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Re: Interpretation of RDF reification

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Thu, 23 Mar 2006 10:44:56 -0500 (EST)
Message-Id: <20060323.104456.31963442.pfps@research.bell-labs.com>
To: henry.story@bblfish.net
Cc: brian.mcbride@hp.com, larsga@ontopia.net, semantic-web@w3.org

From: Henry Story <henry.story@bblfish.net>
Subject: Re: Interpretation of RDF reification
Date: Thu, 23 Mar 2006 16:36:35 +0100

> 
> On 23 Mar 2006, at 15:58, Peter F. Patel-Schneider wrote:
> > Date: Thu, 23 Mar 2006 13:47:17 -0000
> >>>
> >>> I don't understand the difference.  RDF just does not have the  
> >>> expressive
> >>> power to do this sort of thing.   It just like asking whether  
> >>> propositional
> >>> logic could express something like "All students are people."
> >>
> >> I think that answers my question.
> >> I'm confused about how semantics of [OWL] relate to the semantics  
> >> of RDF.
> >
> > Roughly in the way that propositional logic relates to predicate  
> > logic (at
> > least if you think of propositional logic in a certain way).  There  
> > are
> > "more" constructs in OWL, and these constructs have meaning that  
> > cannot be
> > expressed in RDF.  The only strangeness is that the extra OWL  
> > constructs
> > are actually written as (collections of) RDF triples, but  
> > nonetheless the
> > added expressive power is still real.
> 
> 
> I hope that is wrong. 

Nope, it is correct.

> My understanding is rather that RDF gives you a  
> framework.

Well, just *what* is a framework?

> OWL just gives you specialised relations with certain specific  
> inferential properties.

Not really.  OWL in RDF gives you much more than specialized relations.
Much of the power of OWL comes from syntax that is more than just single
relations.  It so happens that it is possible to embed OWL in RDF in a
certain manner.  (This is not always possible, by the way, and OWL is very
close to the maximum expressiveness that can be so embedded in RDF.)

> So if you take the relation owl:inverseOf then this is just an [RDF]  
> relation.

Well, sure, owl:inverseOf is *just* an RDF property, in RDF.  In OWL, on
the other hand, owl:inverseOf is a special property - it has a extra
meaning provided by the OWL semantics.

> But it is linked to the following well known rule:
> 
> { ?r1 owl:inverseOf ?r2 .
>    ?a ?r1 ?b .  } => { ?b ?r2 ?a . } .

Not all all.  There are *no* rules in RDF, nor in OWL.  The above is not
even legal RDF syntax, nor legal OWL syntax.  

> All of the [OWL] vocabulary can be defined in terms of such rules. The  
> OWL terms have been very carefully selected to make certain types of  
> inferencing easier. But otherwise OWL just is a specialised vocabulary.

Not at all.  OWL is a a different logical language from RDF.  It just
happens that OWL (Full) was designed as a same-syntax extension of RDF
Schema.

> Henry

peter
Received on Thursday, 23 March 2006 15:45:18 UTC

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