Common Knowledge: Why Is It Not Enough That Everyone Knows?

Common knowledge is not merely a fact that many people happen to know. Each person must also know that the others know it, and know that the others recognise this mutual awareness. The recursive structure may sound cumbersome, but it changes whether people can confidently act on the fact together.

Suppose a manager separately emails five colleagues to say that a meeting has moved to three o’clock. Even if all five read the message, each may still wonder whether everyone else received it and hesitate to change a shared arrangement. If the change is announced in a meeting where everyone can see that the others heard it, the information is much closer to common knowledge. People no longer need to guess privately who knows what, so coordination becomes easier.

Common knowledge differs from shared information. Shared information means that the same content has reached several people; common knowledge also includes mutual recognition that it is shared. Nor is it simply common sense. Something may be widely known without members of a particular group knowing that the others know it. Public announcements, shared witnessing and visible rules often matter because they turn scattered private judgements into a basis for joint action.

The concept connects epistemology, game theory, organisational communication and multi-agent cooperation. The central question is not only whether information arrived, but whether each participant can reasonably expect the others to act on it too.

https://plato.stanford.edu/entries/common-knowledge/