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

From: pat hayes <phayes@ai.uwf.edu>
Date: Mon, 26 Aug 2002 00:35:48 -0700
Message-Id: <p05111b06b98f8a75d643@[65.212.118.251]>
To: Piotr Kaminski <piotr@ideanest.com>
Cc: www-rdf-comments@w3.org

>A minor nitpick:
>
>From: "pat hayes" <phayes@ai.uwf.edu>
>>  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.

Yes, you are absolutely correct. I forgot about empty classes. Sorry 
if my email was misleading.

Pat
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Received on Monday, 26 August 2002 13:32:52 GMT

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