Knave or Knight?

A little puzzle for you before the evening begins (with help from several crappy old books and that infallible source, Wikipedia, which I draw on below).

You are stranded on an island whose residents are either Knaves or Knights. All Knaves are Liars. All Knights are Truth-tellers. You wander the island looking for Freedom, knowing full well Death is a possibility.

You come to a fork in the road. One path ahead leads to Freedom, the other to Death. Each of the two paths is guarded, one by Robert, one by Paul. You know that one of the two is a Knave, the other a Knight. But you do not know who is which. By asking one yes/no question, can you determine the road to Freedom?

The answer is, "Yes, you can." And now the question becomes, "What is that one question?" [Hint: There are two possible questions.]

Think about it and after you've scratched your head, continue reading.


The Wikipedia entry provides two alternate questions. With some major grammatical editing, here they are.

"Will the other man tell me that your path leads to freedom?" If the man says "No", then the path does lead to freedom, if he says "Yes", then it does not. 

The following logic is used to solve the problem. If the question is asked of the Knight and the Knight's path leads to freedom, he will say "No", truthfully answering that the Knave would lie and say "No". If the Knight's path does not lead to freedom he will say "Yes", since the Knave would say that the path leads to freedom. 

If the question is asked of the Knave and the Knave's path leads to freedom he will say "no" since the Knight would say "yes" it does lead to freedom. If the Knave's path does not lead to freedom he would say "Yes" since the Knight would tell you "No" it doesn't lead to freedom.

The reasoning behind this is that, whichever guard you ask, one would not know whether the guard was telling the truth or not. Therefore you must create a situation where each receives both the truth and a lie applied one to the other. Therefore if you ask the Knight, you will receive the truth about a lie; if you ask the Knave then you will receive a lie about the truth. 

Note that the above solution requires that each of them knows that the other is a knight/knave. 

"What would your answer be if I asked you if your path leads to freedom?" If the man says "Yes", then the path leads to freedom, if he says "No", then it does not. 

If you ask the Knight if his path leads to freedom, he will answer truthfully, with "yes" if it does, and "no" if it does not. He will also answer this question truthfully, again stating correctly if the path led to freedom or not. 

If you ask the Knave if his path leads to freedom, he will answer falsely about his answer, with "no" if it does, and "yes" if it does not. However, when asked this question, he will lie about what his false answer would be, in a sense, lying about his lie. He would answer correctly, with his first lie canceling out the second. 

This question forces the Knight to say a truth about a truth, and the Knave to say a lie about a lie, resulting, in either case, with the truth.