- From: Ian Horrocks <horrocks@cs.man.ac.uk>
- Date: Sun, 16 Jun 2002 08:10:39 +0100 (BST)
- 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 UTC