Religion is among the most powerful engines of common knowledge. A private belief is real but socially weak, because no one can see it; what ritual does, through spoken prayer, shared song, and the visible badges of membership, is turn "I believe" into "we can all see that we believe." We are sensitive to being the odd one out, and the worst position of all is uncertainty: not knowing whether others believe, or whether others have noticed our own hesitation. Ritual removes the doubt. It is like signing a contract under a spotlight, with the whole room watching. And, as I argued in discussing sacrifice and behavioural restrictions, the more costly and awkward the display, the harder it is to fake.
But the same logic explains the silences of social life as much as its ceremonies, because people do not always want a thing made common knowledge. A family may know perfectly well that one of its members is drinking himself to ruin, yet the talk glides around it for years, because once the words are said the knowledge becomes a fact everyone must act on. A congregation may be quietly full of doubt about a doctrine while each member, assuming the others are more convinced, says nothing, and the silence keeps up an appearance of unanimity that no one actually feels. Social psychologists call this pluralistic ignorance: a state in which most members of a group privately reject a norm while believing, wrongly, that most others accept it. In the classic studies, most university students privately disliked the heavy-drinking culture around them while assuming that nearly everyone else was comfortable with it, and so the culture rolled on, endorsed by almost no one. A community often survives less by answering its hardest questions than by managing which ones may be asked out loud.
The moment a doubt is voiced, it stops being a private flicker and becomes a public event that demands a response. This is the whole force of The Emperor's New Clothes: the child discovers nothing; he simply says aloud what everyone can already see, and so turns private knowledge into the far more dangerous public kind. Dictatorships understand this, which is why free assembly frightens them more than private grumbling: each citizen may know the regime is corrupt, and may even suspect that others know it too, but until people can gather and see one another, no one can be certain that everyone knows that everyone knows. The child's sentence, scaled up, is a revolution. The same tale reminds me of present-day partisan movements, in which many people surely harbour private misgivings about their own side's conduct or its leaders, yet will not say so, because to speak would be to risk exile from the only community they have. But all partisan movements value heroism—for me, one essential component of heroism is the willingness to speak your private knowledge of the truth to a group that might, at least at first, reject you for it.
These effects travel along the lines of our friendships. Behaviour and mood appear to spread through social networks—friend to friend to friend, out to about three degrees of separation—so that we are shaped by people we will never meet. A church congregation, seen this way, is a web of friendships with rituals attached. Few people are reasoned into faith; far more often a trusted friend draws them toward a group where belief is already on display. Doubt moves the same way in reverse: rarely through an argument, usually through watching someone you respect begin to question the story. This is why religious authorities have always feared open dissent far more than private unbelief: the private doubter is containable, the visible one is contagious.
It is also why the "New Atheist" writers, such as Richard Dawkins, who treated religion as a set of false claims to be knocked down one by one, so often failed to move believers: the thing they were attacking was not just an idea but a social world. To leave a faith is to risk becoming a stranger to your own people, and in the harshest communities to be cast out; the change of mind about the age of the earth is the least of it. The mind treats that as a real danger. It is also why some politicians perform piety their private lives do not support: the photograph with the holy book is aimed not at God but at the crowd. Common knowledge can make kindness visible, and it can do the same for contempt—a shared scorn for outsiders is far more dangerous than a private grudge, because cruelty comes easiest when made public, repeated together, and rewarded.
The secular answer to religion cannot stop at refuting supernatural claims. It must also build other, non-religious ways of making our shared virtues visible to one another. Religion binds beauty, morality, memory, and belonging into one shared practice; these are the nonspecific goods this book has traced from the beginning, and common knowledge is what turns them from private comforts into communal ones. This is where secular life falls short. The next chapter examines what happens when machinery this powerful is captured whole.
Christakis, N. A., & Fowler, J. H. (2009). Connected: The surprising power of our social networks and how they shape our lives. Little, Brown.
— A synthesis of the authors' research on social contagion, proposing the "three degrees of influence" rule: that behaviours, moods, and states—obesity, smoking, happiness, cooperation—spread through networks of friends to roughly three degrees of separation. How far this reflects genuine transmission rather than the clustering of similar people is still debated, but the broad claim that conduct is socially contagious is well supported.
Chwe, M. S.-Y. (2001). Rational ritual: Culture, coordination, and common knowledge. Princeton University Press.
— The foundational application of the game-theoretic idea of common knowledge to ceremony and ritual. Chwe argues that public rites—coronations, holidays, public demonstrations, communal worship, even Super Bowl advertising—work not merely by conveying a message to each onlooker but by letting each onlooker see that all the others receive it at the same moment.
Pinker, S. (2025). When everyone knows that everyone knows…: Common knowledge and the mysteries of money, power, and everyday life. Scribner.
— Pinker's recent, book-length account of common knowledge: the difference between privately knowing something and everyone publicly, mutually acknowledging it. He shows how it underlies coordination (driving conventions, paper currency, rallying behind a leader), how we generate it through signals such as laughter, tears, and blunt speech, and how we often labour to avoid it through hypocrisy, innuendo, and declining to name the elephant in the room.
Pinker, S., Nowak, M. A., & Lee, J. J. (2008). The logic of indirect speech. Proceedings of the National Academy of Sciences, 105(3), 833–838. https://doi.org/10.1073/pnas.0707192105
— The peer-reviewed research behind that book's account of indirect speech: a game-theoretic analysis of why people so often communicate through innuendo, euphemism, and the veiled request, arguing that indirection preserves "plausible deniability" and keeps mutually suspected facts from hardening into common knowledge.
Prentice, D. A., & Miller, D. T. (1993). Pluralistic ignorance and alcohol use on campus: Some consequences of misperceiving the social norm. Journal of Personality and Social Psychology, 64(2), 243–256. https://doi.org/10.1037/0022-3514.64.2.243
— The classic demonstration of pluralistic ignorance in everyday life. Across four studies, university students privately reported more discomfort with the campus drinking culture than they attributed to the average student—everyone believing themselves the exception—and over the course of a semester the men's private attitudes drifted toward the misperceived norm. The result is a community upholding a standard that most of its members privately reject: the situation of the silent congregation in this chapter.
A note on sources (handled as references rather than formal citations): the concept of common knowledge was given its modern philosophical form by David Lewis in Convention (1969) and analyzed in game theory by Thomas Schelling and Robert Aumann; The Emperor's New Clothes is Hans Christian Andersen's tale of 1837.
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