One of the world's greatest thinkers investigates the logic and psychology of common knowledge As a cognitive scientist, the ultimate subject of Steven Pinker's fascination is how we think about each other's thoughts, ad infinitum. It sounds impossible, but Steven Pinker shows that we do it all the time. This awareness, which we experience as something that is public or \"out there,\" is called common knowledge, and it has a momentous impact on our social, political, and economic lives. 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. In exploring the paradoxes of human behaviour, When Everyone Knows that Everyone Knows... invites us to understand the ways we try to get into each other's heads, and the harmonies, hypocrisies, and outrages that result.