Monday, March 3, 2008

Logic Problem

A genealogist is researching two non-intersecting family trees. Members of one family always tell the truth, and members of the other family always lie. The genealogist comes to a brick wall. Standing at this wall is a member of each family, but the genealogist can't tell which family each belongs to. What question should she ask to break through the wall?

Sigh…to belong to families of all truth-tellers, or all liars. That would make research so much easier.


Chris said...

Finally, my philosophy education comes in handy! She should ask either one, "If you were a member of the other family, how would you answer the question, 'What family are you a member of?'" If she asks the liar, he will answer (untruthfully) "The family that always lies." If she asks the truth-teller, he will answer (truthfully), 'The family that always tells the truth.'"

Anonymous said...

Ask "Are you dead?" The truth-teller will say "No" and the lier will say "Yes". I suppose it would work with the question "Are you alive?" as well. It will point out which is the truth-teller to ask a real question.