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Re: cycles in rdfs:subClassOf, cyc gels, extensionality of classes

From: Stephen Reed <reed@cyc.com>
Date: Wed, 27 Jun 2001 12:22:54 -0500 (CDT)
To: Dan Connolly <connolly@w3.org>
cc: www-rdf-logic@w3.org
Message-ID: <Pine.LNX.4.21.0106271148120.895-100000@balrog.cyc.com>
On Wed, 27 Jun 2001, Dan Connolly wrote:

In order to accomodate the various DAML tools available at
http://www.daml.org, I have very recently regenerated the Cyc upper
ontology at: http://opencyc.sourceforge.net/daml/cyc.daml.

I sidestepped the Cyc genls property and rdfs:subClassOf issue by not
stating them as equivalent.  I did however generate a subClassOf property
for each genls link in the Cyc class (collection) hierarchy.

We do not permit cycles in the class (collection) hierarchy.  Cyc's few
coextensional classes are not considered the same class as each other
because class level assertions about them can differ.

> ...
> But more substantively... because cyc's genls is
> explicitly *not* extensional, and rdfs:subClassOf,
> per the DAML semantics, is explicitly extensional:
> 
> [[[
> In addition, two
>      instances of #$Collection can be co-extensional (i.e., have all the
> same elements)
>      without being identical, whereas if two arbitrary sets had the same
> elements, they would
>      be considered equal.
> ]]]
> 
> --        Cyc Fundamental Vocabulary
> http://www.cyc.com/cyc-2-1/vocab/fundamental-vocab.html#Collection
> Wed, 10 Feb 1999 16:51:07 GMT
> 
> vs.
> 
> [[[
> Warning: The RDF Schema specification demands that the subclass-relation
> between
>      classes must be acyclic. We believe this to be too restrictive,
> since a cycle of subclass
>      relationships provides a useful way to assert equality between
> classes. Consequently,
>      DAML+OIL (March 2001) places no such restriction on the subClassOf
> relationship between
>      classes;
> ]]]
> 
> --        DAML+OIL (March 2001) reference description
> http://www.daml.org/2001/03/reference.html#subClassOf-def
> Wed, 11 Apr 2001 19:52:35 GMT
> 
> note the <=>, as opposed to =>
> 
> [[[
> Ax32.    (<=> (PropertyValue subClassOf ?csub ?csuper) 
>            (and (Type ?csub rdfs:Class)
>                 (Type ?csuper rdfs:Class)
>                 (forall (?x) (=> (Type ?x ?csub) (Type ?x ?csuper)))))
> ]]]
> 
> --        DAML+OIL Axioms
> http://www.daml.org/2001/03/axiomatic-semantics.html
> Sun, 04 Mar 2001 17:32:03 GMT
> 
> 
> So... in saying that cyc gels is the same property
> as rdfs:subClassOf, technically, you are licensing
> the inference from
> 	(and (cyc:genls X Y) (cyc:genls Y X))
> to
> 	(daml:sameClassAs X Y)
> 
> Was that on purpose? Were you aware of DAML's interpretation
> of rdfs:subClassOf?
> 
> Perhaps you sort of avoided the issue by making sure
> there are no cycles in the genls tree in the
> ontology you released?
> 
> This is particularly important/timely because the
> RDF Core WG is starting to discuss some nearby issues...
> 
>   Opening RDF Schema issues: rdfs-domain-and-range,
>      rdfs-domain-unconstrained Dan Brickley (Tue, Jun 26 2001) 
>   http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2001Jun/0482.html
> 
> We haven't gotten to this particular issue yet, but
> we're likely to soon:
> 
>   Issue rdfs-no-cycles-in-subClassOf: Cycles of subClassOf properties
> are prohibited.
>   raised Wed, 14 Jun 2000 by Michel Klein
>   http://www.w3.org/2000/03/rdf-tracking/#rdfs-no-cycles-in-subClassOf
> 
> *1 re the syntax misfeature, as evidence that this is
> a known bug/misfeature, and for backlink purposes...
> http://www.w3.org/2000/03/rdf-tracking/#rdfms-qnames-as-attrib-values
> 
> 

-- 
===========================================================
Stephen L. Reed                  phone:  512.342.4036
Cycorp, Suite 100                  fax:  512.342.4040
3721 Executive Center Drive      email:  reed@cyc.com
Austin, TX 78731                   web:  http://www.cyc.com
===========================================================
Received on Wednesday, 27 June 2001 13:22:59 GMT

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