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Re: [ontolog-forum] When to use pure subclassing and when to use subclas

To: "[ontolog-forum] " <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Christopher Menzel <cmenzel@xxxxxxxx>
Date: Mon, 27 Jul 2009 14:20:29 +0200
Message-id: <34796CC9-5BC1-44B2-8617-A393F317AA5A@xxxxxxxx>
On Jul 26, 2009, at 8:17 PM, Dave McComb wrote:
> We teach a class in OWL.  My sense from our students confusion is  
> that the main point of departure between what you understand and  
> what's occurring in an ontology comes down to the distinction  
> between intensional and extensional sets.    (01)

Dave, I think the distinction here is better cast as the difference  
between intensional and extensional *definitions* of sets.   
"Intensional set" seems to me to be an oxymoron, as sets are by  
definition extensional, i.e., Set1 = Set2 iff every member of Set1 is  
a member of Set2.  However, one and the same set can be *defined*  
intensionally, i.e., by means of a description (e.g., {x | x is a US  
Senator from Texas}) that might in fact pick out *different* sets at  
different times, or extensionally, i.e., by simply listing its members  
(e.g., {John Cornyn, Kay Bailey Hutchison}).    (02)

-chris    (03)

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