Answer:
C.S. Lewis states that moral law is not a simply convention . He says "there are two reasons for saying it belongs to the same class as mathematics. The first is, as I said in the first chapter, that though there are differences between the moral ideas of one time or country and those of another, the differences are not really very great — not nearly so great as most people imagine — [...].The other reason is this. When you think about these differences between the morality of one people and another, do you think that the morality of one people is ever better or worse than that of another? Have any of the changes been improvements? If not, then of course there could never be any moral progress. 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."
Then the Law of Human Nature is compared as a standard or universal truth: "he 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. But the standard that measures two things is something different from either. 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."
Reference: Lewis, C.S. “Some Objections .” PBS, Public Broadcasting Service, 1952