CS Lewis, Mere Christianity, & Law of Nature

So I started reading CS Lewis' Mere Christianity. I have previously read The Screwtape Letters, and I love Lewis' simple profundity. The last paragraph of chapter one in Mere Christianity follows this pattern:

These, then, are the two points I wanted to make. First, that human beings, all over the earth, have this curious idea that they ought to behave in a certain way, and cannot really get rid of it. Secondly, that they do not in fact behave in that way. They know the Law of Nature; they break it. These two facts are the foundation of all clear thinking about ourselves and the universe we live in.