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Re: ACTION: task force unasserted triples

From: Dan Connolly <connolly@w3.org>
Date: 23 Apr 2002 08:50:22 -0500
To: "Peter F. "Patel-Schneider <pfps@research.bell-labs.com>
Cc: Guus Schreiber <schreiber@swi.psy.uva.nl>, www-webont-wg@w3.org
Message-Id: <1019569823.5315.2134.camel@dirk>
On Tue, 2002-04-23 at 08:28, Peter F. Patel-Schneider wrote:
> All that said, here is what I consider to be an adequate response to the
> request.
> peter
> The Web Ontology Working Group is producing a language that can be used to
> build ontologies.  This language is supposed to use the syntax of RDF and
> to be compatible with the model theory of RDF.  
> However, taking all three of these together results in severe problems.
> Consider the following RDF graph
>   John rdf:type Student .
>   John rdf:type Employee .
> as the WebOnt language will include an intersection construct, this graph
> should entail
>   John rdf:type |intersection Student Employee| .
> in the WebOnt language (where |intersection Student Employee| is a short
> form for a unnamed node that is connected to some other RDF triples that
> carry the intersection construct applied to Student and Employee).
> However, this cannot be a valid entailment unless there is an object in the
> domain of discourse of the initial RDF graph that is this intersection.
> Therefore the model theory for the WebOnt language will have to include
> comprehension principles for these sort of classes.
> Many more such examples can be constructed, together requiring a powerful
> theory of classes in the WebOnt language.  This theory is so powerful that
> it will contain inconsistencies, leading to a complete breakdown of the
> model theory for the WebOnt language.  (Even if the WebOnt language could
> be expressively limited to disallow the paradoxical classes, the same
> problem will reoccur at higher expressive levels in the Semantic Web
> tower.)

I'm still not convinced of that paragraph; I still expect we can
develop a theory of classes that's powerful enough but not
too powerful. But leaving that aside for now...

> If, however, the triples that are used to encode the syntax of the
> constructs of the WebOnt language are unasserted,


Please spell out the (proposed) solution completely.

Show the actual RDF/XML that is used in the (proposed) solution.

Show how one can differentiate the asserted triples from
the unasserted triples.

Fill in enough details that the RDF Core WG can evaluate
the costs of such a mechanism (and so that they can
consider nearby alternatives).

> then these constructs do
> not need to be part of the domain of discourse.  In this case, there is no
> need for comprehension axioms, and a non-trivial theory can be developed for
> the WebOnt language.
> What is lost?  Well, of course, quite a number of entailments are lost, and
> the details depend on which triples are unasserted.

Which triples do you/we propose are unasserted?

>  However, if nothing is
> done, the empty RDF graph will, in the WebOnt language, entail a
> contradiction.

Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Tuesday, 23 April 2002 09:50:22 UTC

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