Common Knowledge vs Mutual Knowledge

Mutual knowledge concerns everyone knowing a fact; common knowledge includes all higher levels of knowing that others know it.

Mutual knowledge means everyone in a specified group knows a fact. Common knowledge adds every higher level: everyone knows that everyone knows it, everyone knows that everyone knows that everyone knows it, and so on. These levels matter when a person’s decision depends on what others are likely to know and expect.

A genuinely public event can create common knowledge in a model with suitable assumptions about shared observation. In practice, sending a message does not ensure receipt, comprehension, or awareness of everyone else’s receipt. People can also share a belief that is false; shared belief and knowledge are different. Public acknowledgement can aid coordination, but it does not remove conflicting interests, resource constraints, or fear of acting.

When to use it

Useful when participants have information individually but remain unsure how others will act on it.

How it can help

Identify whether coordination is blocked by uncertainty about other people’s information or expectations.