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If your moral ideas can be truer, and those of the Nazis less true, there must be something—some Real Morality—for them to be true about. The reason why your idea of New York can be truer or less true than mine is that New York is a real place, existing quite apart from what either of us thinks. If when each of us said “New York” each meant merely “The town I am imagining in my own head,” how could one of us have truer ideas than the other? There would be no question of truth or falsehood at all. In the same way, if the Rule of Decent Behaviour meant simply “whatever each nation happens to approve,” there would be no sense in saying that any one nation had ever been more correct in its approval than any other; no sense in saying that the world could ever grow morally better or morally worse. I conclude then, that though the differences between people’s ideas of Decent Behaviour often make you suspect that there is no real natural Law of Behaviour at all, yet the things we are bound to think about these differences really prove just the opposite. But one word before I end. I have met people who exaggerate the differences, because they have not distinguished between differences of morality and differences of belief about facts. For example, one man said to me, “Three hundred years ago people in England were putting witches to death. Was that what you call the Rule of Human Nature or Right Conduct?” But surely the reason we do not execute witches is that we do not believe there are such things. If we did—if we really thought that there were people going about who had sold themselves to the devil and received supernatural powers from him in return and were using these powers to kill their neighbours or drive them mad or bring bad weather—surely we would all agree that if anyone deserved the death penalty, then these filthy quislings did? There is no difference of moral principle here: the difference is simply about matter of fact. It may be a great advance in knowledge not to believe in witches: there is no moral advance in not executing them when you do not think they are there. You would not call a man humane for ceasing to set mouse-traps if he did so because he believed there were no mice in the house. Chapter
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C.S. Lewis (Mere Christianity)