"The Foundation of All Clear Thinking"

...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 wrong unless Right is a real thing which the Nazis at bottom knew as well as we did and ought to have practised. 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 colour of their hair...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...If we do not believe in decent behaviour, why should we be so anxious to make excuses for not having behaved decently...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...The laws of nature, as applied to stones or trees, may only mean "what Nature, in fact, does." But if you turn to the Law of Human Nature, the Law of Decent Behaviour, it is a different matter. That law certainly does not mean "what human beings, in fact, do"; for as I said before, many of them do not obey this law at all, and none of them obey it completely. The law of gravity tells you what stones do if you drop them; but the Law of Human Nature tells you what human beings ought to do...

CS Lewis
Mere Christianity pp. 5-17