On Saturday, a judge condemns a prisoner to die at noon any one of the next seven days. The judge declares that the prisoner will be completely unaware of which day that will be until the hangman informs him that morning. Returning to his cell, the prisoner begins thinking. Obviously, he reasons, he can't be executed on Saturday. Why? If he were to be executed on Saturday, he would know on Friday night, before the hangman comes in and tells him. Therefore, he cannot be executed on Saturday. But that means he can't be executed on Friday either, since he would know on Thursday night that he would be executed the next day. Similarly, he rules out Thursday, Wednesday, Tuesday, Monday, and Sunday. Therefore he can't be executed, according to his brilliant logic. However, to his utter dismay, the hangman comes in Thursday morning to inform him of his impending execution, and the prisoner is then executed at noon. The judge's decree is obviously accurate; the prisoner was completely unaware that he would be executed on Thursday until the hangman informed him. On the other hand, the prisoner seemingly ruled out the possibility of his execution through impeccable logic. How can this be?
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