Bertrand Russell: 

There is a village in which all the adult males are clean shaven. In the village is a barber. The barber shaves all and only those adult males who do not shave themselves. So, if Bob shaves himself then the barber does not shave Bob. And, if Bob does not shave himself then the barber does shave Bob. 

Question: Does the barber shave himself? 

Hypothesis: The world and everything in it was created five minutes ago. 

(Russell wants to show that the memories of something are logically independent of that something, but the hypothesis has been used to support skepticism.)