- From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
- Date: Tue, 23 Apr 2002 11:44:13 -0400
- To: connolly@w3.org
- Cc: schreiber@swi.psy.uva.nl, www-webont-wg@w3.org
OK, herein is included a completely worked out example, with the WebOnt language stuff in something close to RDF/XML. (I think that it is completely-valid RDF/XML modulo the ellipses, but I'm certainly not an expert in what is and what is not valid RDF/XML.) peter The Web Ontology Working Group is producing a language that can be used to build ontologies. This language is supposed to use the syntax of RDF and to be compatible with the model theory of RDF. However, taking all three of these together results in severe problems. Consider the following RDF graph John rdf:type Student . John rdf:type Employee . as the WebOnt language will include an intersection construct, this graph should entail John rdf:type |intersection Student Employee| . in the WebOnt language (where |intersection Student Employee| is a short form for a unnamed node that is connected to some other RDF triples that carry the intersection construct applied to Student and Employee). However, this cannot be a valid entailment unless there is an object in the domain of discourse of the initial RDF graph that is this intersection. Therefore the model theory for the WebOnt language will have to include comprehension principles for these sort of classes. Many more such examples can be constructed, together requiring a powerful theory of classes in the WebOnt language. This theory is so powerful that it will contain inconsistencies, leading to a complete breakdown of the model theory for the WebOnt language. (Even if the WebOnt language could be expressively limited to disallow the paradoxical classes, the same problem will reoccur at higher expressive levels in the Semantic Web tower.) If, however, the triples that are used to encode the syntax of the constructs of the WebOnt language are unasserted, then these constructs do not need to be part of the domain of discourse. In this case, there is no need for comprehension axioms, and a non-trivial theory can be developed for the WebOnt language. One way to do this is to put WebOnt language triples into separate documents, with a different extension. Such a file might look like File foo.ont <xml ...> <rdf:rdf xmlns:fowl="..."> <fowl:PrimitiveClass fowl:name="Student" /> <fowl:PrimitiveClass fowl:name="Employee" /> <fowl:DefinedClass fowl:name="Student-and-Employee"> <fowl:super rdf:resource="Student" /> <fowl:super rdf:resource="Employee" /> </fowl:DefinedClass> </rdf:rdf> Then information about base objects would be in .rdf files File foo.rdf <xml ...> <rdf:rdf ...> <Person rdf:about="John" /> <Student rdf:about="John" /> </rdf:rdf> What is lost? Well, of course, quite a number of entailments are lost, and the details depend on which triples are unasserted. However, if nothing is done, the empty RDF graph will, in the WebOnt language, entail a contradiction.
Received on Tuesday, 23 April 2002 14:36:58 UTC