Throughout history, different cultures have had distinct customs and rules regarding specific matters, such as marriage arrangements, food, and property. But no culture has ever held up selfishness as a virtue, or taught that you should simply take whatever you want. Beneath cultural differences lies a striking moral agreement.

And if you’ve ever been in an argument where you found yourself saying something like “that’s not fair,” you need to know that it’s only you who appeals to fairness in such situations. There exists a universal Moral Law. It’s a standard of right and wrong that every human being, in every culture and every era, already knows and appeals to, even when they’re busy breaking it.

The Moral Law’s not similar to the laws of physics, since gravity, for example, doesn’t ask a stone for its opinion; the stone simply falls. The Moral Law tells us what we ought to do, not what we naturally do. We are free to obey it or ignore it. Yet when someone wrongs us, we instinctively appeal to it. This is the contradiction of human nature: we know the standard, but we continually fail to live by it.

There is a difference between the Moral Law and mere social convention or evolved instinct. Think of your hunger and protective instincts as the keys on a piano keyboard. They’re all there, ready to be played. But the Moral Law is the sheet music, which tells you which key to press at any given moment.

Thus, if you hear someone crying for help, you feel two things pulling at once: the instinct to help, and the instinct to protect yourself. The Moral Law quietly insists that you ought to help, even though that’s the weaker impulse.