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Re: OWL equivalentClass question

From: Michael Schneider <schneid@fzi.de>
Date: Sat, 14 Jul 2012 10:43:50 +0200
Message-ID: <50013146.5070304@fzi.de>
To: David Booth <david@dbooth.org>
CC: Alan Ruttenberg <alanruttenberg@gmail.com>, <semantic-web@w3.org>
Hi David!

Am 13.07.2012 19:47, schrieb David Booth:
> On Fri, 2012-07-13 at 13:08 -0400, Alan Ruttenberg wrote:
>>
>>
>> On Fri, Jul 13, 2012 at 10:30 AM, David Booth <david@dbooth.org>
>>
>>          So if the blank nodes are skolemized,
>>          http://dvcs.w3.org/hg/rdf/raw-file/default/rdf-concepts/index.html#section-skolemization
>>          then the OWL 2 mapping breaks?  This sounds like a bug in the
>>          mapping algorithm.
>>
>>
>> That would be a bug in the skolemization spec, if there is a bug. The
>> idea is that new stuff is considered a bug if it breaks old stuff, not
>> the other way around.
>
> But that would render skolemization impossible,

?

> and it would conflict
> with the treatment of blank nodes as existentially qualified variables
> http://www.w3.org/TR/rdf-mt/#unlabel
> since it would be like saying "there exists an x, but you're not allowed
> to name x with a URI".

You can do whatever you want. The (reverse) RDF mapping just specifies 
which RDF graphs are legal OWL 2 DL ontologies in RDF graph form and how 
such graphs are mapped into ontologies in the the syntax of OWL 2 DL. 
After being mapped, the resulting OWL 2 DL ontology can then be 
interpreted under the OWL 2 direct semantics. To be clear: the OWL 2 
Direct Semantics doesn't apply to RDF graphs but to OWL 2 DL ontologies 
in the OWL 2 structural specification (or the concrete OWL 2 functional 
syntax). In the OWL 2 DL syntax, there are no blank node for datatype 
restrictions, the blank node is only an artefact of the RDF encoding. 
Hence, in OWL 2 DL (under the OWL 2 direct semantics), there is no 
question whether this blank node should be interpreted as an existential 
variable, skolemized, or whatever, because it simply doesn't exist there.

Of course, you can directly apply the RDF semantics (or its OWL'ish 
semantic extension, the OWL 2 RDF-based semantics) to such an RDF graph, 
which means that you do /not/ understand the RDF graph as an OWL 2 DL 
ontology anymore, but as an RDF (or an OWL 2 Full) ontology. In this 
case, the RDF graph is interpreted literally, and no mapping takes 
place, so the reverse RDF mapping is not at all relevant in this case. 
In this situation, the blank node does indeed represent an existential 
variable, and, if you desire, you can replace it by a URI, which will 
change the RDF graph, but will still have a perfect meaning under the 
RDF or OWL 2 Full semantics. The only "negative" thing that will happen 
then is that the resulting RDF graph is no longer an OWL 2 DL ontology 
in RDF graph form and cannot be mapped into a corresponding OWL 2 DL 
ontology in OWL 2 functional syntax, so it cannot be interpreted under 
the OWL 2 direct semantics. But, as you apparently argue from a strict 
RDF syntax and semantics perspective, this shouldn't be much of a 
concern for you, right?

The bottom line is: The RDF mapping defines an RDF encoding for OWL 2 DL 
ontologies and only uses the RDF /syntax/ for this purpose - the RDF 
semantics doesn't play a role for the mapping, but the meaning of the 
ontology is given by the OWL 2 direct semantics. You could likewise 
define an RDF encoding for Java programs (might look strange, but would 
be possible technically), and you would then probably not insist that 
such an RDF graph gets its meaning from the RDF model theory, would you?

Still, you /may/, of course, apply the original RDF semantics to such an 
RDF graph, and there is even the OWL 2 RDF-based semantics which will 
give you an interpretation of such an RDF graph that is (a) perfectly 
compatible with the RDF semantics, i.e., provides all entailments of the 
RDF(S)-MT, is (b) very similar to the meaning of the corresponding OWL 2 
DL ontology under the OWL 2 direct semantics, and (c) applies to any RDF 
graph, so you can modify the original graph in whichever way you want.

Michael

-- 
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Dipl.-Inform. Michael Schneider
Research Scientist, IPE / WIM

FZI Forschungszentrum Informatik
Haid-und-Neu-Str. 10–14
76131 Karlsruhe, Germany
Tel.: +49 721 9654-726
Fax: +49 721 9654-727

michael.schneider@fzi.de
www.fzi.de

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Vorstand: Dipl. Wi.-Ing. Michael Flor, Prof. Dr. Ralf Reussner,
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Received on Saturday, 14 July 2012 08:44:16 UTC

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