In Chapter 2 Lewis addresses the difference between “instinct” and the “Law of Human Nature or Moral Law”; “Isn’t what you call the Moral Law simply our herd instinct?”
Lewis addresses this objection as a feeling or strong desire, say to help another person. Feeling a desire to help is quite different from feeling you ought to help whether you want to or not. He uses the scenario where you hear a cry for help. You probably feel two desires, the desire to help-the herd instinct and another strong desire to keep out of danger due to the instinct for self preservation. But inside you probably find a third thing which tells you that you ought to ignore the instinct for self-preservation and help. This thing that judges between to instincts and decides which should be encouraged cannot be either of them. The Moral Law tells us which note to play on a piano it is not one of the notes.
The other principal objection presented in Chapter 2 is the Moral Law is simply social convention put in each of us through education and therefore is a human invention.
Lewis addresses this objection through a comparison to mathematics. We are taught mathematics but a child who grows up on a desert island does not know mathematics. While some things we learn are conventions, like the clothes we wear or driving on the left or right side of the road develop differently from place to place, mathematics is a truth and is the same everywhere and not changing. And so is the Moral Law, is at its core the same everywhere and across time has remained unchanged.
Fr. Barron on Conscience and Morality:
Comments or Questions?