I think that you guys may be missing the important implication when you say that morals are relative. If morals are relative to a society, then who am I to say that pedophilia is wrong and we should prosecute all those who commit it? What if it is acceptable in his culture? How can I prove that my viewpoint is the valid one?
Sorry John, I'm just using your post as a springboard to vent, since some of this is not relevant to what you asked.
The way out is to say that morals are relative not to societies but to rational beings -- us. Take the issue of slavery. We can apply rationality to the issue and see that slavery under a utilitarian or deontological position (the two biggies that we use to discuss rationally this matter) is immoral. Those slave holding societies in the past were simply wrong. Is this judging their society by modern standards? Yep, probably. But it may also be that, with the economic necessities removed, we may more clearly see the immorality of the practice and not be blinded by our rationalizations.
Morality makes no sense except to rational beings. Sure, there are forms of proto-morality in other animals that allow particular forms of action -- allow them to coexist in groups -- but this is nowhere near the full expression of morality in which humans engage and which depends critically on the existence of language.
We must always recall that morality is not what people do -- not the actions in which different societies engage -- but what people
should do. Examination of a society's actions is anthropology, not morality. Attempts at justification are what constitute morality.
We may all agree that moral thinking has been perverted in the past and that we are capable of moral progress. We may disagree. I don't think, as many have already stated, that to point to the existence of slavery in the past (and justifications for slavery in the past) constitutes great evidence for moral relativism between societies. Why does it not simply demonstrate that what we actually do is subject to our whims and desires (and rationalizations) and that we are not particularly moral creatures? Why does it not simply reveal that we need to be more careful in deciding what is and what is not ethical?
As to the OP, it cannot be the case that morality proper arises in some set of divine rules. Those rules might possibly exist, but they would not constitute morality, or a moral sense. We feel what is moral within us and we decide if rules are ethical or not based on those feelings. Morality proper, then, must derive from the type of creatures that we are and the ways that we think and feel. So, yes, it has an evolutionary origin; it is the means by which we are able to form together into large groups. We needn't sink into pure emotivism, however, or "morality is what we feel must be right" because that is only the origin. Our internal sense that we should not kill innocents is not morality. It is a feeling. The rational expression of this feeling into a larger moral imperative, expressed through language (which necessarily implies that is not an individual, but a group concern) is what constitutes morality. It is a construction, and it is relative. But it is not relative to the individual's wants or to the larger society's wants. It is relative to human interaction, human construction, rational thinking.
Or, the easy answer would be to tell him to read the Euthyphro, Meno, and Protagoras of Plato.
The other option is to conclude that morality is a sham, and it's all just emotivism and rationalization.
