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From: hopp@nbs-amrf.UUCP (Ted Hopp)
Newsgroups: net.puzzle
Subject: Re: A logic(?) Puzzle (SPOILER)
Message-ID: <100@nbs-amrf.UUCP>
Date: Wed, 1-Jan-86 13:52:17 EST
Article-I.D.: nbs-amrf.100
Posted: Wed Jan  1 13:52:17 1986
Date-Received: Thu, 2-Jan-86 05:01:57 EST
References: <114@drutx.UUCP>
Organization: National Bureau of Standards
Lines: 27

>     Here's the situation:
> 
> You're walking along a road and you come to a fork where the road splits
> into two paths, one to the right and one to the left. You don't know 
> which way to go, but you must find out. 
> 
> 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