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

From: Uli Sattler <sattler@cs.man.ac.uk>
Date: Thu, 8 Nov 2007 11:27:01 +0000
Message-Id: <853A9F38-E9CA-4566-B530-73B972AE4F06@cs.man.ac.uk>
Cc: Boris Motik <boris.motik@comlab.ox.ac.uk>, public-owl-wg@w3.org, "Carsten Lutz" <clu@tcs.inf.tu-dresden.de>
To: <gstoil@image.ece.ntua.gr>

On 8 Nov 2007, at 11:12, <gstoil@image.ece.ntua.gr> wrote:

> Boris Motik <boris.motik@comlab.ox.ac.uk> said:
>> Hello,
>> The OWL 1.1 Member Submission does not contain anonymous  
>> individuals for the
> reasons I explain below. These reasons are related to
>> ISSUE-46: Unnamed Individual Restrictions
> (http://www.w3.org/2007/OWL/tracker/issues/46). It might make sense  
> to discuss
> both issues
>> together.
>> In short, we did not include the anonymous individuals into the  
>> Member
> Submission because they significantly affect computational
>> aspects of the logic (explained under item 1 below). Furthermore,  
>> anonymous
> individuals are usually used in practice with a weaker
>> semantics (explained under item 2 below). Therefore, we did not  
>> introduce
> anonymous individuals in the Member Submission and wanted
>> to discuss this in the working group.
>> 1. Why can nontree-like "true" anonymous individuals be dangerous?
>> Nontree-like "true" anonymous individuals in the ABox cause  
>> undecidability
> of ontology entailment, which is the basic inference
>> problem for OWL. An ABox containing anonymous individuals can  
>> actually be
> understood as a conjunctive query. It is well known that
> Hi Boris,
> Is it a conjunctive query or a union of conjunctive queries?

Hi Giorgos,

this dangerous stems from single conjunctive queries: see


> BTW, can you explain more how you can view anonymous individuals as  
> CQs?

simply because, if you allow them in an ontology, then you can reduce  
entailment of CQs to entailment between ontologies: simply view the  
CQ as an ontology with anonymous individuals!

The reason why skolem constants are more harmless is because they are  
simply names for domain elements like normal constants (but the  
"anonymous individuals as skolem constants" would free you from  
having to invent a proper name for them)  -- and, when you are trying  
to see whether an interpretation is a model of an ontology, you don't  
need to find an appropriate mapping!

Cheers, Uli

> Best,
> G. Stoilos
Received on Thursday, 8 November 2007 11:27:13 UTC

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