The Law of Nature as Lewis describes it is very different from a law of nature, understood in the ordinary sense. When we speak of a law of nature such as gravity, we usually say that the law describes the world as it must be. When we drop a stone, the stone doesn’t “remember” gravity or “choose” to obey it; the law of gravity describes what all stones do, all the time. Nobody could ever criticize a stone for falling the “wrong” way, except as a joke. With the Law of Nature as Lewis defines it, however, humans have a choice. They can choose to obey the Law, or not. Put differently, there is a “gap” between the way people should behave and way they do behave.
The laws of morality are similar to laws of science, but with the one crucial difference that humans are capable of breaking laws of morality. The challenge of being a human being, at least from a Christian perspective, is that humans are aware of the moral law, but can never obey it perfectly—they will always sin or misbehave in some way.
It might be objected that the Law of Nature is just a reflection of what is and isn’t harmful; in other words, society invented the idea of “evil” as a way of prohibiting actions that caused harm to other people. To prove this theory wrong, Lewis imagines a person who tries to trip him and fails. Naturally, Lewis would be irritated with this person, even though no harm came to him. In other words, the Law of Nature does not merely reflect what is and isn’t harmful to us—there’s more to it than that.
Lewis continues to rebut objections to the idea that morality is universal. One could argue that morality is just a system for avoiding pain and harm. However, it’s clear that morality encompasses far more than just pain—when we pass moral judgment on people, we take things like intent into account, suggesting that morality is rooted in more complex criteria than the mere avoidance of harm.
Another, similar objection: the so-called laws of Right and Wrong are just reflections of what is and isn’t harmful to society as a whole. So even if, on specific occasions, Lewis might get angry with someone who tried to trip him and failed, it could be argued that society as a whole benefits from the presence of a rule that discourages tripping people—overall, fewer people will fall on their faces. In response, Lewis argues that the “good for society” explanation of morality is circular. To say that the point of morality is that it’s good for society is like saying that the point of football is to score goals—such a statement is technically true, but tautological; it doesn’t really tell us anything new. Similarly, it could be argued that people “should” be generous because it’s good for society. But why, then, should people care about what’s good for society? The Law of Human Nature isn’t a law in the same sense as gravity, but it’s not a human invention, either—it must be “a real thing.”
Some thinkers have argued that Lewis’s rebuttal here isn’t strictly logical; perhaps the truth is that, in a Darwinian sense, societies that practiced some form of collective morality survived over time, while their “immoral” counterparts collapsed and died out (from not taking care of each other, probably). In this way, morality could be considered a kind of random “mutation,” which leads certain societies to be more successful in the long run.