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Re: defaults

From: Lynn Andrea Stein <lynn.stein@olin.edu>
Date: Tue, 22 Jan 2002 17:43:22 -0500
Message-ID: <3C4DEB0A.FE1A268A@olin.edu>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
CC: Frank.van.Harmelen@cs.vu.nl, www-webont-wg@w3.org


"Peter F. Patel-Schneider" wrote:

> 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.

Actually, appearances notwithstanding, I didn't mean them as perjorative.


                                  Main Entry: rar·e·fied
                                  Variant(s): also rar·i·fied  /'rar-&-"fId, 'rer-/
                                  Function: adjective
                                  Date: 1941
                                  1 : of, relating to, or interesting to a select
group...

-- http://www.m-w.com/cgi-bin/dictionary

I meant that Peter wants the OWL world to be its own separate world and not the world
of messes like e-commerce (though it might be useful to step in and out of Peter's OWL
world while making e-commerce messes).

> 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.

Yup.  As I've said repeatedly, I think that we (the Semantic Web project) are stuck
with the reality of living in a world with paradox, contradiction, and other logical
difficulties.  Not to mention ambiguity of meaning, socially constructed usage,
partial knowledge, temporal embeddedness, retraction, and outright lies.

(Argument for layering deleted, though read and, I think, understood.)

> 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.

I guess I'm less sanguine than you that we can solve the job at hand using the tools
that you propose.  But I eagerly await your demonstrations (while simultaneously
trying to design alternatives that involve document manipulation at the base 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.

What do you do when it changes?  Presumably you are suggesting that you withdraw your
conclusions (i.e., "unfeed them" to the OWL reasoner?).  This of course makes your
conclusions nonmonotonic without even introducing a default mechanism.  (Or, if you
prefer, you can make them conditional with a need to continuously recheck the
condition.  But since you can never know what'll be true by the time you finish your
inferencing, it's hard to know how you could live with such a condition.)

That these sorts of things are a part of the new world we're in is precisely my point.

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

I don't think that they're considerably harder, but that's because I think that the
without-ontologies problem is pretty darn hard, not because I think that the
with-ontologies problem is easy.

> > > 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.

Hmm....This means that (Peter's) OWL is precisely what I said at the beginning,
rarified, and exists in a world of absolute truth and absolute falsity, rather than a
world of assertions by parties.  I think that this pretty directly contradicts TimBL's
vision, FWIW, and I am inclined to agree rather strongly with TimBL on this one.

Lynn
Received on Tuesday, 22 January 2002 17:43:35 GMT

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