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Re: SEM: "natural" entailments

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Sun, 8 Sep 2002 22:50:02 +0100
Message-ID: <15739.50698.575482.796046@merlin.oaklands.net>
To: pat hayes <phayes@ai.uwf.edu>
Cc: Jeremy Carroll <jjc@hplb.hpl.hp.com>, www-webont-wg@w3.org

On September 6, pat hayes writes:
> >
> >I don't see any a priori reason why set theoretic truths have any 
> >greater need to be included in Owl than arithmetic truths.
> >  Both set thery and arithmetic are really somebody else's concern.
> >
> >
> >Jeremy
> One of the things that is so misleading about the entire 
> description-logic way of thinking is that it treats some *logical* 
> truths as set-theoretical truths. Look, to infer
> John is a student and John is an employee .
> from
> John is a student.
> John is an employee .
> is a *logical* matter. It has to do with the meaning of 'and'. It has 
> nothing to do with sets or classes or any of this paraphanalia. But 
> in the warped world of description logics, to assert a simple 
> conjunction requires us to invent a conjunctive class (an 
> intersection) and say that John is in it.  The reason for this 
> intellectual warping goes back to an influential, but I think 
> perverse, publication by Ron Brachman about 20 years ago, which 
> invented the distinction between two 'kinds' of reasoning: that to do 
> with meanings, which is supposed to be done in one place (the 
> 'A-box') and the other to do with facts, supposed to be done 
> elsewhere (the 'T-box'). The same kind of distinction is sometimes 
> expressed by the distinction between 'data' (mere facts, assertions 
> which are true) and 'meta-data' (schemas; things that 'define the 
> meanings' of the terms used in the facts.)  None of these 
> distinctions make any logical sense or have even the slightest basis 
> in semantics: they are pragmatic distinctions invented to facilitate 
> effective use of database technology with simple reasoners. By using 
> them as the basis for the *semantic* framework of the semantic web we 
> are making the entire future of the Web hostage to the intellectual 
> apparatus of a technology designed for the efficient use of 
> regularized, commercial databases, not to the emergent world of a 
> social semantic web.  We should be focussing on the issues that will 
> matter, not on how to transfer old technology into a world where it 
> is probably going to be largely irrelevant.


I know that we can all get a bit overheated sometimes (at least I know
that I can), but I think that it is a bit unfair to take it out on
poor Ron Brachman. One of Ron's main contributions was to point out
that KR languages are pretty useless unless they have a clear and
unambiguous semantics - which is one of the few things that the WG
seems to be in unanimous agreement about.

I should also correct a few factual errors. In modern DLs, if Tbox and
Abox are talked about at all, it is only as a sometimes convenient way
to group different kinds of axiom (by the way, in this context the
Tbox is concerned with "meanings" and the Abox with "facts"). No
"logical" distinction is made between different types of axiom, and
implemented reasoners for expressive DLs treat them in a completely
uniform manner. In languages as expressive as OWL, the distinction is
even more blurred as individuals can be used in class descriptions.

Having said all that, I believe that in some applications it can still
be conceptually useful to separate ontologies (which roughly
correspond to a Tboxes) from assertions that use terms from those
ontologies, e.g., in semantic annotations (which roughly correspond to
an Aboxes).

Finally, I would like to point out (again) that DLs are nothing more
than decidable subsets of FOL with useful computational
properties. So, in as much as FOL is an old technology, yes, DLs are
an old technology (and proud of it).

Regards, Ian

> Pat
> -- 
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Received on Sunday, 8 September 2002 20:22:17 UTC

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