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Re: [OPEN] and/or [PORT] : a practical question

From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 30 Mar 2004 19:55:36 -0800
Message-Id: <p06001f29bc8fcd741f04@[]>
To: Ian Horrocks <horrocks@cs.man.ac.uk>
Cc: SWBPD list <public-swbp-wg@w3.org>

>On March 24, Christopher Welty writes:
>>  Jeremy wrote on 03/24/2004 04:24:16 AM:
>>  >
>>  > Yes, like Bernard, I have been thinking more about this, and about Ian's
>>  > insistence in WebOnt that classes-and-instances was almost always raised
>>  by
>>  > people wanting to mismodel their world. (cc Ian, wondering if I have
>>  learnt
>>  > my lessons well!, or misrepresented him)
>You can go to the top of the class :-)
>>  Well, "mismodelling their world" is not limited to classes as instances. I
>>  find it rather dangerous to make such statements.  People use subclass
>>  incorrectly, too, but that wasn't a reason to remove that axiom from OWL
>>  DL.
>I would say that there is a big difference. Like any part of the
>language, subClass may occasionally be abused

Arguably, as it is in OWL, for example. The point being that even 
with apparently such a simple idea as 'subclass' there is more than 
one way to legitimately understand what it means (extensionally - 
identical to 'subset', logically equivalent to an implication - or, 
intensionally, analogously to 'subcategory of' - implies but is not 
implied by 'subset', implies but is not implied by a logical 
implication).  The advantages are not entirely obvious either way. 
The extensional notion has an established mapping into FOL, but the 
intensional one has some computational advantages as well.

>, but it is used very
>widely and most people seem able to use it more or less

Actually, I think it is often used with an intended meaning closer to 
the intensional RDFS reading than the OWL-sanctioned extensional one. 
The differences in reasoning only arise in odd corner cases, but the 
fact that the extensional (stronger) consequences are often found 
puzzling or unintuitive itself suggests that intuition is not in 
exact correspondence with what Ian means by 'correct'. And the fact 
that identifying concepts with their class extensions produces 
'paradoxes' has been noted and discussed for almost a century now. 
Like any other example, one can get used to thinking in 'extensional 
class' terms, and once you do then it seems 'correct' and even 
'obvious', but it is important to realize (or remember) that this 
sense of correctness has to be learned, and the learning process 
itself involves taking over an entire way of thinking that is in fact 
at odds with many natural ways of thinking. I agree that extensional 
thinking has its benefits, and it certainly produces neater formal 
logics: but one can agree to all that and still concede that 
intensional thinking is not incoherent and has its own advantages.

>Moreover, subClass fits into a family of logics which are
>theoretically well understood and for which there is considerable
>implementation experience.

True, but the same degree of theoretical understanding is available 
for a variety of other logics, and in some cases they are much more 
closely related to natural-language semantic issues. DLs are a very 
recent invention, after all, and their relationships to intuitively 
adequate conceptual modelling has not been very fully investigated.

>In contrast, classes as instances are relatively rarely used (most
>forms of conceptual modelling, databases etc., seem to have managed
>perfectly well without them), and hardly ever used

I find this an amazingly arrogant claim - although I suppose that I 
should not really be amazed, at this stage - and one that there is 
very little evidence for.   Most of the people I know have been doing 
it successfully for years.

>Moreover, the resulting logics are much less well
>understood and there is little implementation experience.

FOL is hardly 'less well understood' than DLs. If we must have such 
displays of partisanship, please let it be restricted to meaningful 



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Received on Tuesday, 30 March 2004 22:55:28 EST

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