Lewis talks about some objections to the idea of a “Moral Law” raised by some of the listeners to his radio talks.
Some people say that this “Law of Human Nature” or “Moral Law” is nothing but our instincts — or “herd instinct” — which we have developed as we evolved as a species. Lewis argues that it’s not human or herd instinct. We have many instincts (for example, the instinct to fight or flee under certain circumstances, or to save someone or save ourselves under others, etc), but there’s still a “Moral Law” that “presses upon us” the obligation to choose one instinct over another, or one kind of action/behavior over another. This “Law” guides us to behave in particular ways. It is like the music sheet that guides the musician to choose to play a particular set of notes over another so that harmonious sound (a melody) is produced. Therefore, this “Law” is not itself our instincts. It lies beyond us as a transcendent and objective standard of behavior.
Other people say that the “Law of Human Nature” or the “Moral Law” is simply a social convention, like traffic rules. Moral laws differ from society to society because different societies invent different moral rules. Lewis argues that that is not true. The “Moral Law” is actually objectively real — like mathematics is objectively real.
I agree with Lewis. (Here, I’ll stop putting the Moral Law in quotation marks.) The Moral Law is objectively real. It is not itself human or herd instinct. It is not invented by individuals, societies, or cultures. It lies beyond each of us as a transcendent and objective standard of morality.