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Re: Lang: owl:ontolgy

From: Jim Hendler <hendler@cs.umd.edu>
Date: Fri, 13 Sep 2002 18:48:44 -0400
Message-Id: <p05111708b9a80cc95649@[129.2.176.36]>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
Cc: webont <www-webont-wg@w3.org>

>From: Jim Hendler <hendler@cs.umd.edu>
>Subject: Re: Lang: owl:ontolgy
>Date: Fri, 13 Sep 2002 14:41:31 -0400
>
>[...]
>
>>  >What happens if a class is ``claimed'' by no ontology?  What happens if it
>>  >is claimed by more than one ontology?
>>
>>  That's easy, it is available on the web if someone wants to link to
>>  it in their ontology, but it's not part of these ontologies -- i.e.
>>  ontology = document is no longer true.
>
>What happens if it is claimed by more than one ontology?

so what?  Unless you assume a single universal ontology, many 
facts/classes may be claimed by multiple ontologies - that is why we 
call it the semantic Web.  Also, we have some early mathematical 
results showing overlap of ontologies is very useful for semantic 
interoperability in the large.  D+O doesn't rule this out, and we 
should allow it as well.

>
>>  >What is the meaning of this?  What aspects of :foo, etc., are part of this
>>  >ontology?
>>
>>  Well, this seems clear to me - since :foo is a class, and all the
>>  descriptions that relate to it must be linked to it (that's why the
>>  URI thing is important), then everything relate to that class URI is
>>  contained in the ontology by reference.   All we are doing here is
>>  removing "lexical proximity" from being the way we decide whether
>>  carrots or potatoes are in the same ontology.
>
>  But everything that relates to it is probably everything on the entire
>  web.  You have to say which axioms and facts you are including.  In fact,
>  as RDF/XML is the exchange language, you have to say which *triples* you
>  are including.

yes, but we have exactly that same problem in the current system.  It 
is perectly legal to have a document with nothing on it but a class 
definition, another document that has some other class definition 
that points to it, and a third document that claims to be an ontology 
and has something which is a subclass of one of these.  That's 
perfectly legal, and I have a number of tools which take advantage of 
the fact (letting one document add properties to an ontology in 
another, for example).  In point of fact, we could have all the 
triples on the ontology be in a closed list - but I don't like that - 
I htink it better to have some general ideas of how the pointers 
work, and leave the rest unspecified for now.  If you want 
decidability/consistency, you can insist that people only use local 
referents or something like that.

A view of ontologies that cannot point to each other, and instances 
that only point to one ontology, is like the web without the links -- 
it's still a lot of documents, but it ain't the Web.

>
>>  >  > The beauty of this is that I could now handle imports in various ways
>>  >>  - I could import an entire ontology with an imports statement within
>>  >>  this
>>  >>
>>  >>  <owl:Ontology ...>
>>  >>     <owl:imports URI2 />
>>  >>
>>  >>  and extend it
>>  >>
>>  >>     <owl:Defines ...>
>>  >>       <owl:ontologyClass :notInURI2>
>>  >>     </owl:defines>
>>  >>  ...
>>  >>  <owl:Ontology>
>>  >
>>  >How would this be different from any other mechanism for ontologies?
>>
>>  because there is no other mechanism currently on the table, so this
>>  is a solution to the open issue (with other desirable properties)
>
>I bring your attention to the abstract syntax document, available at
>http://www.w3.org/TR/owl/ab-syn/, which includes a treatment of imports.
>DAML+OIL also includes a treatment of imports.

yes, but we have an issue open asking for a better way to handle 
imports - I'm perfectly fine with going with the D+O approach (magic 
syntax) but the issue was raised as to whether there is a better way, 
and I think this is one.

