Three Logicians Walk into a Bar

Friday, October 7th, 2011

Three logicians walk into a bar:

Waitress: Does everyone want beer?

Logician 1: I don’t know.

Logician 2: I don’t know.

Logician 3: Yes!

If you don’t get it, keep puzzling for a minute.

Give up? Here’s your hint: If you’re not a logician, “Does everyone want beer?” means something like, “Each of you, confirm or deny that you want a beer.” If you are a logician, it means, “Is it true that every single one of you wants a beer?” — which any one person can deny.

(Hat tip to Ian Ayres.)


  1. Doctor Pat says:

    I understood the original joke, it took me about 10 seconds.

    But I must admit I did not understand your explanation.

  2. Alrenous says:

    Blonde: If I didn’t want a beer, I could conclude that not-everyone wants a beer. I don’t know what Raven and Brunette want, however.

    Raven: I likewise, cannot conclude that not-everyone wants a beer.

    Brunette: Nobody else could safely conclude that at least one person wants no beer. I want a beer. That means nobody can confirm that a person wants no beer. That means nobody wants no beer. That means everyone wants a beer.

    How much of this is new to you? If none of it is, how did you not get the explanation? If some of it is, what joke did you get?

    If only I could do this in real life. Sadly this would require people to be straightforwardly logical, and that would require sociality not to reward deception.

    Counterpoint: if you understand the logic of deception, you can do this kind of thing in real life. The results are depressing, though, precisely because of what they’re effective at.

  3. Doctor Pat says:

    Your much more detailed explanation was exactly what I had worked out. I guess I just didn’t read your hint closely enough. I didn’t interpret “deny” adequately.

    Explanation for my lack of understanding: it was past my bedtime.

  4. Alrenous says:

    A fair excuse, one of my own favourites. I basically spend all day trying to teach myself to logic better, and I still make that exact interpretation mistake.

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