Re: RDF Issue rdfs-clarify-subClass-and-instance

A minor nitpick:

From: "pat hayes" <>
> What makes something a class is just that it has some
> things in it (that is, in its extension)

A class can have an empty extension.  If I say:

pk:EmptyClass rdf:type rdfs:Class

And there are no statements like this anywhere in my model:

_:anything rdf:type pk:EmptyClass

Then pk:EmptyClass still qualifies as a class, right?

So I think it's more accurate (and perhaps simpler?) to say, just like in
the Model Theory:

A resource is a class if and only if it has rdf:type rdfs:Class (implicitly
or explicitly).
Every resource that is the object of an rdf:type statement is implicitly of
rdf:type rdfs:Class.
Any resource can be a class (implicitly, explicitly, or both).

Of course, to make a coherent picture with extensions of rdfs:Class, we need
to say that rdfs:subClassOf is transitive, etc.  But I think the rules above
make a solid foundation for newcomers.

        -- P.

  Piotr Kaminski <>
  It's the heart afraid of breaking that never learns to dance

Received on Wednesday, 21 August 2002 17:06:11 UTC