Nowadays, when we talk of the “laws of nature” we usually mean things like gravitation, or heredity, or the laws of chemistry. But when the older thinkers called the Law of Right and Wrong “the Law of Nature,” they really meant the Law of Human Nature. The idea was that, just as all bodies are governed by the law of gravitation and organisms by biological laws, so the creature called man also had his law—with this great difference, that a body could not choose whether it obeyed the law of gravitation or not, but a man could choose either to obey the Law of Human Nature or to disobey it. We may put this in another way. Each man is at every moment subjected to several different sets of laws 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 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 color-blind or have no ear for a tune. But taking the race as a whole, they thought that the human idea of decent behavior 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 wrong unless Right is a real thing which the Nazis at bottom knew as well as we did and ought to have practiced? If they had 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 color of their hair.
I know that some people say the idea of a Law of Nature or decent behavior known to all men is unsound, because different civilizations and different ages have had quite different moralities.
But this is not true. There have been differences between their moralities, but these have never amounted to anything like a total difference. – from Mere Christianity (1944)