I would like to bring to discussion/debate what I believe is a flaw in the design of the OWL web ontology language, at least if OWL will also serve a purpose in representing large scale ontologies. I'll start the discussion by listing some general examples of OWL 'properties' from the OWL tutorials: HasFinger, HasSpouse, HasTopping, HasVintageYear, HasBrother, HasParent, HasUncle, HasMaker, HasSugar, etc ... We have a problem with this, seeing that these are not properties, but concatenated Property - Target combinations. For a large scale (formal) ontology, imagine the amount of duplication would occur if we would treat every parthood relation as such; HasFinger, HasToe, HasNail, HasHair, HasSkin, HasVein, HasCell, HasBone, HasMuscle, etc... In large scale ontologies, one link should suffice, HasPart, and whether the part is a finger, toe, nail, muscle or anything else is not a task for the property to describe, but for the target, e.g. the ISA hierarchy where the respective parts are situated in. Now this might seem trivial, and it might seem that OWL allows the second approach as well, but the fact is that OWL propagates this line of thought in how properties should be constructed and restricted: Some of the existing expressivity of OWL (mainly: assigning cardinalities) can not be applied if we decide to generalize property types (formal modeling) to represent the nature of the relation, e.g. without a description of what the target is (which is a task for the target hierarchy).
Lost expressivity: cardinality. In OWL optional properties can be used by setting mincardinality to 0: <Hand> Has6thFinger mincardinality 0 See how in the above example we do NOT state a target. As a matter of fact the above named property only has a meaning when the range is specified, in this case the range might be <6th finger>. So we can use optional properties, but only if we create a unique property for every property - target combination. In formal ontology you could express this relation on a general level of parthood: <Hand> HasPart <6thfinger>, cardinality 0. This is not possible in OWL. In OWL absence of properties can be modeled by setting cardinality to 0: <Undiagnosed hyperbilirubinemia> IsDiagnosisParticipant cardinality 0 <Untreated hyperbilirubinemia> IsTreatmentParticipant cardinality 0 Again in these examples we do NOT state the target, and again these above stated examples might be referring to anything, the only way we (and a reasoner) are able to figure that the properties are about <Diagnosis> and <Treatment> is if we set the range of those properties as such. In formal ontology you would express this relation on a general level of participation: <Undiagnosed hyperbilirubinemia> IsParticipantOf <Diagnosis> Cardinality 0 <Untreated hyperbilirubinemia> IsParticipantOf <Treatment> Cardinality 0 Conclusion: OWL offers great expressivity, but only IF we use the above mentioned approach of proliferating redundant properties at a large scale. This is not a luxury we have in (formal) modeling of large scale ontologies. As an example, the core domain of LinkBase (our inhouse formal, medicine-oriented ontology) exists of about 700.000 concepts (classes) which are related to each other with more than 700.000 relation instantiations. The OWL approach would lead to hundred thousands of properties, which in many ways would duplicate the classhierarchy. It would be great to receive some feedback on this matter. David Decraene PH.D. BioMedical sciences/Ontology modeller. Language and Computing
