from CS Lewis: "We may put this in another way. Each man is at every moment subjected to several 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 anymore 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 colour-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 everyone. 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 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 then for the colour 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. If anyone will take the trouble to compare the moral teaching of, say, the ancient Egyptians, Babylonians, Hindus, Chinese, Greeks and Romans, what will really strike him will be how very like they are to each other and to our own. Some of the evidence for this I have put together in the appendix of another book called The Abolition of Man; but for our present purpose I need only to ask the reader to think what a totally different morality would mean. Think of a country where people were admired for running away in battle, or where a man felt proud of double-crossing all the people who had been kindest to him. You might just as well try to imagine a country where two and two make five. Men have differed as regards what people you ought to be unselfish to--whether it was only your own family, or your fellow countrymen, or everyone. But they have always agreed that you ought not to put yourself first. Selfishness has never been admired. Men have differed as to whether you should have one wife or four. But they have always agreed that you must not simply have any woman you liked. But the most remarkable thing is this. Whenever you find a man who says he does not believe in a real Right and Wrong, you will find the same man going back on this in a moment later. He may break his promise to you, but if you try breaking on to him he will be complaining "It's not fair" before you can say Jack Robinson. A nation may say treaties do not matter; but the next minute, they spoil their case by saying that the particular treaty they want to break was an unfair one. But if treaties do not matter, and if there is not such thing as Right and Wrong--in other words, if there is no Law of Nature--what is the difference between a fair treaty and an unfair one? Have they not let the cat out of the bag and shown that, whatever they say, they really know the Law of Nature just like anyone else? It seems, then, we are forced to believe in a real Right and Wrong. People may be sometimes mistaken about them, just as people sometimes get their sums wrong; but they are not a matter of mere taste and opinion any more than the multiplication table. Now if we are agreed about that, I go on to my next point, which is this. None of us is really keeping the Law of Nature. If there are any exceptions among you, I apologize to them. They had much better read some other work, for nothing I am going to say concerns them. And now, turning to the ordinary human beings who are left: I hope you will not misunderstand what I am going to say. I am not preaching, and Heaven knows I do not pretend to be better than anyone else. I am only trying to call attention to a fact; the fact that this year, or this month, or, more likely, this very day, we have failed to practice ourselves the kind of behavior we expect from other people. There may be all sorts of excuses for us. That time you were so unfair to the children was when you were very tired. That slightly shady business about the money--the one you have almost forgotten--came when you were very hard up. And what you promised to do for old So-and-so and have never done--well, you never would have promised if you had known how frightfully busy you were going to be. And as for your behavior to your wife (or husband) or sister (or brother) if I knew how irritating they could be, I would not wonder at it--and who the dickens am I, anyway? I am just the same. that is to say, I do not succeed in keeping the Law of Nature very well, and the moment anyone tells me I am not keeping it, there starts up in my mind a string of excuses as long as your arm. The question at the moment is not whether they are good excuses. The point is that they are one more proof of how deeply, whether we like if or not, we believe in the Law of Nature. If we do not believe in decent behavior, why should we be so anxious to make excuses for not having behaved decently? The truth is, we believe in decency so much--we feel the Rule of Law pressing on us so--that we cannot bear to face the fact that we are breaking it, and consequently we try to shift the responsibility. For you notice that it is only for our bad behaviour that we find all these explanations. It is only our bad temper that we put down to being tired or worried or hungry; we put our good temper down to ourselves. 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. (Mere Christianity)" Sorry for the length. Lewis goes on to argue that this "Law of Nature" is our conscience. We have an innate knowledge or sense of right and wrong, which he argues comes from God. But we often choose to do wrong, or "sin" in Christian terms.
