A catalogue of things I've written (or collected from elsewhere) about common knowledge.
In Rational Ritual, Michael Chwe illustrates how the phenomenon of common knowledge underlies a lot of our ritual activities.
For a fact to be common knowledge, it's not enough simply for everyone to know the fact. Everyone must also know that everyone else knows it, and know that they know that they know it, and so on.
Consider life under an oppressive regime — in North Korea, say. It's possible that every North Korean citizen knows, from firsthand experience, that the ruling regime is oppressive and terrible — but such knowledge would not be common knowledge. It would be (to coin a phrase) closeted knowledge. It's something that everyone knows privately, but no one is certain that everyone else knows as well. And crucially for the North Koreans, when knowledge is closeted it prevents everyone from acting together, e.g., in a rebellion.
How does knowledge come to be so closeted? In part, it's because leaders use massively public rituals to create the impression that everyone is happily obedient. When hundreds of thousands of citizens show up to celebrate Dear Leader, it's hard not to suppose that a good fraction of that sentiment is genuine. It's an elaborate, modern-day version of the "kiss the ring" ritual.
The Pledge of Allegiance, by the way, as well as singing the national anthem at a ballgame — these are similar tactics, and they have a similar effect. The two primary differences are that (1) loyalty in the U.S. is directed primarily to the nation rather than the President, and (2) the loyalty among Americans is more genuine than that of the North Koreans (or so we suspect).
But regimes that live by the sword, die by the sword, and rituals of mass public participation can just as soon topple as prop up a precarious regime. The reason public protests and demonstrations are so potent isn't just because the crowd might turn angry and storm the gates. It's because when you get all those people together — people who are risking injury or jail just by showing up — it creates instant common knowledge of how unpopular the regime has become. Everyone who shows up at the protest knows that everyone else knows that... it's time for change.
A fig leaf of popularity, backed by an implied threat of violence, can cow every citizen individually, keeping their true feelings closeted in their thoughts, homes, and small social circles. But a fig leaf is no match for a large chanting mob.
From Ads Don't Work That Way:
For a fact to be common knowledge among a group, it's not enough for everyone to know it. Everyone must also know that everyone else knows it — and know that they know that they know it... and so on.
So for an ad to work by cultural imprinting [i.e., common knowledge], it's not enough for it to be seen by a single person, or even by many people individually. It has to be broadcast publicly, in front of a large audience. I have to see the ad, but I also have to know (or suspect) that most of my friends have seen the ad too. Thus we will expect to find imprinting ads on billboards, bus stops, subways, stadiums, and any other public location, and also in popular magazines and TV shows — in other words, in broadcast media. But we would not expect to find cultural-imprinting ads on flyers, door tags, or direct mail. Similarly, internet search ads and banner ads are inimical to cultural imprinting because the internet is so fragmented. Everyone lives in his or her own little online bubble. When I see a Google search ad, I have no idea whether the rest of my peers have seen that ad or not.
Personhood breaks down in severe drug addicts. Because the addict lacks self-control, he can't be reasoned with or trusted to keep promises. At first the addict is simply marginalized, pushed away from polite society. But if his condition deteriorates, and if he has friends and loved ones who care enough (notably, those bound to him by relationships other than personhood), they may stage an intervention. This is tantamount to stripping the addict of his personhood (rendering him persona non grata), and it's telling that an intervention is a communal affair involving mutual recognition (common knowledge) of the consensus. This will be embarrassing and uncomfortable for everyone — both the addict and his loved ones — because of how much we rely on the contract of personhood in our normal, everyday interactions.