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Three logicians walk into a bar. The barman says, 'Does everybody want a drink?'

The first logician says, 'I don't know.'

The second logician says, 'I don't know.'

What does the third logician say?


Please provide a clear explanation of why each of the logicians reply in the way they do.

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    $\begingroup$ I think this is a very old joke/puzzle. It appears various places in a variety of forms. $\endgroup$ – Bob Apr 19 '15 at 12:31
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    $\begingroup$ As a former logician, I would like to point out that, in reality, the first logician would say "no." There is at least one person in the world who does not want a drink. $\endgroup$ – David Richerby Apr 19 '15 at 20:26
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    $\begingroup$ spikedmath.com/445.html $\endgroup$ – Abr001am Apr 19 '15 at 22:29
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    $\begingroup$ @DavidRicherby As another former logician, I would like to point out that, in reality, although the odds would definitely be in the favor of at least one person in the world not wanting a drink, it would still be an assumption unless he indeed knows at least one person in the world who does not want a drink... $\endgroup$ – Warlord 099 Apr 20 '15 at 14:16
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    $\begingroup$ @reirab I would love to know one person who has never ever in their life drank some water... There is nothing to imply that the drink has to be alcoholic (which I am assuming is your premise). I don't drink alcohol and I have been to bars with friends where I have had food and water. $\endgroup$ – Warlord 099 Apr 20 '15 at 20:47
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Question:

Does everybody want a drink?

The third logician answers:

"YES" if he wants a drink, "NO" if doesn't want it.

Reason:

The first says "I don't know" because he wants a drink, but doesn't know if everybody wants one. If the first didn't want a drink, he would have answered "No".
Same for the second, he wants a drink but doesn't know if the third wants one. So, the third answers "Yes" if he wants a drink, "No" if he doesn't.

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    $\begingroup$ He might also say "I don't know" if he interprets "everybody" as more than just the three logicians. $\endgroup$ – Ian MacDonald Apr 19 '15 at 12:05
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    $\begingroup$ Actually, if "everybody" meant more than the logicians, the first one could safely answer "No", as the barman is not allowed to drink on the job. $\endgroup$ – Nigralbus Apr 21 '15 at 12:19
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    $\begingroup$ @Nigralbus Not being allowed doesn't mean he doesn't want one! The logician would have to be aware of someone who never wants a drink, which seems trivial -- assuming a newborn doesn't want alcohol is a safe bet. $\endgroup$ – Matthew Read Apr 21 '15 at 12:31
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    $\begingroup$ @MatthewRead But there is no reason to think that the drink has to be alcoholic... $\endgroup$ – Warlord 099 Apr 21 '15 at 13:14
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    $\begingroup$ If the first one really wants a drink, he will say "Yes", knowing that if the premise of all three being logicians is true, then their answers will not contradict. In that situation, either the premise is false or all three say "Yes" and the first one gets the drink he wanted.. $\endgroup$ – Anon May 20 '15 at 22:23
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We'll name the logicians A, B, and C to simplify.

If A wants a drink, A doesn't yet know whether B and C want a drink. Thus A cannot answer "yes".

If A does not want a drink, A would say "no" because at least one of A, B, and C don't want a drink, making "does everybody want a drink" false.

By saying "I don't know" A indicates their personal desire for a drink, but his lack of knowledge about the other two.

B is essentially in the same situation. B can deduce that A wants a drink, but still doesn't know the preference of C, therefore must answer "no" if B doesn't want a drink, and must answer "I don't know" if B wants a drink.

C, however, can now deduce that A and B both want drinks.

So if the third logician wants a drink, they say:

Yes, everybody wants a drink.

If the third logician doesn't want a drink, they say:

No, not everyone wants a drink.

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    $\begingroup$ At which point the barman says, "Fooled you. I'm not thirsty." $\endgroup$ – ruffin Apr 20 '15 at 19:14

protected by leoll2 Apr 20 '15 at 20:06

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