- From: <gstoil@image.ece.ntua.gr>
- Date: Thu, 8 Nov 2007 11:12:41 -0000
- 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? Best, 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 constraints, > 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