When we encounter the phrase “Law of Nature,” we tend to assume that it refers to the workings of the natural world (e.g. the law of gravity). Lewis, however, uses it to refer to a set of universal moral standards. As Lewis points out, there is a significant difference between the two usages: where it refers to forces and matter in the natural world (such as trees and stones) it’s simply an observation about how things work. Human beings, however, are sentient and have the capacity to either obey or disobey moral laws.

Some might argue that this law isn’t really a law at all, but instead depends on cultural context—that is, that morality is relative rather than absolute. In response, Lewis acknowledges that there are minor differences between societies, but he believes that these belie deeper similarities. He observes, for instance, that people generally do not feel proud of running away in battle or betraying those who have been kindest to them. Furthermore, Lewis argues, those who claim not to believe in the reality of moral law don’t really act in a way that’s consistent with their beliefs; if someone cheats them, for instance, they’re likely to appeal to a shared set of moral standards.