W3C home > Mailing lists > Public > semantic-web@w3.org > July 2012

Re: OWL equivalentClass question

From: Alan Ruttenberg <alanruttenberg@gmail.com>
Date: Sat, 14 Jul 2012 13:15:59 -0400
Message-ID: <CAFKQJ8mLkNVMbkPkS9hFOQUcSe3XtedTEhTUxnoUzx764Lpcig@mail.gmail.com>
To: Pat Hayes <phayes@ihmc.us>
Cc: David Booth <david@dbooth.org>, Michael Schneider <schneid@fzi.de>, semantic-web@w3.org, nathan@webr3.org
On Sat, Jul 14, 2012 at 12:35 AM, Pat Hayes <phayes@ihmc.us> wrote:

> Alan has drawn my attention to this thread., which I confess I find rather
> confusing.

I've used the "i'm confused" thing too. But I doubt you are. It's perfectly
normal for the chair of one group, on seeing a change in a specification on
which it depends, trying to figure out what the implications are for their
spec. Second, David showed a misunderstanding of what the situation was
from a logical point of view, and this made me worry that others (even the
editors of the RDF 1.1 spec) might also share such misconceptions.

> 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 graph GS where the bnode has been replaced by a URI, then G does not entail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.

Good. That's important, as it means that the RDF document needs to specify
in which situations, and with what consequences, skolemization may be done.

> 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.

It can't by what was specified - that the skolem URI should not appear
anywhere else.

> 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.

I will re-read the draft spec to see how this is stated.

> 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.

Agreed for OWL2 under the DL semantics. However OWL according to the RDF
semantics is a different story and that is part of the spec I worry about

> 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.

It is not only engines that may need OWL-DL/RDF, but users of those
engines. A sanction for engines to do skolemization at will will affect
those users inadvertently. Sometimes triple stores are used solely to
*store* OWL.

> 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.

Yes, but they also define OWL using the RDF Semantics as a semantic
extension of RDF (as it existed when the spec was finalized). Changes
thereafter that change RDF semantics will, and should be, examined

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

I'm not so sure. For example, looking at the current draft we see.

"Blank nodes do not have identifiers in the RDF abstract syntax. The blank
node identifiers introduced by some concrete syntaxes have only local scope
and are purely an artifact of the serialization."

It is incorrect that blank notes are purely an artifact of serialization.
In *any*  serialization that I am aware of. Please correct me if I am wrong.


"In situations where stronger identification is needed, systems may
systematically transform some or all of the blank nodes in an RDF graph
into IRIs [IRI]. Systems wishing to do this should mint a new, globally
unique IRI (a Skolem IRI) for each blank node so transformed."

This amounts to, from my point of view, entailing: systems may
systematically change OWL ontologies (under the DL-semantics) stored in
them to become RDF that is no longer an OWL (under the DL semantics)

That sounds bad to me. It definitely sounds more broken then things were
before. Before I could put an OWL ontology into a named graph and get it
out unscathed. Now I can't count on that.

"This transformation does not change the meaning of an RDF graph, provided
that the Skolem IRIs do not occur anywhere else."

This also seems just wrong. Under what sense of "meaning" would this be
true? You say above that this operation is not a valid inference model.

"Systems may wish to mint Skolem IRIs in such a way that they can recognize
the IRIs as having been introduced solely to replace a blank node, and map
back to the source blank node where possible."

Where would it not be possible?
Wouldn't this "feature" better be specified as part of the SPARQL
specification? There you could say that given some keyword, blank nodes in
the result should be skolemized, and that subsequent queries which retrieve
the same blank nodes, asking them to be skolemized, MUST get the same
skolems back each time.

Sanctioning such changes for any process that handles RDF looks to me to be
a bad idea.

I'm happy to hear explanations of how I am wrong in each case I list above
- I'm anxious to learn. But let's stay away from the "I'm confused"
rhetoric, please.


> 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 17:17:00 UTC

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