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Re: ISSUE-3: REPORTED: Lack of anonymous individuals

From: Jim Hendler <hendler@cs.rpi.edu>
Date: Thu, 8 Nov 2007 10:42:05 -0500
Message-Id: <90D79F2E-1C32-4BFD-952A-7C4C78CD10BB@cs.rpi.edu>
Cc: OWL Working Group WG <public-owl-wg@w3.org>
To: Boris Motik <boris.motik@comlab.ox.ac.uk>

Apologies - Boris pointed out to me that he did have an example in an  
earlier message - it was at the end of a long message, so I have cut  
it out and pasted it below - Thanks Boris!


 From Boris:

Let me give you a concrete example. Assume that O1 contains the  
following ABox assertion:

(13) hasParent(Bob,Mary)

As long as O2 contains named individuals (Bob, Mary, and so on), you  
will get exactly the same answers. Now let O2 be an ontology
containing the following ABox assertion:

(14) hasParent(Bob,_:1)

Here the difference becomes important. Under the standard "true"  
semantics, O2 follows from O1. This is because, in first-order
logic, hasParent(Bob,Mary) entails \exists x : hasParent(Bob,x).

Under the approximative semantics, O2 *does not* follow from O1. This  
is because (14) is actually equivalent to the following ABox
O2':

(14) hasParent(Bob,some-invented-constant)

Now in first-order logic, it is not the case that hasParent(Bob,Mary)  
entails hasParent(Bob,some-invented-constant), so O1 does not
entail O2.




The last example might suggest that the approximative semantics might  
be undesirable. I would just like to point out, however, that
the anonymous individuals in the ABox make ABoxes a kind of a query  
language. It might be cleaner not to mix the two roles of an
ABox. An ABox might be thought of as just the data; for querying, we  
might use languages such as SPARQL, which would then be
designed appropriately such that we get everything we want (such as  
the correct answer to (14)) without the drawbacks. For example,
if we modify SPARQL to disallow inequalities in the query, we can be  
decidable for SHIQ.

Regards,

	Boris
Received on Thursday, 8 November 2007 15:42:26 GMT

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