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Re: ISSUE 131 (OWL R Unification): Different semantics on syntactic fragment

From: Ian Horrocks <Ian.Horrocks@comlab.ox.ac.uk>
Date: Tue, 12 Aug 2008 16:08:18 +0100
Message-Id: <161E1572-906F-448B-ADB3-6D4A8A774F56@comlab.ox.ac.uk>
Cc: <public-owl-wg@w3.org>
To: Michael Schneider <schneid@fzi.de>


Past design errors and infelicities do not add up to a good reason  
for us to engage in bad design.

I note that in spite of your claims to be arguing only from a  
technical point of view you have started to introduce pejorative  
remarks such as "this might look inconvenient from a description  
logics perspective". This is nothing to do with DL. We already have  
two semantics, one of which is an RDF semantics. These two semantics  
are very closely related (in terms of entailments). You want to  
introduce a third one that isn't at all closely related to either of  
the other two. The only motivation for this seems to be to allow  
implementations based on the OWL R rules to be able claim to be sound  
and complete w.r.t. this semantics. This isn't a particularly  
enviable claim, and it completely misses the point of OWL R, which is  


On 12 Aug 2008, at 15:05, Michael Schneider wrote:

> I will answer this question:
> Ian Horrocks wrote:
>> Michael,
> [...]
>> As I understand it, your proposed "solution" to this problem is to
>> retain OWL RL Full,
> Yes.
>> and make its semantics be defined by the rule set
> Yes.
>> (which, as a result of a very poor decision we made that the 2nd F2F,
>> is implicitly the case in the existing spec).
> Indeed, that's how I understand the status quo (I disagree that  
> this was a
> poor decision).
>> This means introducing a 3rd semantics for OWL:
> Without wanting to dive into this recurrent "this is counter  
> intuitive"
> argument again, I want to only say this:
> The W3C recommendations "Simple Entailment", RDF, RDFS and OWL 1  
> Full all
> have pair wise different semantics, and all of them will have a  
> different
> semantics compared with OWL 2 Full. And that's also the case for  
> pD*, and
> all of Jena's rule based OWL sublanguages on the non-standards side.
> "Different semantics" means that for every two such languages there  
> always
> exists an RDF graph, which is interpreted differently by the two  
> languages.
> It's a distinguishing property of RDF based languages that they differ
> w.r.t. their *semantics*. Unlike description logics, RDF based  
> languages
> cannot be distinguished by their respective syntax, because they  
> always have
> the same syntax: RDF.
> This might look inconvenient from a description logics perspective.  
> But it
> is natural from an RDF semantics perspective. And it's not uncommon  
> for
> certain other families of logics, either, btw. So, nothing "absurd"  
> is going
> on here with a "third semantics", at least not technically.
> Cheers,
> Michael
> --
> Dipl.-Inform. Michael Schneider
> FZI Forschungszentrum Informatik Karlsruhe
> Abtl. Information Process Engineering (IPE)
> Tel  : +49-721-9654-726
> Fax  : +49-721-9654-727
> Email: Michael.Schneider@fzi.de
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Received on Tuesday, 12 August 2008 15:08:58 UTC

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