Re: subclasses (RDF vocabulary definitions)

Richard H. McCullough wrote:

> I don't think intension vs. extension is the issue here; the whole point 
> is that the rdfs:subClassOf property does not rule out the alternative 
> that the two Classes are identical, i.e., subsume the same group of 
> individuals.
>  
> In reality, intension and extension are inseparable.


They may be inseparable, but they're different.  And you've addressed 
the first sentence of my reply:


> 
>     My impression is that the basic problem here is trying to consider
>     subclass as specifying an intensional rather than an extensional
>     relationship.  


but not the rest:

>     In RDFS, a class is a resource that represents the
>     set of
>     things which have that class as the value of their rdf:type property.
>     Given that definition, it certainly could be true that at any given
>     point, the class Man (i.e., the set of things that have class Man as
>     the
>     value of their rdf:type property) could be the same as (have the same
>     members as) the class Animal (the set of things that have class Animal
>     as the value of their rdf:type property).  However, there is no
>     specification that the two classes are (or could be) intensionally
>     identical.  All the Semantics spec (entailment rdfs9) says is that
> 
>     Man rdfs:subClassOf Animal
>     aaa rdf:type Man
> 
>     entails
> 
>     aaa rdf:type Animal
> 
>     It does NOT say that
> 
>     Man rdfs:subClassOf Animal
>     aaa rdf:type Animal
> 
>     entails
> 
>     aaa rdf:type Man
> 
>     Could you cast what you see as the problem is these terms?
> 


To amplify on this a bit, it seems to me that when you say that the 
rdfs:subClassOf property does not rule out the alternative that the two 
Classes are identical, you're effectively saying that it does not rule 
out the entailment, in the above example, that:

      Man rdfs:subClassOf Animal
      aaa rdf:type Animal

      entails

      aaa rdf:type Man

But RDF does not license that entailment.

--Frank




-- 
Frank Manola                   The MITRE Corporation
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Received on Friday, 22 November 2002 09:56:14 UTC