From C.S. Lewis, Mere Christianity (1943), chapter three, “The Reality of the Law.”
Some people say that though decent conduct does not mean what pays each particular person at a particular moment, still, it means what pays the human race as a whole; and that consequently there is no mystery about it. Human beings, after all, have some sense; they see that you cannot have real safety or happiness except in a society where every one plays fair, and it is because they see this that they try to behave decently. Now, of course, it is perfectly true that safety and happiness can only come from individuals, classes, and nations being honest and fair and kind to each other. It is one of the most important truths in the world. But as an explanation of why we feel as we do about Right and Wrong it just misses the point If we ask: "Why ought I to be unselfish?" and you reply "Because it is good for society," we may then ask, "Why should I care what's good for society except when it happens to pay me personally?" and then you will have to say, "Because you ought to be unselfish"—which simply brings us back to where we started. You are saying what is true, but you are not getting any further. If a man asked what was the point of playing football, it would not be much good saying "in order to score goals," for trying to score goals is the game itself, not the reason for the game, and you would really only be saying that football was football—which is true, but not worth saying. In the same way, if a man asks what is the point of behaving decently, it is no good replying, "in order to benefit society," for trying to benefit society, in other words being unselfish (for "society" after all only means "other people"), is one of the things decent behaviour consists in; all you are really saying is that decent behaviour is decent behaviour. You would have said just as much if you had stopped at the statement, "Men ought to be unselfish."
And that is where I do stop. Men ought to be unselfish, ought to be fair. Not that men are unselfish, nor that they like being unselfish, but that they ought to be. The Moral Law, or Law of Human Nature, is not simply a fact about human behaviour in the same way as the Law of Gravitation is, or may be, simply a fact about how heavy objects behave. On the other hand, it is not a mere fancy, for we cannot get rid of the idea, and most of the things we say and think about men would be reduced to nonsense if we did. And it is not simply a statement about how we should like men to behave for our own convenience; for the behaviour we call bad or unfair is not exactly the same as the behaviour we find inconvenient, and may even be the opposite. Consequently, this Rule of Right and Wrong, or Law of Human Nature, or whatever you call it, must somehow or other be a real thing—a thing that is really there, not made up by ourselves.
And yet it is not a fact in the ordinary sense, in the same way as our actual behaviour is a fact. It begins to look as if we shall have to admit that there is more than one kind of reality; that, in this particular case, there is something above and beyond the ordinary facts of men's behaviour, and yet quite definitely real—a real law, which none of as made, but which we find pressing on us. . . .
