Posted by Robert Dilley on November 30, 2005 at 21:29:03 from 65.39.13.2 user rdilley.
In Reply to: Re: Paradox posted by Charles on November 29, 2005 at 11:26:47:
Charles' answer will not work.
As I understand the riddle, you are imprisoned in a room with two exits. One exit leads to your freedom, the other to your execution.
There are two guards. You may ask one question of only one of them. You are told that one guard always lies, one always tells the truth, but not which guard is which.
If you ask "Is this (pointing to either door) the door to freedom" the truthful guard will tell the truth and the untruthful one will lie. As you do not know which is which you are no further forward.
The solution is to say "If I were to ask you if this is door to freedom, what would you reply?".
If it is the door to freedom, the truthful guard will reply "Yes", because it is, and he is telling the truth about what he would say. If it is not, he would say "No" for the same reason, and you would exit by the other door.
If it is the door to freedom, the untruthful guard would say "No" to a direct question. But he must lie consistently, so he must say "Yes" in order to lie about the lie he would tell.
So, in either case you get the right answer. Whether anyone would actually be able to think of that in such a situation....
Lewis Carrol would have enjoyed this. Nancy would probably have slugged both guards and battered her way out.