Re: AW: {Disarmed} Re: blank nodes (once again)

On Fri, 2011-03-25 at 12:19 +0000, Michael Schneider wrote:
> So far, all examples only depend on the existential blank node
> semantics as it is already defined by RDF Simple entailment. The
> larger flexibility and expressive power in OWL 2 Full queries compared
> to RDFS only stems from the additional language features provided by
> OWL 2 Full. However, as said, the semantics specification of OWL 2
> Full itself has some language features that provide certain
> entailments only under existential blank node semantics. One of these
> features is the semantics of /negative property assertions/. Here is
> an example: 
>     If Alice is married with Bob, then she cannot also be married with
> Charly,
>     provided that one can be married with at most one person 
>     and that Bob is distinct from Charly.
>     Premise:
>         ex:isMarriedWith rdf:type owl:FunctionalProperty .
>         ex:alice ex:isMarriedWith ex:bob .
>         ex:charly owl:differentFrom ex:bob .
>     Conclusion:
>         _:x rdf:type owl:NegativePropertyAssertion .
>         _:x owl:sourceIndividual ex:alice .
>         _:x owl:assertionProperty ex:isMarriedWith .
>         _:x owl:targetIndividual ex:charly .
> Here, the new blank node "_:x" is introduced in the conclusion,
> corresponding to an existential variable, following the semantics
> given in
> <>
> So, even if one would dismiss my claim that the above ASK queries are
> useful, then dropping existential blank node semantics would most
> probably break some of the functionality of OWL 2 Full, and would make
> it weaker in expressivity than OWL 2 DL, since the above example is
> also a valid OWL 2 DL entailment.

Pat called this 3rd example out as different, so I wanted to respond to
it particularly.   What I'm proposing (perhaps unlike Pat) is not that
blank nodes be removed from RDF, just that they be disallowed in certain
protocols.   That is, I suggest that the normal way RDF gets published
on the open Web, at least, should be without any blank nodes.   I'm fine
with them being used inside authoring systems, reasoners, etc, but I
think when you go to tell the world a bunch of triples you think they
should know about, it's probably best to avoid using blank nodes.    

This is an idea I've just getting happy with in recent days/weeks, so
maybe there's still a fatal flaw in it, but I think the merits are
considerable, so it's worth some hassle.

     -- Sandro

Received on Friday, 25 March 2011 15:27:28 UTC