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From: jeff@rtech.UUCP (Jeff Lichtman)
Newsgroups: net.puzzle
Subject: Re: A logic(?) Puzzle (SPOILER)
Message-ID: <823@rtech.UUCP>
Date: Tue, 7-Jan-86 00:49:21 EST
Article-I.D.: rtech.823
Posted: Tue Jan  7 00:49:21 1986
Date-Received: Wed, 8-Jan-86 20:14:57 EST
References: <114@drutx.UUCP> <100@nbs-amrf.UUCP>
Organization: Relational Technology, Alameda CA
Lines: 40

> > 
> > You see two people nearby, and you find out that one of them always lies,
> > and the other always tells the truth. They know which way to go.
> > 
> > You can find out which way to go by asking either one of them ONE certain
> > question. What is the ONE question?
> 
> Point at one of the roads and ask either one, "If I were to ask you if this
> is the road to Xanadu [that's where you're going, right?], would you say
> 'yes'?"  If the answer is "yes", that's the road to Xanadu, regardless of
> whether you asked the truth-teller or the liar.
> 
> 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.
> 
> -- 
> 
> Ted Hopp	{seismo,umcp-cs}!nbs-amrf!hopp

Someone once wrote Martin Gardner a letter discussing the problem of the nature
of lying.  It appears in the book "Mathematical Puzzles & Diversions" by Martin
Gardner.  It comes to the conclusion that the most general solution is to ask
the question: "Did you know that they are serving free beer in the village?"
(or wherever it is that you want to go).  The truth-teller will say "no" and
head for the village.  The non-deceptive liar will say "yes" and head for the
village.  The deceptive liar will say, "Ugh! I hate beer!", and also head for
the village.

It's really worth it to get a buy a copy of this book, if only to have the
letter.  It's the funniest response to a logic problem I have ever read.
-- 
Jeff Lichtman at rtech (Relational Technology, Inc.)
"Saints should always be judged guilty until they are proved innocent..."

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