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

From: <gstoil@image.ece.ntua.gr>
Date: Thu, 8 Nov 2007 11:12:41 -0000
Message-Id: <200711081112.lA8BCf9d013340@manolito.image.ece.ntua.gr>
To: Boris Motik <boris.motik@comlab.ox.ac.uk>, public-owl-wg@w3.org, "Carsten Lutz" <clu@tcs.inf.tu-dresden.de>

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?

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

G. Stoilos

> answering conjunctive queries over SHOIQ TBoxes is undecidable if you allow
the query to contain inequalities. Details can be found
> in the following paper, but I can give further explanation if needed.
>     Diego Calvanese, Giuseppe De Giacomo, and Maurizio Lenzerini:
>     Conjunctive query containment and answering under description logics
>     ACM Trans. on Computational Logic, 2007. To appear.
> Finally, DifferentIndividuals ABox assertions are actually inequalities. To
summarize, if you allow nontree-like anonymous
> individuals in DifferentIndividuals ABox assertions, you easily get
undecidability of the basic reasoning problem.
> Even if you were to forbid arbitrary anonymous individuals in the
DifferentIndividuals assertions, ontology entailment would still
> require answering conjunctive queries over DL TBoxes. Currently, we only
know that this problem is decidable for SHIQ; however, it
> is not clear whether this is the case for SHOIQ as well.
> The bottom line is that nontree-like "true" anonymous individuals are
computationally hard, so it might be prudent to avoid them.
> 2. We could interpret anonymous individuals as Skolem constants
> In practice, the semantics of anonymous individuals as true existentially
quantified variables is rarely needed. Usually, anonymous
> individuals are used just as a convenience, saving the ontology modeler from
the trouble of inventing a name for the individual. I
> am not aware of any practical system (OWL or RDF) that implements the
semantics of anonymous individuals as true existentially
> quantified variables.
> Therefore, rather than introducing "true" anonymous individuals, we might
simply interpret them as Skolem constants. In this case,
> we do not need the restriction to tree-like connections, and we could indeed
process a larger fragment of RDF data.
> Here is a concrete proposal how to reflect this in the specification documents:
> - The structural specification would be changed to imbue the Individual
class with an "anonymous" flag. This might be useful for the
> presentation of an ontology.
> - The semantics document would make no special provisions for the anonymous
individuals. Thus, individuals with the "anonymous" flag
> set would be interpreted as all other individuals.
> - The mappings to the XML and RDF syntaxes would be extended to update the
"anonymous" flag correctly during parsing.
> - The semantics of ontology entailment would not change.
> Regards,
> 	Boris

Received on Thursday, 8 November 2007 11:13:41 UTC

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