From Deep Roots Newsletter
Whenever you find a person who says they do not believe in a real Right or Wrong, you will find the same person 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 tastes and opinion any more than the multiplication table.
[Mere Christianity by CS Lewis]