In the exert we read from Mere Christianity, C.S. Lewis talks about human nature. He says that when most people think about the laws of nature they think of things like the laws of gravity, but what they should actually think of is the laws of right and wrong. Lewis analyzes what kind of sense of this nature we have.
He talks about how most laws we cannot choose whether we want to obey them or not such as the law of gravity, but we can choose whether or not we want to do right or wrong. These choices define who we are. Lewis says that these laws are universal such as it is wrong to murder or steal. Other people would say that certain cultures have different morals than other cultures and that is okay. Lewis would say that that is not true; cultures might have morals that vary a little, but they have the same basic laws of nature.
Lewis helps people realize that even though small laws like traffic violations might vary from nation to nation. We all have a certain sense of morality imprinted in us. It is what we choose to act on that matters.