WHEN EVERYONE KNOWS THAT EVERYONE KNOWS . . .

Pinker continues his explorations of the hidden social functions of language and thought with this look at common knowledge.

Common knowledge, as cognitive science Pinker lays it out here, lends itself to the game theory beloved of economists and strategists: Do I know something that you don’t? Can we agree, in order to produce best outcomes, that X means X and Y means Y? Pinker explains at the outset that “with private knowledge, person A knows something, and person B knows it. With common knowledge, A knows something, and B knows it, but in addition, A knows that B knows it, and B knows that A knows it”—a shared set of data, facts, and assumptions that enable not only cooperation but also coordination. That coordination, Pinker holds, is essential to the functioning of society: Our shared visions allow us to build economies, polities, communities, for which reason he adds that a better term for “common knowledge” would be “mutual knowledge,” which more easily accommodates agreeing on “nonphysical realities.” Pinker offers a vivid example when he notes that people don’t rise up against dictators easily because of the unspoken assumption that everyone else supports the regime, but things can get rolling when a public demonstration yields “the common knowledge needed to coordinate resistance.” Timely, that. Common knowledge has its negative aspects, he adds, as when people come to consensus about some point or another by way of shared media and then move together to punish transgressors. The swiftest way to be misunderstood, he notes, is to use irony or indirect speech, much as they help disguise our intentions. Pinker writes fluently, though there’s plenty of arcana from neuroscience, linguistics, and other fields floating around here. While it’s not necessary to have read Noam Chomsky, Antonio Damasio, Daniel Kahneman, and other cognitive scientists to follow Pinker’s arguments, it helps.

A revelatory, if sometimes challenging, look at the traps and rewards that lie within our words.