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From: ice@trwrba.UUCP (Douglas L. Ice)
Newsgroups: net.puzzle
Subject: Re: puzzle
Message-ID: <1822@trwrba.UUCP>
Date: Wed, 12-Feb-86 13:29:10 EST
Article-I.D.: trwrba.1822
Posted: Wed Feb 12 13:29:10 1986
Date-Received: Fri, 14-Feb-86 06:45:01 EST
References: <1644@hound.UUCP>
Reply-To: ice@trwrba.UUCP (Douglas L. Ice)
Organization: TRW EDS, Redondo Beach, CA
Lines: 43
Summary: Truthtellers and liars (NON-ROT SPOILER!!!)

In article <1644@hound.UUCP> 5125kaf@hound.UUCP (K.FRIEDMAN) writes:
>
>     You are staggering down a road ready to give up on life
>when you come to a fork in the road where there are two people
>standing. You have heard about this place. One of the roads goes
>straight to a hospital and the other to a alligator-infested
>swamp. One of these people always tells the truth and the other
>always lies. You can ask either person one question and only
>one question in order to determine which way the hospital is.
>What would be the question and to whom would you ask it?
>How would this question help you to determine which way to go?
>

	Ask either person, "If I were to ask the other person
which way to the hospital, what would they say?"  Then go on
the other path.  If you asked a truthteller, he would truthfully
reply that the liar would provide the wrong path.  If you asked
a liar, he would falsely reply that the truthteller would provide
the wrong path.  By going the other way, you reach the hospital.

	Another method to use (in case one of the two takes a
coffee break, and you don't know which) is to ask the person,
"If I were to ask you the way to the hospital, what would you say?"
If you ask a truthteller, he will truthfully reply that he would
provide the correct path if you were to ask him.  If you ask a
liar, he would lie about what he would say to you (which would have
been the incorrect path), thus providing the correct path.

	These and many MANY other logic puzzles can be found in
Raymond Smullyan's clever book, "What is the Name of This Book"
(available in paperback from Prentice-Hall).  He calls truthtellers
"knights", and liars "knaves".  He also introduces "normals",
who cannot be depended on to tell truth or lies.  Smullyan also
has written several other puzzle books, the titles of which I
cannot provide.

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                                                 --- Doug Ice