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Re: LANG: owl:ontology

From: pat hayes <phayes@ai.uwf.edu>
Date: Tue, 17 Sep 2002 19:18:30 -0500
Message-Id: <p05111b0eb9ad716e14d0@[65.217.30.172]>
To: Jim Hendler <hendler@cs.umd.edu>
Cc: www-webont-wg@w3.org

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 
itself.

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?

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.

Pat
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Received on Tuesday, 17 September 2002 20:18:33 GMT

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