C.S. Lewis, in his essay "The Law of Human Nature," said,
"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 do not 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?
(Source: C.S. Lewis, Mere Christianity, 1943 Macmillan Pub Co, NY. From Clark Tanner's Sermon "God Calls to Account")