Why is your head not exploding? Steven Pinker can explain. - Harvard Gazette

"According to Steven Pinker, Johnstone Family Professor of Psychology, humans are constantly occupied with whether information has achieved this potent status. It's just that the deliberations usually happen below the level of awareness. In "When Everyone Knows That Everyone Knows ...," Pinker explains why common knowledge is nonetheless central to everything from planning a coffee date to driving on the correct side of the road."

""I also suggest it's the basis of our social relationships," he said. "Two people are friends not because they signed a contract but because each one knows that the other one knows that the first knows that the second knows that they're friends. Or lovers. Or neighbors. Or transactional partners." In an interview with the Gazette, edited for length and clarity, the veteran author and professor identified the undercurrents of common knowledge in everyday language, laughter, and social media use."

"There's a "Friends" episode in which Phoebe says: "They don't know that we know they know we know. Joey, you can't say anything." And the dull-witted Joey replies: "I couldn't even if I wanted to." It captures the humor in the very idea that we struggle to keep track of each other's thoughts about our thoughts. So if common knowledge is so essential to human affairs, how do we do it without our heads exploding?"

Common knowledge is the mutual awareness that everyone knows a fact and that everyone knows that everyone knows it, iterated indefinitely. That iterated awareness enables coordination in everyday activities such as planning meetings and following traffic conventions. Social relationships rely on shared mutual expectations rather than formal contracts. Cultural humor and conversational moves exploit layered mutual knowledge to produce and detect irony or secrecy. People use mental shortcuts and conceptual metaphors to represent nested knowledge without conscious recursion. Common knowledge also influences patterns of laughter, language use, and behaviors on social media.

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