmonious societies, man is naturally inclined to codify and follow them. I'd ask either of you to call me out on this interpretation though, because I've done no study whatsoever on the work, and so cannot in any way lay claim to accurately interpreting it. Hell, I haven't even perused the damn thing for almost a decade.
Of course, if I'm interpreting Plato correctly, that still leaves gaping holes in the question of moral universality; I'd like to hear how Kant addresses some of these, if he does. Most notably, what of moral absolutes that are not necessarily required for the 'good' of society (essentially, things that you don't find in the 10 Commandments, which by and large can be read as a laundry list of things people shouldn't do if they want their society to hold together)? Plato for one probably thought nothing of slavery, while Greek society of that time is, when you boil it all down, one big NAMBLA recruitment tool. It's only later that we as a society, heavily influenced by those self-same Greeks, decided that slavery and sexual relations with children were not only wrong, but were morally repugnant on a level equalled by little, if anything else. At the time both were viewed as integral pieces of a functioning civilization.
Given that then, can there be a universal morality independent of both a higher power and the ethical mores of a particular society?
