Describe the two ways Lewis argues that the moral law is not simply a social convention. Some things we learn in school are good manners and social conventions. Others are Truths (like math). That is, why is there a good reason to believe the Law of Human Nature is a universal truth?

Taking into account the two concepts C. S. Lewis explores concerning the moral, it can be inferred that there are indeed universal truths, and the Law of human nature is not the exception mainly because morals are the same between communities despite time and place.

Explanation:

To support the aforementioned statement, Lewis establishes that moral conceptions were not better or worse between countries or different periods of time. The author also says that 2 sides may always be measured by a third side and the cycle may continue.

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