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RE: ISSUE: owl:Class name misleading; try owl:Set?

From: Jim Hendler <hendler@cs.umd.edu>
Date: Thu, 2 Jan 2003 18:12:17 -0500
Message-Id: <p05200f75ba3a7244b9e4@[]>
To: Dan Connolly <connolly@w3.org>, Ziv Hellman <ziv@unicorn.com>
Cc: Jeff Heflin <heflin@cse.lehigh.edu>, "Peter F. "Patel-Schneider <pfps@research.bell-labs.com>, www-webont-wg@w3.org


Whatever the outcome of this discussion is, some points I'd like to 
raise in a chair role:

  1. The WG decided that the Semantics document will contain the 
normative semantics of our language - so what it says goes, or else 
it must be changed.  My preference would be to use what it says 
currently unless we see very strong reason to fix it.

2. once this is resolved, please be sure that other documents are 
fixed to agree with Semantics - that is, in each document, where 
"class" is first defined, and if some point is made about its use, 
please check to be sure you are consistent with the semantics 

3. We made a decision at that owl:class is a subclass of rdfs:class 
but not identical to rdfs:class (per closing of issue 5.22 and the 
"semantic consensus"). The RDF Schema document [1] reads:

>RDF distinguishes between a class and the set of its instances. 
>Associated with each class is a set, called the class extension of 
>the class, which is the set of the instances of the class. Two 
>classes may have the same set of instances but be different classes. 
>For example, the tax office may define the class of people living at 
>the same address as the editor of this document. The Post Office may 
>define the class of people whose address has the same zip code as 
>the address of the author. It is possible for these classes to have 
>exactly the same instances, yet to have different properties. Only 
>one of the classes has the property that it was defined by the tax 
>office, and only the other has the property that it was defined by 
>the Post Office.

so if we decided to go with a purely set based approach, the we would 
not be a subset, but a different animal, and  we would have to reopen 
this discussion and issue (which I'm not inclined to do).

My belief at the moment is that Semantics is consistent with the RDF 
S use of class, and thus I'm reluctant to introduce a different 
semantics at this late date.  I believe it would require a strong 
(STRONG) consensus of the WG if the decision is made to change this 
(esp. as the current also  seems to be consistent with DAML+OIL 
according to Peter in [2]).

I may be misunderstanding the issue, in which case I apologize, the 
above are not meant to force a particular decision but to express 
reluctance by the chair to introduce any major change on something 
this basic.


[1] http://www.w3.org/2001/sw/RDFCore/Schema/200212bwm/
[2] http://lists.w3.org/Archives/Public/www-webont-wg/2003Jan/0011.html


At 15:54 -0600 1/2/03, Dan Connolly wrote:
>On Thu, 2003-01-02 at 13:34, Ziv Hellman wrote:
>>  My two sense:
>>  1. Classes in OWL _ARE NOT_ the same as sets,
>Hmm... if I write YES, THEY ARE, does it convince you?
>i.e. please explain why you think owl:Classes
>are not the same as sets. (I know that rdfs:Classes
>are different.)
>Please provide an example of two owl classes that
>have the same members but differ in some other
>property; i.e. some ?C1, ?C2, ?P, and ?X
>such that
>	?C1 a owl:Class.
>	?C2 a owl:Class.
>	?C1 rdfs:subClassOf ?C2.
>	?C2 rdfs:subClassOf ?C1.
>	?C1 ?P ?X
>but not
>	?C2 ?P ?X.
>>   as the word is generally defined and used in mathematics,
>>  so it would be disastrously wrong to change owl:class to owl:set
>>   -- this distinction ought to be kept clear to the general public.
>>  2. The matter of how misleading sameClassAs can be as mentioned
>>   below is indeed tricky, because it exactly highlights the
>>   intensional/extensional distinction -- thinking of intension
>>   and extension as two functions on the class of classes, we mean
>>   by sameClassAs that the two items have different values from the
>>   perspective of the intension function,
>What's the intension function? I don't think this line of
>argument helps me much. I'm trying to get a strictly
>formal understanding.
>>   but the same value from the
>>   perspective of the extension function, so owl:sameExtensionAs is
>>   more precise. But is it really requiring too much understanding
>>   of logic to use that name when at the same time we expect users
>>   to grok intensional and extensional distinctions if they are going
>>   to use full OWL correctly?
>sameMembersAs seems intuitive and correct.
>Dan Connolly, W3C http://www.w3.org/People/Connolly/

Professor James Hendler				  hendler@cs.umd.edu
Director, Semantic Web and Agent Technologies	  301-405-2696
Maryland Information and Network Dynamics Lab.	  301-405-6707 (Fax)
Univ of Maryland, College Park, MD 20742	  240-731-3822 (Cell)
Received on Thursday, 2 January 2003 18:12:27 UTC

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