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Re: Model Theory

From: Jim Hendler <hendler@cs.umd.edu>
Date: Thu, 3 Jan 2002 14:39:50 -0500
Message-Id: <p05101007b85a61777daa@[192.168.0.102]>
To: Ian Horrocks <horrocks@cs.man.ac.uk>, www-webont-wg@w3.org
At 6:55 PM +0000 1/3/02, Ian Horrocks wrote:
>To expand on the point I made at the end of the teleconf, I really
>don't believe that this emphasis on model theory helps much w.r.t. the
>point that Lynn was making, i.e., the question as to why I should
>care.
>
>As far as I am concerned, model theory is just one way (some would say
>a very elegant way) of precisely specifying the meaning of the
>language (its semantics). I think that the reason for needing such a
>specification is pretty obvious: a language (particularly an ontology
>language that is intended to be interpretable by "automated agents")
>is of limited (zero?) utility if we can't be sure of the meaning of
>what we write down in that language (yes, I know that natural language
>works reasonably well for humans without satisfying this
>requirement). Moreover, people writing/using software for the language
>need a precise specification as to how it should behave and when it is
>broken.
>
>There is an argument that goes something like "the web will be full of
>inconsistencies, so logical reasoning will be useless (everything will
>be inconsistent), so why should we care about formal semantics". I
>don't believe that this holds water for a variety of reasons including:
>
>1) Usually we will only be dealing with a (very small) part of the
>web. It is reasonable to ask if the information there is logically
>consistent and useful to know if it isn't.
>
>2) There is a big difference between an inconsistent ontology and web
>pages that state contradictory facts w.r.t. an ontology - if I am
>going to use/trust an ontology I would like to know that it is
>logically consistent. As far as the web pages are concerned, if I know
>that they are contradictory then I can make an informed decision about
>how to deal with them.
>
>Regards, Ian



All
    without taking a side in the particulars, I would like to point 
out that there are many kinds of semantics and languages and the 
notion of knowing "he meaning of what we write down in that language" 
is not as clear as you say -- consider C++ -- seems to be a language 
that is quite heavily used, yet I'm having trouble finding either a 
model theory or a formal semantics for it.  I think it is important 
for all of us to keep in mind that the language we are trying to 
create needs to satisfy (as best as possible) many conflicting needs
  1) Need for being able to represent information (KR issues)
  2) Need to be used on the web
  3) Need to be usable by computer for processing in various ways 
(implementation issues)
  This working group is trying to make sure we come up with a language 
that all of us can use for any of these and sometimes the needs will 
be in conflict.  Please remain very aware that we are engaging in a 
CONSENSUS process, and sometimes that requires some give and take on 
all sides.
  -JH
p.s. Note the W3C Guide [1] is titled "The Art of Consensus"

[1] http://www.w3.org/Guide/






-- 
Professor James Hendler				  hendler@cs.umd.edu
Director, Semantic Web and Agent Technologies	  301-405-2696
Maryland Information and Network Dynamics Lab.	  301-405-6707 (Fax)
AV Williams Building, Univ of Maryland		  College Park, MD 20742
http://www.cs.umd.edu/users/hendler
Received on Thursday, 3 January 2002 14:39:56 GMT

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