"Each man is at every moment subjected to different sets of law but there is only one of these which he is free to disobey. As a body, he is subjected to gravitation and cannot disobey it, if you leave him unsupported in mid-air, he has no more choice about falling than a stone has. As an organism, he is subjected to various biological laws which he cannot disobey any more than an animal can. That is, he cannot disobey those laws which he shares with other things; but the law which is peculiar to his human nature, the law that he does not share with animals or vegetables or inorganic things, is the one he can disobey if he chooses.

This law was called the Law of Nature because people thought that every one knew it by nature and did not need to be taught it. They did not mean, of course, that you might not find an odd individual here and there who did not know it, just as you find a few people who are colour-blind or have no ear for a tune. But taking the race as a whole, they thought that the human idea of decent behaviour was obvious to every one. And I believe they were right. If they were not, then all the things we said about the war were nonsense. What was the sense in saying the enemy were in the wrng unless Right is a real thing which the Nazis at bottom knew as well as we did and ought to have practised? If hey had no notion of what we mean by right, then, though we might still have had to fight them, we could no more have blamed them for that than for the colour of their hair.

I know that some people say the idea of a Law of Nature or decent behaviour known to all men is unsound, because of different civilisations and different ages have had quite different moralities... But this is not true... Men have differed as regards what people you ought to be unselfish to... But they have always agreed that you ought not put yourself first. Selfishness has never been admired. Men have differed as to whether you should have one wife or four. But they always agreed that you must not simply have any woman you liked.

Human beings, all over the earth, have this curious idea that they ought to behave a certain way, and cannot really get rid of it... And they do not behave that way. "

- C.S. Lewis - Mere Christianity