This riddle was used by Bertrand Russell to derail one of the greatest mathematical projects of the 20th century. It may also make your head hurt.
There is a town with one barber. The barber shaves only those men who do not shave themselves. In this town, who shaves the barber?
Just like that the logical trap is sprung. If the barber does not shave himself, then the barber must shave himself, but if he shaves himself, then he cannot shave himself.
In other words, if the barber shaves himself, then he doesn’t, but if he doesn’t, then he does.
Believe it or not, this whimsical tale about an imaginary barber made it impossible to ground mathematics in purely uncontroversial logical principles, and caused a debate that still rages to this day.
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