he will either be surprised to find he is to be hanged on Thursday, or surprised to find it will be Friday The judge says the prisoner would be surprised exact

he will either be surprised to find he is to be hanged on Thursday, or surprised to find it will be Friday The judge says the prisoner would be surprised exactly when the knock on the door happens (not before). As a consequence, the prisoner is right to believe he won't be hanged at all (following his reasoning), but as a consequence of that, he is surprised when the knock happens. In turn, as a consequence of that, the judge is right. Therefore, both the prisoner, and the judge are correct. Thus the paradox.