Common knowledge, Pinker shows, can make sense of many of life’s enigmas: financial bubbles and crashes, revolutions that come out of nowhere, the posturing and pretence of diplomacy, the eruption of social media shaming mobs and academic cancel culture, the awkwardness of a first date. But people also go to great lengths to avoid common knowledge—to ensure that even if everyone knows something, they can’t know that everyone else knows they know it. And so we get rituals like benign hypocrisy, veiled bribes and threats, sexual innuendo, and pretending not to see the elephant in the room.