The Fear of Common Knowledge

by Eliezer Yudkowsky1 min read9th Jul 200837 comments


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Followup toBelief in Belief

One of those insights that made me sit upright and say "Aha!"  From The Uncredible Hallq:

Minor acts of dishonesty are integral to human life, ranging from how we deal with casual acquaintances to writing formal agreements between nation states.  Steven Pinker has an excellent chapter on this in The Stuff of Thought, a version of which can be found at TIME magazine’s website. What didn’t make it into the TIME version is Pinker’s proposal that, while there are several reasons we do this, the most important reason is to avoid mutual knowledge:  "She probably knows I just blew a pass at her, but does she know I know she knows? Does she know I know she knows I know she knows?"  Etc.  Mutual knowledge is that nightmare where, for all intents and purposes, the known-knows can be extended out to infinity.  The ultimate example of this has to be the joke "No, it wasn’t awkward until you said, 'well, this is awkward.'"  A situation might be a little awkward, but what’s really awkward is mutual knowledge, created when someone blurts out what’s going on for all to hear...

The story of the Emperor’s New Clothes is another example of the power of mutual knowledge...

The power of real deception - outright lies - is easy for even us nerds to understand.

The notion of a lie that the other person knows is a lie, seems very odd at first.  Up until I read the Hallq's explanation of Pinker, I had thought in terms of people suppressing uncomfortable thoughts:  "If it isn't said out loud, I don't have to deal with it."

Like the friends of a terminal patient, whose disease has progressed to a stage that - if you look it up online - turns out to be nearly universally fatal.  So the friends gather around, and wish the patient best hopes for their medical treatment.  No one says, "Well, we all know you're going to die; and now it's too late for you to get life insurance and sign up for cryonics.  I hope it isn't too painful; let me know if you want me to smuggle you a heroin overdose."

So even that is possible for a nerd to understand - in terms of, as Vassar puts it, thinking of non-nerds as defective nerds...

But the notion of a lie that the other person knows is a lie, but they aren't sure that you know they know it's a lie, and so the social situation occupies a different state from common knowledge...

I think that's the closest I've ever seen life get to imitating a Raymond Smullyan logic puzzle.

Added:  Richard quotes Nagel on a further purpose of mutual hypocrisy: preventing an issue from rising to the level where it must be publicly acknowledged and dealt with, because common ground on that issue is not easily available.

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