>  > >>  I could include classes from other ontologies (Without importing the
>>  >>  whole thing) by simply including them in my owl:ontologyDefines
>>  >>  collection
>>  >>     <owl:OntologyClass cyc:dog>
>>  >
>>  >What is the impact of this?  That cyc:dog is a resource in this ontology?
>>  >That the axioms about cyc:dog (from where?) are to be included in this
>  > >ontology?
>>
>>  yes, that the URI cyc:dog is a class, it contains class definitions,
>
>Classes do not contain class definitions.  Classes do not even have
>really have definitions.  All there is is triples!

class are referred to by URIs, and are thus "indexes" into larger 
graphs.  Currently some people view those graphs as separable, others 
of us don't. Whatever the semantics would hold for the document view 
will hold just as easily in my view -- simply consider the ontology 
statement plus its defines to be what you called a document, and 
everything we've done to date still works just fine.  Far as I can 
tell it will make absolutely no difference from a logical view.  But 
it makes a lot of difference from an operational or "extra-logical" 
view because ontologies become first class objects on the web (i.e. 
you can point at them) and get URIs instead of URLs, which is useful 
for a number of other things some of us do who build tools.

>
>>  and those class definitions are included in my ontology. 
>
>Even if you fixed this to remove the ``definition'', you haven't said which
>information (triples) you are including.
>
>>  We can make
>>  decisions as a group as to whethr, for example, it also mandates all
>>  superclasses be included or some other things be included - but we
>>  have this same problem anyway in our current systems,
>
>Not so, a solution based on documents does not suffer from this problem.
>
>>  so that doesn't
>>  change anything (i.e. right now I can say
>>
>>  me:pet1 owl:subclass cyc:dog;
>>
>>  and we've never nailed down what this means either) 
>
>Well, technically true, because there is no semantics for any part of OWL
>as of yet, but RDF has a meaning for this, and one that does not depend on
>ontologies or imports.  DAML+OIL has a meaning for this that does not
>depend on ontologies or imports.

and it could mean exactly the same here

>
>>  We also can
>>  decide to leave that issue to a future group if we want
>>
>>  >I don't see any advantage here either.
>>
>>  I do, I see a major advantage - do you see a major disadvantage? Show
>>  me a use case.
>
>What is your advantage?  Show me a use case!!

I did in my original message, and Jeremy did same in his 
carrots/potatoes case.  Please explain how you would solve same.

>I see many problems, some mentioned above.
>  > >>  and I get for free some new properties that seem quite desirable - in
>>  >>  particular, I could create multiple ontologies in a document by
>>  >>  including pointers to different subsets
>>  >
>>  >Is this an advantage?  It seems natural to have a 1-1 correspondence
>>  >between documents and ontologies.
>>
>>  and I disagree, it would be great not to have to that 1-1 which I
>>  find a weakness of D+O.  Again, show me a use case
>
>Again, you are proposing a change.  Show me a use case!!

again, I and Jeremy both did - we both think the ability to do his 
carrots and potatoes, or my pets and felines are important to info 
sharing and ontology use and construction.

>
>>  >>  <owl:ontology rdf:ID="Pets">
>>  >>     ...
>>  >  >    <owl:ontologyDefines>
>>  >>         ... dog
>>  >>         ... cat
>>  >>     </owl:ontologyDefines></owl:ontology>
>>  >>
>>  >>  <owl:ontology rdf:ID="Felines">
>>  >>     ...
>>  >>     <owl:ontologyDefines>
>>  >>         ... lion
>>  >>             cat
>>  >>             tiger
>>  >>     </owl:ontologyDefines></owl:ontology>
>>  >
>>  >Again, which aspects of cat are part of Pets and which are part of Felines?
>>
>>  all of each are part of both
>
>What all?  Do you mean all RDF triples currently on the web that mention
>cat?  Do you mean the intended meaning of whoever wrote the document that
>the URI points to?

whatever solution you would use for DAML+OIL works for this as well
-- 
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)
http://www.cs.umd.edu/users/hendler
Received on Friday, 13 September 2002 18:48:51 GMT

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