The law of nature:

The law of nature is what 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 may find a dew people who are colour-blind or have no ear for a tune. 

Right and wrong are real things that even the Nazis at the bottom knew as well as we did and ought to have practiced. If they had no notion of what we mean by right, then, through we might still have had to fight them, we could no more have blamed them for that than for the colour of their hair. 

But some people may argue that the idea of law of nature to all men is unsound because different civilisations and different ages have had quite different moralities. But this is not true. While there have been differences between their moralities, but these have never amounted to anything like total difference. If we were to compare the moral teaching of different civilizations, we would realise how very alike they are to each other and to our own. 

Selfishness has never been admired. Men have differed as to whether you should have one wife or four. But they have always freed that you might not simply have any women you liked. 

The same person who says he does not believing a real Right and Wrong will be the same man who will complain that its unfair when someone breaks his promise to him. 

Excepts that have been edited from 'Mere Christianity' by CS Lewis