The very idea of something being imperfect, of its not being what it ought to be, has certain consequences.
If you take a thing like a stone or a tree, it is what it is and there seems no sense in saying it ought to have been otherwise. Of course you may say a stone is ‘the wrong shape’ if you want to use it for a rockery, or that a tree is a bad tree because it does not give you as much shade as you expected. But all you mean is that the stone or the tree does not happen to be convenient for some purpose of your own. You are not, except as a joke, blaming them for that. You really know, that, given the weather and the soil, the tree could not have been any different. What we, from our point of view, call a ‘bad’ tree is obeying the laws of its nature just as much as a ‘good’ one.
Now have you noticed what follows? It follows that what we usually call the laws of nature—the way weather works on a tree for example—may not really be laws in the strict sense, but only in a manner of speaking. When you say that falling stones always obey the law of gravitation, is not this much the same as saying that the law only means ‘what stones always do’? You do not really think that when a stone is let go, it suddenly remembers that it is under orders to fall to the ground. You only mean that, in fact, it does fall. In other words, you cannot be sure that there is anything over and above the facts themselves, any law about what ought to happen, as distinct from what does happen. The laws of nature, as applied to stones or trees, may only mean ‘what Nature, in fact, does’. But if you turn to the Law of Human Nature, the Law of Decent Behaviour it is a different matter, That law certainly does not mean ‘what human beings, in fact, do’; for as I said before, many of them do not obey this law at all, and none of them obey it completely. The law of gravity tells you what stones do if you drop them; but the Law of Human Nature tells you what human beings ought to do and do not. In other words, when you are dealing with humans, something else comes in above and beyond the actual facts. You have the facts (how men do behave) and you also have something else (how they ought to behave). In the rest of the universe there need not be anything but the facts. Electrons and molecules behave in a certain way, and certain results follow, and that may be the whole story*. But men behave in a certain way and that is not the whole story, for all the time you know that they ought to behave differently.
*I do not think it is the whole story, as you will see later. I mean that, as far as the argument has gone up to date, it may be.
C.S. Lewis, Mere Christianity (1952, this edition: 2001) 16-18.