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The Meaning of OWL


by Peter F. Patel-Schneider

Abstract

The W3C Web Ontology Working Group is continuing work on the Web Ontology Language, OWL. The syntax of OWL is mostly determined, but the semantics of OWL has been the subject of continuing debate, with an agreement-in-principle on the general outlines of the semantics of OWL being decided only in early October 2002. The semantics of OWL will be determined by means of two model-theoretic semantics, a more-complex semantics for general OWL knowledge bases and a simpler one for OWL knowledge bases that meet certain syntactic restrictions. These two semantics coincide on the restricted OWL knowledge bases.

What is OWL?

  1. A very-expressive frame formalism?
  2. A very-expressive description logic?
  3. A hybrid modal logic?
  4. A useful fragment of some first-order logic?
  5. An extension of RDF and RDFS?
  6. All of the above?

What is the Syntax of OWL?

Frames (like OIL)?
Class(Student partial Person 
  restriction(institution 
              allValuesFrom(EducationalInstitution))
  restriction(ID someValueFrom(xsd:Decimal)))
Description Logics?
Student ≤ Person ∩ 
          &forall institution : EducationalInstitution ∩
          &exist ID : xsd:Decimal

What is the Syntax of OWL? (continued)

RDF Triples (like DAML+OIL)?
<Student rdf:subClassOf _:x> 
<_:x owl:intersectionOf _:l1>
<_:l1 rdf:first Person> <_:l1 rdf:rest _:l2>
<_:l2 rdf:first _:r1> <_:l2 rdf:rest _:l3>
<_:l3 rdf:first _:r2> <_:l3 rdf:rest rdf:nil>
<_:r1 owl:onProperty institution>
<_:r1 owl:allValuesFrom EducationalInstitution> 
<_:r1 owl:onProperty ID>
<_:r1 owl:someValuesFrom xsd:Decimal> 

What are the inferences of OWL?

What are the inferences of OWL? (continued)

What are the inferences of OWL? (continued)

What kind of Semantics to use for OWL?

Possible Kinds of Semantics for OWL

Possible Kinds of Semantics for OWL (continued)

Possible Kinds of Semantics for OWL (continued)

The Semantics of OWL (Desiderata)

The Semantics of OWL

Two-part Semantics:
  1. An extension of RDF semantics to handle OWL KBs that
    • use classes as instances
    • mix individuals and data values
    • play with RDF or RDFS or OWL vocabulary
  2. A standard model theory for OWL KBs that
    • don't use classes as instances
    • don't mix individuals and data values
    • don't play with the RDF vocabulary
    • i.e., that look like Description Logic KBs

The Semantics of OWL (continued)

What if the two semantics disagree?

Benefits of the Two-Part Semantics