The prisonner problem reminds me of one of my favourite stories / paradoxes:

A man is in prison on death row. The judge has told him that he will be executed before the end of the week, but won't tell him which day, only that it will come as a complete surprise to him.

The prisonner is of a logical frame of mind, and starts to think about this.

"They won't execute me on Saturday" he calculates "Because if I'm still alive on Friday, I'll know that Saturday is the day, so it wouldn't be a surprise."

"But if they're definitely not going to execute me on Saturday, they can't execute me on Friday. If I'm still alive on Thursday evening, and I know they won't execute me on Saturday, it wouldn't be a surprise if the execution was on Friday, as that's the only day it could be on. So it's not Friday."

"And, by the same logic, if I'm still alive on Wednesday night, they won't execute me on Thursday, because that is now the only possible day, so it can't be Thursday"

And so his chain of thought continued. Not Wednesday, not Tuesday, not Monday.







They executed him on Wednesday morining. It came as a complete surprise.

Bradypus