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Re: a bnode URI scheme?!

From: Sandro Hawke <sandro@w3.org>
Date: Fri, 12 Mar 2004 05:57:55 -0500
Message-Id: <200403121057.i2CAvtOo025452@roke.hawke.org>
To: Adam Souzis <adam-l@souzis.com>
Cc: www-rdf-interest@w3.org


> Yes of course that is not true 

Okay, good....

> but irrelevant. I am talking about RDF's, 
> not OWL's, semantics -- in RDF the meaning of URIs are intensional -- 
> you need a higher level intepretation theory -- like OWL -- to determine 
> equivalence. Thus there is no way in RDF model theory to assert 
> something like
> 
> <a> owl:differentFrom <d> or <a> owl:sameAs <d> 

Whether you can assert it seems to me different from whether it is
true/believed.  In my engineering world view, the semantics of RDF and
OWL Full are the same, give or take an understanding of the terms in
the OWL namespace.

> But bNodes, as existential variables, are extensional -- if they weren't 
> the graph equivalency rule wouldn't make sense, nor could you do simple 
> entailment.
> 
> You could say, who cares, I'm only interested in a (particular) OWL 
> interpretation -- well, I address whether that approach is desirable in 
> my original email.

Sorry if I'm being a bit slow.  Can you explain again the differences
in how some software is supposed-to/entitled-to behave on getting

    <a> <b> <c>
vs
    :_a <b> <c> 

and thus what the effects (disadvantages) are of (consistently)
Skolemizing the graph, or getting the authors to name their conceptual
entities?

> Yes, bNodes are a real pain to reason about, but I'm not sure that implies 
> RDF tool developers should avoid trying to understand them.

No, I agree tools should understand them.  But you seem to be trying
to give authors ways to work around the challenges bNodes present, and
it seems to be better all around to have the authors just not use
bNodes (or link to content which uses bNodes) in awkward ways.

       -- sandro

> -- adam
> 
> Sandro Hawke wrote:
> 
> >>Consider the address bnode example in the RDF Primer 
> >>(http://www.w3.org/TR/2004/REC-rdf-primer-20040210/#structuredproperties).
> >>There may be times when you want to reference that address externally 
> >>(e.g. from another model) but the common sense approach to enable that 
> >>by just replacing the bNode with a URI reference has a couple of problems:
> >>* it changes the meaning of the model: bNodes serve as existential 
> >>variables -- if you replace two distinct bNodes x and y with 2 different 
> >>URIs you are adding information to the model: because there is nothing 
> >>in the model that says x and y might not be equal but the two URIs that 
> >>replace are indeed not equal (since RDF uses intensional semantics for 
> >>URIs).
> >>    
> >>
> >
> >Not true.   
> >
> >I think you're saying 
> >
> ><a> <b> <c>.
> ><d> <e> <f>.
> >
> >entails
> >
> ><a> owl:differentFrom <d>.
> >
> >... but I really don't think that's the case.  Can you find some
> >supporting text?
> >
> >(My recommendation: bNodes are a real pain to reason about, so avoid
> >them unless doing so is an even bigger pain.)
> >
> >       -- sandro
> >  
> >
Received on Friday, 12 March 2004 05:57:07 UTC

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