- From: Jeremy Carroll <jjc@hpl.hp.com>
- Date: Tue, 21 Jan 2003 11:15:05 +0100
- To: www-webont-wg@w3.org
I have made, a possibly successful, case to my HP colleagues that OWL is about documents: - syntactic forms - semantic consistency The WG seemed minded to restrict our concerns to precisely those when it came to specifying software. In particular there was no desire to specify any more complicated reasoning component, such as one that could actually run our entailment tests. So we have: - a semantics that permits discussion of the meaning of a single document (specifically its consistency) - a semantics that permits discussion of the relationship between two documents (entailment) - three syntactic levels What I have heard we want to have as CR exit criteria is adequate exploration of the first and third of these; and not much on the second. This could possible be better reflected by making the entailment part of the semantics informative rather than normative. Possible text near the beginning of the AS&S: [[ OWL specifies an RDF vocabulary and three syntactic levels: OWL Lite, OWL DL, OWL Full, each of which specifies a set of RDF documents. OWL normatively specifies through the use of model theoretic semantics the consistency of any such document. The model theoretic semantics also permits the discussion of entailment relationship between pairs of documents. These relationships are informative. In particular, implementors of OWL Consistency Checkers are free to use alternative semantics as long as these are logically equivalent to an OWL Consistency Checker built using the semantics specified here. No other constraints on the semantics of OWL implementations are normatively specified. ]] (with appropriate 'normative' and 'informative' markers elsewhere) The motivation here is that much of the difficulty HP has with the current specification of OWL is to do with what is expected of OWL implementations, and an apparent mismatch between that and the sketch of CR exit criteria created at the f2f. Specifically we do not intend to migrate Jena to the OWL Semantics any time soon, nor do we see competing systems planning such a migration either; nor do we see a WG intending to make the migration of such a system to OWL (and its semantics) a CR exit criterion. To give a specific example, the comprehension principle is part of OWL semantics, i.e. that various classes and restrictions necessarily exist. These can be turned into consistency tests at the OWL Full level but not at OWL DL or OWL Lite. e.g. <owl:Restricition> <owl:onProperty> <owl:ObjectProperty rdf:ID="p"/> </owl:onProperty> <owl:hasValue> <owl:Thing rdf:ID="v"/> </owl:hasValue> <rdf:type> <owl:Class> <owl:complementOf rdf:resource="&owl;Class"/> </owl:Class> </rdf:type> </owl:Restriction> This says that a particularly restriction is not an OWL Class, and this is inconsistent. This: (a) seems plausibly straight forward to implement (I wouldn't be surprised if Euler already could prove this) (b) is anyway in OWL Full where we are being explicit about the lower implementability goals. The alternative OWL DL entailment: <owl:ObjectProperty rdf:ID="p"/> <owl:Thing rdf:ID="v"/> DL-entails <owl:Restricition> <owl:onProperty> <owl:ObjectProperty rdf:ID="p"/> </owl:onProperty> <owl:hasValue> <owl:Thing rdf:ID="v"/> </owl:hasValue> </owl:Restriction> seems to present more difficulty, particularly in a forward chaining environment, and the WG does not seem minded to want to see even one complete OWL DL reasoner that could do this before exiting CR. (I note that a unionOf example would be a lot less compelling, partly because both the entailment and the inconsistent file are at the same OWL DL level) Jeremy
Received on Tuesday, 21 January 2003 05:14:40 UTC