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From: gordon@cae780.UUCP (Brian Gordon)
Newsgroups: net.puzzle
Subject: Re: A logic(?) Puzzle (SPOILER)
Message-ID: <1739@cae780.UUCP>
Date: Mon, 6-Jan-86 17:45:31 EST
Article-I.D.: cae780.1739
Posted: Mon Jan  6 17:45:31 1986
Date-Received: Wed, 8-Jan-86 07:13:20 EST
References: <114@drutx.UUCP> <100@nbs-amrf.UUCP>
Reply-To: gordon@cae780.UUCP (Brian Gordon)
Organization: Tektronix, Inc. (CAE Systems Division), Sunnyvale, CA
Lines: 30

In article <100@nbs-amrf.UUCP> hopp@nbs-amrf.UUCP (Ted Hopp) writes:
>An interesting philosophical point concerns the nature of a lie.  If a "lie"
>is something that is logically false, then the above answer works.  On the
>other hand, a "lie" can be any deceptive answer, in which case the "liar"
>could answer "no", even if that is the truth (i.e., the liar WOULD answer
>"no" if you asked the antecedent question).  I don't know how to solve the
>puzzle for liars of the deceptive sort.

In one of his many puzzle discussion books, Martin Gardner attacks exactly
this problem.  Two roads, one question to one "native", who could be a
truth-teller, a liar, or an artful liar (who gives whichever answer he 
believes will be worse for you).  The question MG proposes is, "Did you know
they were giving away free beer in Xanadu?"  A truth-teller will say "No",
and head down the road to Xanadu -- just follow him.  The (normal) liar will
say "Yes", and again head down the correct road -- follow him.  The artful
liar will answer either Yes or No, as seems appropriate, and then head down
the right road -- follow him.

The worst case is the one where the artful liar figures out what you are doing
and, after answering either Yes or No, deliberately goes down the wrong road,
in order to "lie" with his feet!  Even in that case, you at least have the 
satisfaction that the artful liar will never be sure that he didn't miss out on
the free beer ...

FROM:   Brian G. Gordon, CAE Systems Division of Tektronix, Inc.
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