In his previous book, The Better Angels of our Nature, Pinker argued that the years since World War II have been an era of unprecedented peace and stability. The author of many scholarly studies and popular books, including The Language Instinct, Pinker was named one of Time’s “100 Most Influential People in the World” in 2004. He spoke to me by phone.

Steven Pinker: In my book The Better Angels of Our Nature, I had two chapters on the psychology and neuroscience of violence. The first, “Inner Demons,” was on the systems in the human brain that cause people to engage in violence, namely revenge, dominance, sadism and exploitation. The other, “Better Angels,” was on the systems in the human brain that allow us to refrain from violence, namely compassion, self-control, reason, and moral norms.

This passage, taken from Shakespeare’s Measure for Measure, served as the epigraph to “Inner Demons”:

But man, proud man,

Drest in a little brief authority,

Most ignorant of what he’s most assur'd;

His glassy essence, like an angry ape,

Plays such fantastic tricks before high heaven,

As make the angels weep.

Here, the character Isabella pleads for the life of her brother, Claudio, who has been condemned to be beheaded because he had impregnated his fiancée before they were married. Isabella appeals to the mercy and common sense of Angelo, a deputy who has been temporarily put in charge of Venice.

The play reminds us of the ubiquity of puritanical legal codes and barbaric punishments through much of the history of civilization. It’s a reminder that the horrors of the Islamic State are nothing new: our own cultural tradition has had enough of it that it could serve as the backdrop for this famous play. Of course, by the time Measure for Measure was being performed—1604—theatergoers must have perceived decapitation for the offense of fornication as barbaric; that revulsion is the source of the play’s tension, as we sympathize with Isabella’s appeal to Angelo. In this way, the play is propelled by the kind of moral progress I write about. We witness the triumph of nonviolence over violence: Angelo does not, in fact, carry out the harsh sentence.

The passage itself is a mordant reflection on the flaws of the human moral sense. The subtitle of Harold Bloom’s Shakespeare: The Invention of the Human is a bit of an exaggeration, but it captures the fact that the bard was one of our first and greatest psychologists. Isabella compares the administration of an idealized divine justice with the all-too-fallible human justice. She reminds us that humans are capable of meting out patently cruel and pointless punishment judgments with complete confidence they are doing the right thing.

In the lines that precede the excerpt, we hear how Jove reserves the worst punishments for the hardest-hearted wrongdoers:

Merciful Heaven,

Thou rather with thy sharp and sulphurous bolt

Split'st the unwedgeable and gnarled oak

Than the soft myrtle: but man, proud man…

Ideal punishments are proportional to the crime and the criminal. The gods would not use lightning to smite a “soft myrtle” but only the “gnarled oak,” the most obdurate kind of harmdoer. But “proud man” is more vengeful than Jupiter. I interpret the line “drest in a little brief authority” as referring not just to the fact that Angelo has only temporarily been put in charge by the absent Duke, but that all human claims to authority are, despite our pretensions to wise leadership, contingent and fleeting. The authority of rulers is insubstantial, our sense of justice fickle and haphazard.