' "Isn't what you call the Moral Law just a social convention, something that is put into us by education?" I think there is a misunderstanding here. The people who ask that question are usually taking it for granted that if we have learned a thing from parents and teachers, then that thing must be merely a human invention. But, of course, that is not so. We all learned the multiplication table at school. A child who grew up alone on a desert island would not know it. But surely it does not follow that the multiplication table is simply a human convention, something human beings have made up for themselves and might have made different if they had liked? .....Progress means not just changing, but changing for the better. If no set of moral ideas were truer or better than any other, there would be no sense in preferring civilized morality to savage morality, or Christian morality to Nazi morality. In fact, of course, we all do believe that some moralities are better than others.....Very well then. The moment you say that one set of moral ideas can be better than another, you are, in fact, measuring them both by a standard, saying that one of them conforms to that standard more nearly than the other.....You are, in fact, comparing them both with some Real Morality, admitting that there is such a thing as a real Right, independent of what people think, and that some people's ideas get nearer to that real Right than others.....The reason why your idea of New York can be truer or less true than mine is that New York is a real place, existing quite apart from what either of us thinks. If when each of us said "New York" each means merely "The town I am imagining in my own head", how could one of us have truer ideas than the other?.....You find out more about God from the Moral Law than from the universe in general just as you find out more about a man by listening to his conversation than by looking at a house he has built. ' ~C.S. Lewis, from "Mere Christianity"
Sorry to quote C.S. Lewis, but he says things so much better than I ever could! Basically he elaborates on what Scott said, which was very good.