How Common Knowledge Shapes the World, with Steven Pinker

Common knowledge is not merely when everyone knows something, but when 'everyone knows that everyone knows... ad infinitum.' This infinite recursion is a technical definition from game theory and philosophy, crucial for understanding how societies coordinate. It differentiates public knowledge from private knowledge, where individuals might know something but not realize others also know it.

STEVEN: So common knowledge in the technical sense refers to the situation where I know something, you know it, I know that you know it, you know that I know it, I know that you know that I know it, ad infinitum. So it's when everyone knows that everyone knows and the dot, dot, dot is essential.

The concept of common knowledge is fundamental to the existence of civilization, institutions like government, the value of money, and even language itself. Money holds value because everyone knows that everyone else treats it as valuable. Governments wield power because citizens commonly acknowledge their authority. Language works because there's common knowledge about what words signify.

STEVEN: The only reason that a piece of paper with Abraham Lincoln on it is valuable is because other people treat it as valuable. Now, why do they treat it as valuable? Well, because they know that other people will treat it as valuable. That's what makes it a currency. Likewise, for power, there's no way a government can intimidate every last member of its citizenry... because everyone treats them as if they are. It's a social reality.

Conspicuous non-verbal displays like laughing, crying, blushing, and eye contact function as 'common knowledge generators.' They make an internal state publicly visible, ensuring that not only do you know you're experiencing it, but others know, and you know they know, and so on. This shared awareness facilitates social bonding and communication.

STEVEN: Why do you shed tears? I suggest that they are common knowledge generators. So, when you're laughing, you know you're laughing because your breathing is interrupted. Other people know you're laughing because they can hear it. Other people know that you know, you know that other people know, etc.

In social interactions, people often use euphemisms, innuendo, or 'weasel words' to hint at intentions without making them common knowledge. This 'acceptable duplicity' allows parties to maintain a desired relationship (e.g., platonic, professional) by providing plausible deniability, preventing an explicit shift to a different, potentially undesirable, dynamic (e.g., transactional, sexual).

STEVEN: I have a chapter called Weasel Words... Let's say you were trying to bribe a maitre d to jump the queue... You might, holding out a $50 bill in peripheral vision, say, is there anything you can do to shorten my wait? I was wondering if you might have a cancellation. Right. And not, if I give you $50, will you seat me right away?

The story of the Emperor's New Clothes is a prime example of common knowledge. Everyone privately knew the emperor was naked, but it wasn't common knowledge until the child publicly stated it. This act converted private understanding into shared, public awareness, instantly changing the relationship between the people and the emperor from deference to ridicule.

STEVEN: So I actually opened the book with the emperor's new clothes because that's a story about common knowledge. When the little boy said the emperor was naked, he wasn't telling anyone anything they didn't already know, but he was changing their knowledge because when he blurted it out in public, now everyone knew that everyone else knew that everyone else knew.

Societal polarization is exacerbated by the breakdown of shared common knowledge. When people inhabit segregated information networks (e.g., through cable news, social media, residential segregation), they develop disjoint pools of common knowledge. This makes it harder to find common ground, fostering divisiveness and allowing 'vituperative' leadership to demonize 'the other side.'

STEVEN: There has been a segregation into two separate pools of common knowledge. Everyone blames social media... But I think it's also cable news, Fox News, prior to social media had that effect... you stop rubbing shoulders with people from different social classes. Each one then hung out with people like them, and then the common knowledge that they shared started to grow disjoint.