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Re: Non-monotonic

From: Drew McDermott <drew.mcdermott@yale.edu>
Date: Fri, 14 Nov 2003 11:51:53 -0500 (EST)
Message-Id: <200311141651.hAEGprH06094@pantheon-po03.its.yale.edu>
To: sean@smo.uhi.ac.uk
Cc: adrianw@snet.net, minsu@etri.re.kr, www-rdf-rules@w3.org, www-rdf-interest@w3.org


   [Minsu Jang]
   Non-monotonic reasoning is about the ability to retract previous
   conclusions and coming up with new conclusions, as the world
   is changing and facts we are given are changing.

   [Sean Mehan]
   Nonmonotonic reasoning is the ability to mkae assertions about the=20
   domain (push new facts on the stack) which may later be modified or=20
   retracted (pop the facts).

Both of these don't quite state correctly what nonmonotonicity is.
A reasoning algorithm (or logic) is nonmonotonic if there are premise
sets P and Q such that P is a subset of Q, but the algorithm draws (or
the logic licenses) a conclusion C given P that it does not draw or
permit given Q.  In other words, some conclusions can cease to be
inferrable in the presence of more information.

In practical situations, this ability also requires the ability to
retract previous conclusions.  But monotonic systems also require that
ability.  If some of the premises are retracted (by some other agent),
then some of your conclusions may have to be retracted.

   [Sean Mehan]
   The  question I have is how often do we need open world? In other=20
   words, how often would a closed world assumption supporting logic be=20
   good enough?

I hope you realize that a world assumed to be closed is the
nonmonotonic case, in other words, the harder case according to many
web gurus.  By their lights, your query is like asking, Do we really
need to explore the solar system?  Wouldn't exploring the galaxy be
good enough?

However, I tend to agree with you.  Closed-world assumptions work
quite well in practice and are well understood.  It's the purely
monotonic case that's difficult.  

-- 
                                             -- Drew McDermott
                                                Yale University CS Dept.
Received on Friday, 14 November 2003 11:51:55 GMT

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