But human beings are not like this. Human beings live in two worlds, the physical domain and the moral domain. If a person insults your mother, you respond, “You shouldn’t have done that.” When a friend tells you he deceived his business partners or family, you tell him he shouldn’t have lied. These normative statements are fundamentally different from physical laws. It makes no sense to say that the earth ought to revolve around the sun or that it would be unfair if it didn’t. A law of nature may be true or false, but it cannot be broken. As Carl Sagan puts it, “Nature … arranges things so that its prohibitions are impossible to transgress.”

There are parts of our human nature that operate according to these descriptive natural laws. If you tickle me, I will laugh. If either of us eats contaminated food, it will upset our stomachs. If we are dropped from a tall tower, we will plummet to the ground. These are the laws of physics and chemistry working on us, and we have no choice in the matter. On the other hand, there is a part of our human nature that is not descriptive but prescriptive. The simple proof of this is that moral norms and precepts, unlike natural laws, can be violated.

Honor thy father and mother. Thou shalt not murder. Thou shalt not covet thy neighbor’s goods. None of these commandments would make any sense if we had no option. But there is more. When we humans invoke the language of morals—praising and blaming, approving and disapproving, applauding and scorning—we appeal to a shared standard of judgment external to ourselves. Let us call this standard the natural law or the moral law. It differs from the scientific laws of nature in that it tells us not what we do but what we ought to do. Consequently we are free to break these laws in a way that we are not free to violate the laws of gravity.