Re: OWL2 RDF mapping and skolemization [was Re: OWL equivalentClass question]

Correction . . .

On Sun, 2012-07-15 at 19:10 -0400, David Booth wrote:
> Hi All,
> 
> Thanks for your comments.  To get to the crux of the matter . . . 
> 
> On Fri, 2012-07-13 at 23:35 -0500, Pat Hayes wrote:
> > [ . . . ] If you start with a graph G containing a bnode and skolemize
> > it to get another graph GS where the bnode has been replaced by a URI,
> > then G does not entail GS. 
> 
> Unless we're making a closed world assumption, that sounds wrong to me,

I read Pat's statement backwards, so my response above said the opposite
of what I meant (as Peter pointed out).  What I *meant* was that I think
GS should entail G . . . 

> because giving a (guaranteed new) name to a previously unnamed node is
> merely *adding* information -- no information has been removed -- so all
> previous entailments should still hold.  If the semantics are not
> currently defined that way then it seems to me that we've made a
> mistake.  
> 
> After owl:sameAs entailments are applied to both graphs, it seems to me
> that this graph G1:
> 
>   _:bnode :p 5 .
>   _:bnode owl:sameAs :x .
> 
> should have the exact same semantics as this graph G2:
> 
>   :x :p 5 .
>   :x owl:sameAs _:bnode .
> 
> and the same thing should be true of RDF graphs that happen to encode
> OWL2 constructs.
> 
> The point of mapping OWL2-->RDF-->OWL2 is not merely pedantic, it is to
> allow OWL2 constructs to be used and manipulated by standard RDF tools
> -- including doing inference, graph merging, etc. -- and then allow
> specialized OWL2 tools to be used on that subset of the result that
> matches OWL2 idiomatic encoding patterns.  
> 
> Unless information has been *removed* from the graph, this should
> include all encoding patterns that were produced when the original OWL2
> was mapped to RDF, but it may also include OWL2 encoding patterns that
> were intentionally created during RDF processing.
> 
> Alan mentioned (about Skolemizing an OWL2 encoding in RDF):
> 
> > It would be like saying, you can't change an expression "there exists
> > an x" to "x". They don't mean the same thing.
> 
> Agreed, because skolemization does *add* information to the graph.  But
> sameness is not the goal, although there should be a certain kind of
> *equivalence* between an unskolemized graph and its skolemized version
> -- basically, the same semantics after ignoring the names of the
> skolemized nodes.
> 
> But beyond that, it should always be possible to *add* information to an
> RDF graph that contains OWL2 encodings, and still have those encodings
> be recognized when mapping back from RDF to OWL2.  If we cannot do that
> then I don't think we have the specs quite right yet.
> 
> 

-- 
David Booth, Ph.D.
http://dbooth.org/

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Received on Monday, 16 July 2012 02:18:20 UTC