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Re: Lang: owl:ontolgy

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Fri, 13 Sep 2002 20:18:39 -0400 (EDT)
Message-Id: <20020913.201839.123562506.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:ontolgy
Date: Fri, 13 Sep 2002 18:48:44 -0400

> >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.  

But what are the effects of being claimed by multiple ontologies?

> 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.  

Not at all.  Ontologies are documents, so everything in a document is
included.   No other determination is needed.

> 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).  

This does nothing to determine which information to import.

> In point of fact, we could have all the 
> triples on the ontology be in a closed list - but I don't like that - 

This would work, in some sense at least.  An ontology is a collection of
triples.   But then how is this significantly different from an ontology
being a document that is a collection of triples?

> I htink it better to have some general ideas of how the pointers 
> work, and leave the rest unspecified for now.  

Huh?  Leave the determination of which triples to import unspecified?  How
could anyone use imports then?

> If you want 
> decidability/consistency, you can insist that people only use local 
> referents or something like that.

How did decidability or consistency get in here?   Decidability or
consistency of what?

> 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.

Why?  I see absolutely nothing to back up this claim.  Ontologies being
documents and ontology documents pointing to other ontology documents sure
looks like the Web to me!


> >>  >  > 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.

Sure we want a better way.  But there *is* a mechanism from DAML+OIL, and
other proposals have already surfaced.
> 
> >  > >>  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.  

Not at all.  The problem is determining boundaries.  You haven't said where
the boundaries between classes are supposed to be, so you haven't produced
a viable proposal for imports.

> 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.

Useful for what things?  Useful for what things that can't easily be done
using tools that use the view of ontologies as documents?


> >
> >>  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

Sure, but the RDF and DAML+OIL solutions do not bring in any aspects of
ontologies or imports.  You seem to be saying that the meaning of the above
triple needs a definition of ontologies and importing, but both RDF and
DAML+OIL provide meaning for it without needing any notion of ontologies or
importing.

> >>  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 don't see any use case there.  I see a proposal, an example, and several
properties, but no use case.  Please point out the use case.

> >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.

Ah, now we are getting closer to a use case.  It is not a full use case, as
there is no attempt to show what the information can be used for.

So, let's look at Jeremy's proto use case, which might be characterized as
the ability to determine the source of information about a class, for some
unspecified purpose.  How might this be solved?

Well, it is not adequate to have a relationship just from a class, as
information about classes need not be given in one place.  It also seems
inadequate to have a relationship from triples, as much of the meaning of OWL
KBs is in combinations of triples that have no OWL meaning independantly.

So it appears that what is needed is a connection from the OWL abstract
syntax to its source.  Why not have this just being the document that the
information appears in?  OWL tools could record this information completely
outside of RDF.  This should solve the problem without needing ontologies,
or even any syntactic constructs for this extra-logical information.


> >>  >>  <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

Sure, the solution in DAML+OIL is to identify ontologies as certain kinds
of documents. 

[...]

peter
Received on Friday, 13 September 2002 20:18:48 GMT

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