From C.S. Lewis, Mere Christianity (1943), chapter three, “The Reality of the Law.”
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.