Common Knowledge
Information that everyone knows, knows that everyone knows, and so on infinitely, enabling coordination without communication.
Also known as: Mutual knowledge, Shared knowledge, Common knowledge logic
Category: Decision Science
Tags: game-theory, coordination, knowledge, communications, decision-making
Explanation
In game theory and epistemic logic, common knowledge is a stronger condition than shared knowledge. A fact is common knowledge among a group when everyone knows it, everyone knows that everyone knows it, everyone knows that everyone knows that everyone knows it, and so on through every level of recursive awareness. This infinite regress of mutual awareness is what makes coordination possible without explicit communication. If two strangers must meet in New York City and choose Grand Central Station, the location works as a Schelling point only because each person believes the other knows it is salient, and believes the other believes they know it, indefinitely. Common knowledge underwrites currencies, traffic laws, language, and social conventions: a dollar bill has value because everyone treats it as valuable, everyone knows everyone treats it as valuable, and so on. Public ceremonies, announcements, and rituals function partly to manufacture common knowledge by ensuring that what is broadcast is unambiguously seen by all. The distinction matters because shared knowledge alone is fragile: people may individually know a fact but hesitate to act on it if they are uncertain whether others know it. Common knowledge unlocks collective action, which is why authoritarian regimes work hard to suppress public expressions of dissent even when private dissent is widely known.
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