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Re: LANG: owl:ontology

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Sun, 15 Sep 2002 20:43:14 -0400 (EDT)
Message-Id: <20020915.204314.60041695.pfps@research.bell-labs.com>
To: hendler@cs.umd.edu
Cc: www-webont-wg@w3.org

From: Jim Hendler <hendler@cs.umd.edu>
Subject: Re: LANG: owl:ontology
Date: Sun, 15 Sep 2002 15:16:15 -0400

> I was going to wait for others to weigh in before deciding to 
> continue this, but I realized that there is some ambiguity and that 
> perhaps Peter and I are making different assumptions:
> 
> >Peter wrote:  [snip]
> >In the current systems (DAML+OIL) ontologies are documents, at least as far
> >as I can see.  Having ontologies be documents is a viable solution in my
> >eyes, and provides an elegant solution to which information to import.
> >[snip]
> >
> >The DAML+OIL documents are rather vague.  However, an imports refers to a
> >URI and it seems to me that this means to import the contents of the
> >document, as there is no other potential thing to import.
> 
> He is right that the document is vague - what it says is:
> 
>  From the DAML+OIL TR [1]
> >Each daml:imports statement references another DAML+OIL ontology 
> >containing definitions that apply to the current DAML+OIL resource. 
> >Each reference consists of a URI specifying from where the ontology 
> >is to be imported from. See the example above. Imports statements 
> >are transitive, that is, if ontology A imports B, and B imports C, 
> >then A imports both B and C. Importing an ontology into itself is 
> >considered a null action, so if ontology A imports B and B imports 
> >A, then they are considered to be equivalent.
> 
> I have been assuming that when I define a daml:ontology (which gets 
> its own URI) then an import statement points at that definition, not 
> at the document which it appears in.   Thus, when Peter says "this 
> means to import the contents of the document" I don't think I agree. 
> I think it means to import those statements that appear associated 
> with the ontology URI (not the document URL) and that it may be an 
> unwarranted assumption to assume that all the triples of any type 
> appearing in the rdf:RDF of that document are necessarily assumed to 
> be part of the ontology.

But statements are not, and cannot be, nested within an ontology element in
DAML+OIL because that would not be valid RDF.  

Instead an ontology is explicitly defined as zero or more headers
(daml:Ontology elements), followed by zero or more class elements, property
elements, and instances.  Now this definition *is* rather suspect, because
any way of generating the same triples is supposed to be equivalent, and
collections of triples are unordered in RDF.  However, the only way of
making this work in RDF is for an ontology to be an RDF document (an
rdf:RDF element and its constituents in RDF/XML), so importing an ontology
must refer to importing the entire contents of the document.

> Jeff's use of the daml:imports as magic syntax [2] primarily focused 
> on people wanting to import everything, but some of the mail on the 
> original thread (see [3] 

[3] is about multiple levels for OWL, not about ontologies.

> and the many threads around it that it 
> spawned and/or spawned it) pointed out the desire to only include 
> some properties without importing the whole ontology (using cyc as an 
> example). For an example of someone using stuff from other ontologies 
> without importing see [4] 

Using resources from other ontologies does not depend on importing.
Statements (triples) have no identity in RDF, so they do not depend on
importing either.  

> and there's lots of other examples out in 
> the real web (this is one of my students, so I got to see it, and 
> then went seaching and found many more - in fact, I can find very few 
> ontologies defined since the original Darpa set that use 
> "daml:imports")

> I'm looking for a solution that makes it clearer what is imported, 
> and allows some control over that.

Well, then I strongly suggest that you make clear what *is* imported in
your proposal.  I do not see any way of taking what you have presented so
far and determining what to import, or even determining what is in an ontology.

>   -JH

> p.s. I have answers to Peter's challenge of what gets included given 
> my scheme, but I'll wait until I see whether other people care about 
> this issue before sending yet another long note...

Well until you have answers to this question I don't think that you have a
viable proposal.

> [1] http://www.w3.org/TR/daml+oil-reference#imports-def
> [2] 
> http://www.w3.org/2001/sw/WebOnt/webont-issues.html#I5.6-daml:imports-as-magic-syntax
> [3] http://lists.w3.org/Archives/Public/www-rdf-logic/2002Apr/0060.html
> [4] http://glue.umd.edu/~aloomis/498x/hw1-ont.n3

peter
Received on Sunday, 15 September 2002 20:43:23 GMT

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