Being a Professional Mathematician

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Spiked Math "Three logicians walk into a bar" comic

Transcript and Explanation

Three logicians walk into a bar. The bartender asks "Does everyone want beer". The first and second logicians each say "I don't know", and then the third answers "Yes".

The point is that each logician knows only whether they themselves want beer. The statement "Everyone wants beer" is true only if all three want beer. If the first logician didn't want beer, they could answer "No" to the bartender's question, so their reply of "I don't know" proves that the first logician does want beer, Similarly for the second, so after hearing these two answers, the third logician can deduce that each of the first two does want beer, and is therefore able to answer the question with certainty.

This is Spiked Math Comic number 445 and is licensed under a Creative Commons License.

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This project was supported by the MSOR Network, the Institute of Mathematics and its Applications and the Universities of Greenwich and Birmingham as part of the National HE STEM Programme and was completed in May 2012. 

Creative Commons Licence
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

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