Siddhant Tiwary
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Pl. XI Black-headed uakari. Climbs with its whole body.
Pl. VII Red-handed tamarin. Black all over except the hands.
Pl. X L’Hoest’s monkey. A white beard and a chestnut back.
Pl. XXV Bearded saki. A black beard and one arm raised.
Pl. XXI Tufted capuchin. A dark cap and a ringed tail.
Pl. XIV Monk saki. A shaggy cloak with a face peering out.

Idea · 19 Oct 2026 · 1 min read

Everyone knows that everyone knows

The gap between shared knowledge and common knowledge.

There’s a difference between everyone knowing something and everyone knowing that everyone knows it. The first is shared knowledge. The second, stacked all the way up, is common knowledge, and it changes how people act. Picture a crowd where every person has privately noticed the emperor has no clothes. Nothing happens. Then a child says it out loud. Nobody learned anything new about the emperor, but now each person knows that everyone else knows, and the whole crowd can laugh at once. David Lewis used this idea to explain conventions, like which side of the road we drive on. Robert Aumann showed that two rational people who start from the same prior beliefs, and whose opinions are common knowledge, can’t “agree to disagree”. I keep seeing it in markets and in group chats. A lot of coordination isn’t about information at all. It’s about the moment information becomes public.

References: David Lewis, Convention (1969); Robert Aumann, “Agreeing to Disagree” (1976)

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