"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 a moment later. He may break his promise to you, but if you try breaking one to him he will be complaining 'It's not fair' before you can say Jack Robinson. A nation may say treaties don't matter; but then, 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 no 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.
"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 behaviour 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 behaviour 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 it or not, we believe in the Law of Nature. If we do not believe in decent behaviour, 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."
--C.S. Lewis, Mere Christianity