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Re: ISSUE 5.6 - daml:imports as magic syntax

From: Jeff Heflin <heflin@cse.lehigh.edu>
Date: Tue, 10 Sep 2002 17:48:44 -0400
Message-ID: <3D7E68BC.9B2AF2B5@cse.lehigh.edu>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
CC: www-webont-wg@w3.org

"Peter F. Patel-Schneider" wrote:
> 
> From: Jeff Heflin <heflin@cse.lehigh.edu>
> Subject: Re: ISSUE 5.6 - daml:imports as magic syntax
> Date: Tue, 10 Sep 2002 16:22:04 -0400
> 
> > Peter,
> >
> > Please see my responses inline below...
> >
> > "Peter F. Patel-Schneider" wrote:
> > >
> > > A very good summary of daml/owl:imports.
> > >
> > > From: Jeff Heflin <heflin@cse.lehigh.edu>
> 
> [...]
> 
> > > A treatment of imports can be done completely syntactically, by replacing
> > > imports foo, where foo is a URI (or whatever) by the contents of the
> > > document pointed at by foo.   This is the way I would handle it in the
> > > abstract syntax and direct semantics.
> >
> > Although imports could be treated syntactically (kind of like a
> > "#include" directive), I think that would be a big mistake.  The point
> > of imports is that knowledge from another source applies to the resource
> > in which it is expressed. The Semantic Web is fundamentally about
> > distributed ontologies and data sources, and as such its semantics
> > should discuss these things explicitly. A syntactic fix obscures one of
> > the things that differentiates the Semantic Web from traditional logic
> > approaches. Ontologies and the interrelationships between them are
> > important; they aren't just things to be swept under the rug.
> 
> Oh agreed, they are important.  However, making ontologies elements of the
> domain of discourse opens up a large number if issues, including very
> dangerous things like self-reference.  N3 has (or had) something along
> these lines, and it is easy then to create the liar's paradox and other,
> nastier, problems.

I wasn't suggesting that ontologies should be in the domain of
discourse. I'd actually prefer that they weren't. That's the whole point
of the "magic syntax." Still, they can play a role in the semantics
without being objects that you can make assertions about.

> > > > In Pat Hayes' recent OWL model theory, it should
> > > > be sufficient to say that owl:imports is not an instance of owl:Object,
> > > > owl:DatatypeValue, owl:Class, owl:ObjectProperty, or
> > > > owl:DataTypeProperty. However, it would still be an rdfs:Property, but
> > > > any RDF graph that uses it as a subject or object would not be
> > > > well-formed OWL/RDF. Note, owl:imports doesn't fit into any of the
> > > > classifications in the table in section 2 of the Hayes model theory, so
> > > > perhaps a new classification called metaproperty or such is needed.
> > >
> > > Even in this separated treatment, imports is problematic.  It would require
> > > a completely new kind of semantic rule, and one that brings new kinds of
> > > things into the semantics.
> >
> > Sure this semantic rule is a little different from the kind that we
> > already have considered, but I don't think it would break things or make
> > them more confusing. In fact, I think it would illucidate how knowledge
> > sharing actually works on the Semantic Web.
> 
> > > > Semantics:
> > > > ----------
> > > > It is important that the semantics of owl:imports be added to any
> > > > semantics documents. To do this, we need to be able to refer to sets of
> > > > OWL statements (such as a web page, a database with an OWL interface,
> > > > etc.) This could probably be called a resource, but that term is also
> > > > used to describe RDF instances, so for lack of a better term, I will
> > > > choose the term graph for the time being. Let graph be a function from a
> > > > URI (URL?) to an RDF/OWL graph. Each OWL graph has a set of entailments
> > > > that are determined by the model theory. The semantics of a statement:
> > > >
> > > > A owl:imports B.
> > > >
> > > > are:
> > > >
> > > > if graph(B) |= X then graph(A) |= X
> > > >
> > > > (Note: Here, "|=" is the OWL entailment relation)
> > >
> > > This adds in a lot of error-prone machinery.
> >
> > What is error prone about it? This simply says that anything entailed by
> > the ontology denoted by B is also entailed by the ontology denoted by A.
> 
> > > > One question is whether we need something more specific in the model
> > > > theory that, for example, uses ICEXT and IEXT.
> > >
> > > Something more is needed.  In particular, a relationship between the
> > > current graph and its name is needed.  Otherwise how can the condition
> > > be discharged in the conditional above?
> >
> > The relationship can be implmented by using the HTTP GET function to
> > fetch the contents at the URL and then parsing it based on OWL
> > semantics. I don't see what the problem is.
> 
> Well you now have to incorporate the meaning of the HTTP GET function in
> the semantics itself.  How are you planning on doing this?  This is the
> sort of machinery that is hard to specify and easy to pervert.

Why would you have to incorporate that meaning into the semantics
itself? It is a simple, computable function. The results of the function
aren't important to the basic theory, the only matter when determining
the entailments of a particular document.

> [...]
> 
> peter
Received on Tuesday, 10 September 2002 17:48:47 GMT

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