Picture a packed commuter train, stalled between stations for the third time this month. Every single passenger is fed up. Every single passenger knows the service is a disgrace. And every single passenger sits there in silence, staring at their phone, doing absolutely nothing. The grievance is total — and the platform stays quiet.
Now imagine one person stands up and says, loudly, “This is unacceptable — who’s with me?” — and heads turn, and murmurs spread, and suddenly a carriage that was a hundred lonely complaints is one annoyed crowd. Nothing about the trains changed. What changed is that a private truth everyone was carrying alone became a public one: not just everyone knows, but everyone knows that everyone knows.
That second, recursive layer of knowing is the whole subject of this course. It has a precise name — common knowledge — and once you can see it, you’ll spot the same hidden switch behind bank runs, revolutions, viral trends, Super Bowl ads, and why dictators lose sleep over public squares.
What you’ll actually walk away with
By the end you’ll be able to tell three kinds of knowing apart — what I know, what we each know, and what everyone knows that everyone knows — and use the difference to explain why crowds freeze, why they suddenly move, and why the signal that makes knowledge public often matters more than the underlying truth. Here’s the map:
- The ladder of knowledge — private, mutual, and common knowledge, and the “everyone knows that everyone knows…” recursion made concrete.
- The emperor’s new clothes — why a single public signal coordinates a crowd that a thousand private signals never could, plus focal points. This lesson has an interactive crowd you can freeze and unfreeze yourself.
- Coordination games — how beliefs about other people’s beliefs, not payoffs alone, pick which outcome a group lands on (with the Stag Hunt).
- Bank runs, revolutions & crowds — the model’s real faces: self-fulfilling panics, hidden dissent that erupts overnight, advertising, and the beauty contest.
- Where common knowledge breaks — the idealisation’s cracks: almost-common knowledge, the two-generals problem, and manufactured false common knowledge.
The one-sentence version
Coordinated action doesn’t hinge on what everyone privately knows — it hinges on what everyone knows that everyone knows. Make a shared truth public and a frozen crowd can move as one.
Before you read — take a guess
Before we start — take a guess. What does 'common knowledge' most precisely mean here?
A quick taste of the idea
Back to the silent train. Why does the grievance sit there doing nothing? Because each passenger is thinking the same nervous thought: I’m furious — but am I the only one? If I stand up and no one joins me, I just look like the unhinged person shouting on the 8:15. Each person privately knows the trains are awful, but nobody is sure the others feel the same, or would act. The knowledge is real but trapped — everybody has it, nobody knows that everybody has it.
The person who stands up doesn’t add a single new fact. Everyone already knew the trains were late. What the shout does is make that shared feeling common knowledge: now you know the others are angry, and you know they can see that you’re angry, and you know they know you can see it. The fear of being alone evaporates, because being first is no longer a risk anyone is taking by themselves.
Where we're headed
That trapped-then-released feeling is the entire course in miniature. “Everyone’s angry but nobody’s sure the others are” is mutual knowledge — and it’s why the crowd freezes. “Now everyone knows that everyone’s angry” is common knowledge — and it’s why the crowd moves. The rest of the course is just this idea, made precise and then let loose on banks, regimes, and markets.
On the silent train, why does nobody act even though everybody is furious?
How to use this course
Every lesson opens with a quick guess, teaches the idea through a concrete story, and checks that it stuck. Don’t skip the guesses — trying to answer before you know is one of the most reliable ways to actually remember. There’s no heavy maths here: one lesson has payoff tables with numbers you can add up by hand, and the emperor lesson hands you an interactive crowd whose behaviour you control with a slider and a switch.
This is an expert-tier course, so it assumes you’ve met a few earlier models — you’ll get more from it if you’re comfortable with game-theory basics, Nash equilibrium, and critical mass. When you’ve finished the five teaching lessons, a graded final exam pulls it all together. It’s one-way — once you submit an answer it’s locked — so treat the practice questions along the way as exactly that: practice.
One habit to build as you go
Whenever you see a crowd do something dramatic — a run, a protest, a stampede into a stock — train yourself to ask not “what did they learn?” but “what did they suddenly all know that everyone else knew?” That second question is the one this course teaches you to answer.
Ready? The next lesson builds the ladder carefully — private, mutual, common — and shows exactly why that top rung is the one that moves the world.