Since spirits are eternal I ask each one if they always existed; the honest spirit would say yes, the dishonest spirit could only say no. Then I would ask the honest spirit: which door?

Here a similar puzzle: "On a fictional island, all inhabitants are either knights, who always tell the truth, or knaves, who always lie." "John and Bill are standing at a fork in the road. You know that one of them is a knight and the other a knave, but you don't know which. You also know that one road leads to Death, and the other leads to Freedom. By asking one yes/no question, can you determine the road to Freedom?" "There are several ways to find out which way leads to freedom. One alternative is asking the following question: "Will the other man tell me that your path leads to freedom?" If the man says "Yes", then the path does not lead to freedom, if he says "No", then it does. 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 his path doesn't 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 guardian the questioner asks, one would not know whether the guardian was telling the truth or not. Therefore one must create a situation where they receive both the truth and a lie applied one to the other. Therefore if they ask the Knight, they will receive the truth about a lie; if they ask the Knave then they will receive a lie about the truth. Note that the above solution requires that each of them know that the other is a knight/knave. An alternate solution is to ask of either man, "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. The reason is fairly easy to understand, and is as follows: If you ask the knight if their path leads to freedom, they will answer truthfully, with "yes" if it does, and "no" if it does not. They will also answer this question truthfully, again stating correctly if the path led to freedom or not. If you ask the knave if their path leads to freedom, they will answer falsely about their answer, with "no" if it does, and "yes" if it does not. However, when asked this question, they will lie about what their false answer would be, in a sense, lying about their lie. They would answer correctly, with their 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. Another alternative is to ask: "Is either one of the following statements correct? You are a Knight and at the same time this is the path to freedom; or you are a knave and this is not the path to freedom". More alternatives for a question to ask can be found using Boolean algebra." Source and further information: http://en.wikipedia.org/wiki/Knights_and_Knaves