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Re: Krivov's question: "Why RDF?"

From: Jeremy Carroll <jjc@hplb.hpl.hp.com>
Date: Thu, 11 Mar 2004 12:04:55 +0000
Message-ID: <405055E7.7020105@hplb.hpl.hp.com>
To: Sandro Hawke <sandro@w3.org>
Cc: www-webont-wg@w3.org

If you regard OWL as just an ontology language then basing it on RDF (at 
all) is hard to argue for.
But as a web ontology language perceived as layering on top of RDF 
semantically it is plausible that the amount of effort needed to make it 
just more triples (rather than a true syntactic extension) was worth it.

I think it comes down to the W in OWL ...

Having decided that we working on Web ontology, we had to accept a number 
of limitations from other Web languages, RDF being one of them.


Sandro Hawke wrote:

> re: http://lists.w3.org/Archives/Public/public-webont-comments/2004Mar/0020
> I provoked some discussion of this matter on the the DAML/EU Joint
> Committee list recently.  (The question there being whether or how to
> layer a rule language (eg SWRL) on RDF.)
>    Peter F. Patel-Schneider:
>       I argued long and loud in the W3C WebOnt working group about
>       problems that using the RDF syntax caused.  This argument didn't go
>       anywhere, so I gave in and created a partial solution for OWL.  
>    Sandro Hawke: 
>       Do you remember why the WG disagreed with you?
>    Peter F. Patel-Schneider:
>       Because all Semantic Web langauges have to be same-syntax
>       extensions of RDF. 
>    Frank van Harmelen: 
>       Yes, I must support this. The *only* argument for many WebOnt
>       members to accept/put up with the RDF syntax for OWL was
>       political pressure (perceived or real) from W3C.
>          -- http://www.daml.org/listarchive/joint-committee/1639.html
> I wasn't in the WG for those discussion, and I suspect the history
> isn't as important as the future.   For people developing a rule
> language these are important issues for the future; for this working
> group there may be some important explanations or lessons that
> can be offered, perhaps in response to Prof. Krivov's question.
> Here's a strawman answer:
>    > I completely do not understand why RDF is necessary as an
>    > intermediate layer between XML and OWL.
>    The short answer is that it's not theoretically necessary, but
>    after weighing the options and issues, the working group decided
>    that OWL would be most useful for the web community if constructed
>    in this way.  The abstract syntax for OWL, or the XML syntax you
>    mention, would certainly work for expressing OWL ontologies, but in
>    the end they would not support the evolution of the Semantic Web
>    quite as well.
>    The arguments against they layering are fairly well known, but in
>    terms of theoretical challenges and in terms of parsing, as
>    explored in "Parsing OWL in RDF/XML" [1].
>    On the other hand:
>       (1) Relying on RDF poses no additional burden, because the
>       properties and classes offered by an OWL ontology are offered
>       largely for use in RDF instance data.  This means that
>       programmers, software systems, and users working with OWL can be
>       expected to be already comfortable with RDF and convinced of its
>       value.  OWL users are likely to see RDF as a natural part of
>       their system (and visa versa), while XML itself may be
>       irrelevant to them.  (This does not justify the OWL syntactic
>       layering, but it does help explain why the presence of RDF isn't
>       as expensive as it might first seem.)
>       (2) RDF users may want to use small bits of OWL that fit
>       naturally into their RDF.  Their entire "ontology" may consist
>       of an owl:sameAs triple, or one owl:InverseFunctionalProperty
>       statement.  Why should they have to switch to another language,
>       when these simple bits (like all of RDFS) fit elegantly into
>       RDF?  If they go on to define Restrictions and other complex
>       forms, there is no sudden jump to a new language, just a gradual
>       use of more difficult concepts and constructs.
>       (3) RDF systems are expected to become very sophisticated in
>       merging data from web data sources, with caching, provenance
>       tracking, publish/subscribe features, trust reasoning, etc.
>       If OWL ontologies are just more RDF data, they can more easily
>       provide these services for the OWL data needed in reasoning
>       about the RDF data.
>    There may be more arguments, of course.  It's not clear whether
>    these are the arguments that swayed the working group when it first
>    approached this question.  The group was largely following along a
>    path which included RDFS, DAML-ONT, and DAML+OIL, and it may also
>    have been influenced by the vision of RDF as the data-bus of
>    semantic web [1], but one need not subscribe to these notions to be
>    convinced.
>    The bottomw line is that we expect certain practical advantages to
>    result from this layering approach, and we think they outweigh the
>    difficulties in parsing OWL from RDF/XML.  We expect that in due
>    course these difficulties will be well understood and few people
>    will have to deal with them directly.
>    [1] http://www.w3.org/2000/Talks/1206-xml2k-tbl/slide10-0.html
> Or something like that.  :-)
>     -- sandro
Received on Thursday, 11 March 2004 07:05:20 UTC

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