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Re: LANG: owl:ontology

From: Jim Hendler <hendler@cs.umd.edu>
Date: Tue, 17 Sep 2002 21:57:21 -0400
Message-Id: <p05111702b9ad8b230fda@[]>
To: pat hayes <phayes@ai.uwf.edu>
Cc: www-webont-wg@w3.org

At 7:18 PM -0500 9/17/02, pat hayes wrote:
>Can I suggest that in this discussion thread we try to avoid using 
>the word 'definition'? There really is no such thing as a definition 
>in RDF or OWL. There are only assertions; one can never know that 
>one has got *all* the relevant information about something. So the 
>idea of 'containing' some chunk of information to be the 
>*definition* of a class is beside the point here; its like designing 
>a trap for unicorns.
>Seems to me that a lot of the discussion about importing has really 
>been about what counts as an ontology. Let me suggest that we decide 
>this at the syntactic level by saying that an ontology is a set (or 
>possibly a bag) of RDF triples. That is not saying that every set of 
>RDF triples counts as an ontology, but that the triples-store is the 
>appropriate basic syntactic level for defining such things as 
>identity of ontologies, mergings of ontologies, entailments between 
>ontologies and so on.  This is really just following the RDF spec 

actually I'm fine with this but...

>Before the howling starts, let me give some arguments for this. 
>First, we have decided that the interchange syntax for OWL is 
>RDF/XML. But RDF/XML is not a suitable notation for defining 
>RDF-meaningful syntactic operations on: that is, RDF-meaningful 
>notions of merging, containment (of one set of assertions in 
>another) and so on do not correspond to simple syntactic operations 
>on the XML surface syntax. So the aforementioned decision about 
>RDF/XML only makes sense, seems to me, if we agree that this 
>interchange language is in fact being used in the way described by 
>the RDF spec itself, ie as a surface/interchange notation for RDF 
>*graphs*.  A possible objection to this interpretation has always 
>been that the OWL semantics does not agree with the RDF semantics 
>when applied to OWL/RDF, so this RDF-centric perspective is not 
>viable when one wishes to consider semantically meaningful 
>operations on OWL: I think that objection is now refuted, or at 
>least has been demoted from a technical objection to an aesthetic 
>one, so should be discounted. Which brings me to the second point: 
>treating the RDF graph syntax as the basic syntactic level allows us 
>to fairly cleanly define OWL-meaningful operations on ontologies, in 
>a uniform way with how RDFS-meaningful operations are defined on 
>them. And third, this approach preserves the desired 
>interoperability and overall coherence between RDF, RDFS and OWL 
>that we all pray for every evening, right?

ok, but...

>On this view, then, having [imports B] included in A would be saying 
>(semantically) that if if the graph-merge of A and B entails C then 
>A entails C, or (syntactically) that A should be considered to have 
>B graph-merged into it; where 'A' and 'B' throughout are taken to 
>refer to whatever collections of RDF triples the syntactic form 
>being used maps into. In the case of OWL/RDF/XML, that would an OWL 
>closure, which might be quite a large set: OK, but that's OWL life. 
>Implemeters of course can choose to be clever in various ways.

I'm even okay with this, however what I have a problem with is the following

At URI1:
<owl:class rdf:ID="foo" />

(1,000,000 other assertions that appear in the graph)

At URI2:

<:bar owl:subclass URI1:foo />

(put in any owl:ontology and rdf:RDF syntax you want - but no 
owl:imports in URI2:)

In this case I have a real problem with merging the graphs -- the 
user is very unlikely to actually intend that those million facts 
which he or she may not even have read should be included.

This is the case I really care about.  For imports anything that can 
identify and merge graphs makes me happy - for this case, I care that 
we somehow scope what is included.  I would like this to have the 
same semantics as having one URI which contained

<owl:class rdf:ID="foo" />
<:bar owl:subclass :foo />

(i.e. nothing else from URI1: is to be included unless it is 
explicitly  mentioned.)

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)
Univ of Maryland, College Park, MD 20742	  240-731-3822 (Cell)
Received on Tuesday, 17 September 2002 21:57:25 UTC

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