By C.S. Lewis
In this book, Lewis provides a version of the moral argument for the existence of God. His claim is that there is a universal moral standard, an unchanging sense of right and wrong, that runs throughout all cultures and all times.
He points out that, when people denounce others' behavior, they are usually not just saying that those others' behavior does not please them, instead, they are appealing to some type of moral standard which they expect everyone to know about. He further claims that those accused of violating this standard rarely reject its existence outright, but almost always try to give some excuse as to why it does not apply to them in this particular situation. Lewis calls this standard the Law of Human Nature, and says that unlike physical laws such as gravity, which cannot be broken, humans are free to disobey it. Finally, he says that all human beings are constantly breaking this law, and appeals to his readers to recognize and admit their own culpability in this.
The point to which this is leading should be obvious: Lewis claims that only a supernatural power could have instilled this moral sense in us. He claims that the moral sense could not be just another human instinct. Because it is never in complete agreement with any one instinct, but may encourage or discourage any of them based on the situation.
"Now this thing that judges between two instincts, that decides which should be encouraged, cannot itself be either of them. You might as well say that the sheet of music which tells you, at a given moment, to play one note on the piano and not another, is itself one of the notes on the keyboard. The Moral Law tells us the tune we have to play: our instincts are merely the keys" (p.8). He also adds that the moral sense is usually in the position of encouraging the weaker, not the stronger, instinct. Saying "The thing that tells you which note on the piano needs to be played louder cannot itself be that note" (p.9).
Lewis also dismisses claims that the moral sense could be simply a social convention, in two