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Re: OWL equivalentClass question

From: Pat Hayes <phayes@ihmc.us>
Date: Fri, 13 Jul 2012 23:35:09 -0500
Cc: David Booth <david@dbooth.org>, Michael Schneider <schneid@fzi.de>, semantic-web@w3.org, nathan@webr3.org
Message-Id: <68712CA6-805A-4CB7-8347-C9F5C82306C7@ihmc.us>
To: Alan Ruttenberg <alanruttenberg@gmail.com>
Alan has drawn my attention to this thread., which I confess I find rather confusing. 

First, some basics. Regarding skolemization, it is important to remember that skolemization is not a valid inference process, strictly speaking. If you start with a graph G contaning a bnode and skolemize it to get another graph GS where the bnode has been replaced by a URI, then G does not entail GS. 

The relationship between them is subtler: it is that: **if H does not contain the skolem URI **, then G entails H iff GS entails H. Now, GS entails GS, if course, so you might think that this implies that G entails GS, but it doesnt because GS of course *does* contain the skolem URI. 

So, it is not at all surprising that the skolemization of a graph might have logical properties that are not shared by the unskolemized graph. Such a situation does not break RDF entailment, nor does it render skolemization impossible. It just means that you have to use skolemization carefully, as it is not a valid inference mode all by itself; but this always was the case. 

As to the OWL mappings. This is one instance of a general phenomenon, that when a 'higher' language (OWL-DL) is embedded into RDF, there are going to be restrictions on the legal forms that are used to encode the higher language. You will not get perfect freedom to perform even valid RDF entailments on the OWL-DL/RDF without risking making the RDF into something that is no longer a legal RDF encoding of OWL-DL syntax. Put another way, the OWL-DL imposes its own syntactic and semantic retrictions which go beyond those imposed by RDF itself, and engines which need to use the OWL-DL/RDF *as OWL-DL* must be able to respect those OWL-DL-imposed restrictions. This is hardly surprising. The "Full" subfamily is there for people who wish to have complete freedom at the RDF level, but they necessarily pay the price of sacrificing deductive efficiencies available only for the more restricted higher-level language. 

So, overall: nothing here is particularly surprising or alarming, and nothing is (any more) broken (than the world has always been.) 


On Jul 13, 2012, at 1:03 PM, Alan Ruttenberg wrote:

> On Fri, Jul 13, 2012 at 1:47 PM, David Booth <david@dbooth.org> wrote:
> On Fri, 2012-07-13 at 13:08 -0400, Alan Ruttenberg wrote:But that would render skolemization impossible, and it would conflict
> with the treatment of blank nodes as existentially qualified variables
> http://www.w3.org/TR/rdf-mt/#unlabel
> since it would be like saying "there exists an x, but you're not allowed
> to name x with a URI".
> >
> It would be like saying, you can't change an expression "there exists an x" to "x". They don't mean the same thing. If you have "y" then it implies there exists an x. But it doesn't imply "x". Blank nodes, according to the RDF semantics, mean "there exists an x".
> As such, it would seem to break RDF entailment.
> And if this is correct I would expect there to be a formal objection to the proposal.
> Perhaps Micheal could shed some light.
> -Alan
> --
> David Booth, Ph.D.
> http://dbooth.org/
> Opinions expressed herein are those of the author and do not necessarily
> reflect those of his employer.

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Received on Saturday, 14 July 2012 04:35:43 UTC

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