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Re: layering (5.3, 5.10): Sardinia compromise?

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Sun, 16 Jun 2002 08:10:39 +0100 (BST)
Message-ID: <15628.14831.160462.314930@excalibur.oaklands.net>
To: "Jeremy Carroll" <jjc@hpl.hp.com>
Cc: <www-webont-wg@w3.org>

Jeremy's compromise solution looks promising to me. Of course the
details may need some discussion. 

Ian

On June 14, Jeremy Carroll writes:
> 
> There have been numerous conversations between WG members in Sardiia this
> week.
> Some have been about layering.
> My take is that we have been arriving at a potential compromise.
> 
> The goal of the compromise is to postpone the layering issues rather than to
> solve them.
> i.e. we find a solution that is good enough for OWL 1.0; which satisfies
> nobody, but doesn't go over anyone's threshold.
> 
> In essence the compromise would be to make OWL a weak semantic extension of
> a subset of RDF.
> 
> I will unpack that a little and then give an example:
> 
> RDF defines a large set of RDF graphs; for OWL we syntactically define a
> subset W (or alternatively its complement). OWL then only applies to graphs
> in W. We choose fairly arbitrarily what goes in W and what doesn't. The main
> idea is the things that we can agree on go in W, and those things we
> disagree about don't.
> 
> A possible characterisation of  W is that a graph is in W if there is an
> unproblematic mapping of it into some DL syntax. A possible characterisation
> of the complement of W is that a graph is in the complement of  W if it
> contains any bad triples, where a bad triple is one that doesn't really fit
> into a standard description logic framework.
> 
> The "weak semantic extension" is intended to mean, that as long as we
> restrict ourselves to W then whenever g rdfs-entails h, then g owl-entails
> h. However we are not requirng any other relationship between the semantics.
> In particular if properties in the domain of discourse or type as a property
> cause problems for OWL semantics then we do not require OWL semantics to do
> that just because the RDF model theory is constructed like that. We only
> require the externally visible behaviour (i.e. entailment) to conform with
> RDF.
> 
> Finally an example:
> 
> consider the one triple graph:
> 
> g0:
> eg:a eg:p eg:b .
> 
> This is entirely unproblematic, and there is a wide consensus in the group
> as to its meaning. Thus it is in W.
> (A syntactic characterisation of those conditions is needed).
> 
> Under RDF there are a number of other one triple graphs that are entailed
> 
> g1:
> _:x eg:p eg:b .
> 
> g2:
> _:x eg:p _:y .
> 
> g3:
> eg:a eg:p _:x .
> 
> 
> g4:
> eg:p rdf:type rdf:Property .
> 
> g5:
> rdf:type rdf:type rdf:Property .
> 
> Now, g1, g2 and g3 are also unproblematic and so in W, hence g0 is required
> to owl-entail g1, g2 and g3.
> However, g4 and g5 may be problematic, since the first assumes that
> properties are in the domain of discourse and the second assumes that
> class-membership is in the domain of discourse.
> If we choose not to have these things then we ensure by the definition of W
> that g4 and g5 are not in W.
> Then the question does g0 owl-entail g4 becomes syntactically ill-formed,
> and we don't have to answer it. This is a good thing because we don't agree
> on the answer.
> 
> This compromise would involve indicating to the coordination group that we
> had postponed the issue, and they would need to ensure the relevant people
> do continue to move the layering problem forward; but after OWL 1.0 is
> cooked.
> 
> Jeremy
> 
> 
> 
> 
Received on Sunday, 16 June 2002 03:35:29 GMT

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