From: Drew McDermott <drew.mcdermott@yale.edu>

Date: Wed, 11 Feb 2004 18:53:09 -0500 (EST)

Message-Id: <200402112353.i1BNr9i15804@pantheon-po02.its.yale.edu>

To: www-rdf-logic@w3.org

Date: Wed, 11 Feb 2004 18:53:09 -0500 (EST)

Message-Id: <200402112353.i1BNr9i15804@pantheon-po02.its.yale.edu>

To: www-rdf-logic@w3.org

> [Eric Jain] > Is there a way to indicate that a Class should be abstract, i.e. that no > direct instances are allowed to exist? Owl classes are like set-theory classes, not like OOP classes. There is no notion of "direct instance." In mathematics, a class or set is nothing but a ... bunch or ... group or ... (hmmm) set of objects. If x is an element of set S, it's pointless to try to find a set S' such that x elt S' and S' subset S. There will always be such an S', usually quite a large number. If you change the "subset" to "proper subset," then the only time there can fail to be such an S' is if S = {x}. In OOP, classes aren't classes of objects, just convenient packages of methods and such. An object x can be an instance of S', and S' be a subclass of S, without x being an instance of S in any useful sense. Of course, it's an instance by fiat, but what I mean is that if there is a set of properties that is intended to hold for elements of S and fail to hold for elements outside of S, then all those properties could fail to hold for x. -- -- Drew McDermott Yale Computer Science DepartmentReceived on Wednesday, 11 February 2004 18:53:12 UTC

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