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Re: defaults

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Tue, 22 Jan 2002 17:10:06 -0500
To: las@olin.edu
Cc: Frank.van.Harmelen@cs.vu.nl, www-webont-wg@w3.org
Message-Id: <20020122171006Y.pfps@research.bell-labs.com>
From: "Lynn Andrea Stein" <lynn.stein@olin.edu>
Subject: Re: defaults
Date: Tue, 22 Jan 2002 16:30:53 -0500

> "Peter F. Patel-Schneider" wrote:
> 
> > All that said, there will be a part of OWL that is not part of the logic
> > underlying OWL, or, at least, that I hope will not be part of the logic
> > underlying OWL.  This is precisely the part of OWL that deals with
> > ontologies (or documents, or ...).  Yes, this part of OWL interacts with
> > the logic underlying OWL, and, maybe, there could be a formal treatment of
> > it, but it does not inhabit the same conceptual space as interpretations,
> > models, and entailment.
> 
> I may be misunderstanding Peter here, but I *think* he just said that OWL
> statements should exist in some rarified space that is NOT the space of real
> documents and real manipulations of the real web.  

Well, aside from the perjorative use of ``rarified'' and ``real'', I would
go along with this. 

> Whether Peter holds this
> position or not, there certainly are those who do.  I believe that this position
> is fundamentally at odds with our job as WebOnt-WG and will run into trouble as
> soon as we begin to deal with things like 404 and changing web pages and digital
> signatures.

The problem is that powerful theories of ontologies and documents,
particularly theories that have ontologies and documents as first-class
objects, are extremely prone to semantic paradoxes.  (Surprise, surprise.)
Think of documents that assert their own falsity, for example.

If you separate the (powerful) object-level language from a (much
less-powerful) language that concerns itself with ontologies and/or
documents, then these problems become much less likely.

This is not to say that you can't construct paradox-free logics with
ontologies as objects.  You can.  Similarly it is possible to construct
logics that separate ontologies from other things but nonetheless have
paradoxes.

I don't see the separation as causing any (extra) problems.   On the
contrary, I think that the separation can be a powerful tool in solving
problems that come up in web contexts.  For example, a non-referring
pointer can be handled at the ontology level without having to having to
worry about such possibilities at the object level.


> > Such constructs (e.g., daml:imports) can indeed have impact on the
> > behaviour of OWL implementations, of course, but this is generally in terms
> > of determining what pieces of syntax are fed into an OWL reasoner, and
> > definitely not in terms of affecting the OWL reasoner in any other way.
> 
> What happens when the assertion that the OWL reasoner used to draw its conclusion
> is retracted by the previously asserting page?

Well that depends on what you want to have happen.  One (easy) way would be
to have a very simple document level, i.e., only daml:imports.  The
ontology level could condition all object-level conclusions on the
documents remaining the same.

How would you handle it in a formalism that has ontologies as objects?  I
think that the problems there are much harder.


> > It may turn out that there is a way of making some version of defaults fit
> > into this part of OWL.  I expect that any such version of defaults will be
> > a very weak (or very strong) version of something like input completion.
> 
> It may well turn out that this is so.  But that part will have these issues
> (nonmonotonicity, literal incorporation of syntax, etc.) whether we fit defaults
> in there or not.  It is also where the asserter lurks....OWL statements aren't
> true or false, they're asserted (by an agent or by the resolvent of a url or by a
> document) or not.

Yes, and treating this ``asserting'' level as separate from the object
level makes a lot of sense to me.


> Lynn


peter
Received on Tuesday, 22 January 2002 17:11:42 GMT

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