IDEA FOUND // IDEA 200
Common Knowledge and Coordination
“Everyone knowing the time may not be enough; they may need to know that others know it too.”
01 / PLAINLY
What it means, plainly
A fact is common knowledge when everyone knows it, knows that everyone else knows it, knows that everyone knows that, and so on.
02 / CONTEXT
A little more
A fact is common knowledge when it is known, everyone knows that others know it, and so on. Public signals can support coordination by aligning expectations about expectations.
03 / WHY IT MATTERS
Why it matters
A public signal may coordinate better than identical private messages because it aligns expectations about what others will do.
04 / EXAMPLE
A familiar example
A room-wide evacuation announcement coordinates better than identical private messages whose receipt nobody can confirm.
05 / LIMIT
What it does not mean
Common knowledge is not popular information, consensus, or an infinite calculation people consciously perform.
06 / NOTICE
Notice it in your day
Compare a room-wide announcement with identical private messages. Note what each person knows about everyone else's receipt.
FINAL NOTE
The idea worth keeping
When action depends on others, communicate that the message arrived publicly and jointly.
QUESTIONS / 02
Questions people still have
What does common knowledge add to shared knowledge?
Each person knows that others know the fact, with further expectations of the same kind.
Does it mean popular information or consensus?
No. It describes a knowledge structure that can exist without agreement of opinion.