Humans vs the hairless bipeds in OWL

June 29th, 2007

On the W3C Owl Development mailing list there has been an interesting discussion raised by a query from Denny Vrandecic about annotation properties. He asked if ex:A rdfs:label “X” and ex:A owl:sameAs ex:B entail ex:B rdfs:label “X”. The issue is that rdfs:label is defined in OWL as an annotation property. These are special properties that can be used to assert values for classes without requiring the result to be in OWL Full.

The annotation property concept dates back to KL-ONE, I think, and comes with the notion that such properties don’t normally participate in entailments — they’re just extra annotations, useful for properties like lastEditedBy and such.

Denny framed his question as “are annotation property instances connected to the URI or the underlying individual?”. The upshot seems to be that the entailment holds for owl:sameAs but not owl:equivalentClass, which Denny found unintuitive. I thought that an explanation from Pat Hayes was marvelously clear and sheds light on the more philosophical undepinnings of OWL.

“Allow me to suggest the appropriate intuition. In RDFS and OWL-Full there is a distinction between a class and the (set which is the) extension of the class. So two different classes might have the same sets of instances and yet still be distinct classes. (This is often described by saying that RDFS and OWLFull classes are ‘intensional’ as opposed to ‘extensional’. For example, the classes of Human and HairlessBiped have the same members, but one might want to distinguish them since they have different defining conditions on membership. OWL-DL refuses to countenance such a possibility, although this may be rectified in OWL 1.1.) Thus there is an intuitive distinction in meaning between equivalentClass (having the same instances) and sameAs (being exactly the same thing). When, as in RDFS and OWL-Full, classes can have properties, one wants to preserve this distinction by saying that if A sameAs B then all the properties of A are also properties of B (since A and B are the very same thing); but this does not follow for A equivalentClass B, since A and B might still be distinct even if they do have the same members.”

I’m just glad no own brought up birds.