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Re: Reconciliation of concerns, re islands and dataset semantics?

From: Ivan Herman <ivan@w3.org>
Date: Thu, 1 Mar 2012 15:14:08 +0100
Cc: Pat Hayes <phayes@ihmc.us>, W3C RDF WG <public-rdf-wg@w3.org>
Message-Id: <68D16E16-8F95-44A9-A4A0-461649A697F9@w3.org>
To: Antoine Zimmermann <antoine.zimmermann@emse.fr>

On Mar 1, 2012, at 14:31 , Antoine Zimmermann wrote:

> Le 01/03/2012 13:41, Ivan Herman a écrit :
>> 
>> On Mar 1, 2012, at 13:31 , Antoine Zimmermann wrote: [snip]
>>>> 
>>>> If (:a owl:sameAs :b) and (:a owl:differentFrom :b) appeared in
>>>> the same graph, then an OWL reasoner, using the definition of
>>>> the predicates, would deduce that there is a inconsistence. I
>>>> mean: the triples themselves are just fine, it is up to a
>>>> reasoner to find the problem.
>>>> 
>>>> If they are in different graphs, then the inconsistence would
>>>> not occur, because we only care about the models in separate
>>>> graphs, independently from one another.
>>> 
>>> Hmm, this seems to contradict what you said above. If URIs are
>>> interpreted identically in all graphs with overlapping
>>> vocabularies, how can :a be interpreted as the same thing as :b and
>>> at the same time as something different then :b? Either you have an
>>> inconsistency, or you interpret the URIs differently in the two
>>> graphs.
>>> 
>> 
>> "Same thing" does not exist in RDF semantics, afaik. The only thing
>> that exist are triples and other triples that can be deduced thereof.
>> In my understanding, it is perfectly fine to put, into the same
>> graph, the triples
>> 
>> (:a owl:sameAs :b) (:a owl:differentFrom :b)
>> 
>> and an interpretation 'I' can map :a and :b onto *different* elements
>> in the target set (or, even, they have to do that?). It is only when
>> an OWL reasoner looks at these triples that it will shout because the
>> semantic condition for an OWL-interpretation are violated if that
>> happens due to the special semantics of sameAs and differentFrom.
>> 
>> If we have
>> 
>> G1 {(:a owl:sameAs :b)} G2 {(:a owl:differentFrom :b)}
>> 
>> The OWL reasoner will look only at I|G1 and I|G2, respectively, and
>> in those constrained environment no inconsistency occurs.
> 
> You said that the interpretation could be an OWL interpretation, right?
> According to the OWL semantics, :a owl:sameAs :b is satisfied iff the interpretation of :a is equal to the interpretation of :b (said differently, :a denotes the exact same thing as :b). :a owl:differentFrom :b is satisfied if the interpretation of :a is different from the interpretation :b (:a denotes a different thing than :b). So there cannot be an OWL interpretation that satisfies both.

Ah. I just checked in the OWL 2 RDF based semantics and indeed you are right. There is a very special case for these two (and their derivatives like owl:AllDifferent):

From Table 5.9 in [1]

( a1 , a2 ) ∈ IEXT(I(owl:sameAs)) iff a1 = a2
( a1 , a2 ) ∈ IEXT(I(owl:differentFrom)) iff a1 ≠ a2

I am not sure how to answer that issue, I must admit...


Ivan

[1] http://www.w3.org/TR/owl2-rdf-based-semantics/
[2] http://www.w3.org/TR/owl2-profiles/#Reasoning_in_OWL_2_RL_and_RDF_Graphs_using_Rules

> 
> Or, maybe, there is something not stated in your proposal that I don't see.
> 
>> 
>> 
>>>> [...]
>>>> 
>>>>> 
>>>>> Now, if you want to do temporal reasoning, provenance, trust,
>>>>> it's more complicated. But the fierceful rejection by Pat on
>>>>> the mere idea of a multi-interpretation semantics has deviated
>>>>> the discussion away from these issues.
>>>> 
>>>> And I do not think this working group should deal with temporal
>>>> reasoning, provenance, or trust. Just giving the basis in terms
>>>> of that semantics is what should be done.
>>> 
>>> I do not mean the WG should provide a standard for temporal
>>> reasoning etc. I just mean that we have to analyse these use cases
>>> in light of the various options we have for defining a semantics of
>>> datasets/quads/multiple-graph structure.
>> 
>> Ok.
>> 
>> Ivan
>> 
>> ---- Ivan Herman, W3C Semantic Web Activity Lead Home:
>> http://www.w3.org/People/Ivan/ mobile: +31-641044153 FOAF:
>> http://www.ivan-herman.net/foaf.rdf
>> 
>> 
>> 
>> 
>> 
> 
> 
> -- 
> Antoine Zimmermann
> ISCOD / LSTI - Institut Henri Fayol
> École Nationale Supérieure des Mines de Saint-Étienne
> 158 cours Fauriel
> 42023 Saint-Étienne Cedex 2
> France
> Tél:+33(0)4 77 42 83 36
> Fax:+33(0)4 77 42 66 66
> http://zimmer.aprilfoolsreview.com/
> 


----
Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
mobile: +31-641044153
FOAF: http://www.ivan-herman.net/foaf.rdf







Received on Thursday, 1 March 2012 14:14:13 GMT

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