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

From: Dan Connolly <connolly@w3.org>
Date: Wed, 27 Jun 2001 11:26:40 -0500
Message-ID: <3B3A0940.7D1B0AE7@w3.org>
To: Stephen Reed <reed@cyc.com>
CC: www-rdf-logic@w3.org
Stephen Reed wrote:
> 
> In the Cyc ontology ...

speaking of Cyc's ontology and DAML, I was very interested
to see Cyc's upper ontology in DAML:

	http://www.cyc.com/cyc-2-1/cyc.daml

and the surrounding docs:
	http://www.cyc.com/cyc-2-1/toc.html

But this gave me pause:

|  <rdf:Description rdf:about="&o;genls">
|          <samePropertyAs rdf:resource="subClassOf"/>
|  </rdf:Description>

first, because of typo/bug (of a very common variety,
due to an RDF syntax misfeature*1). That should be
	rdf:resource="http://www.w3.org/2000/01/rdf-schema#subClassOf"
or, since you've evidently decided entities aren't too ugly to use,
	rdf:resource="&rdfs;subClassOf"


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

-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Wednesday, 27 June 2001 12:26:51 GMT

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