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Re: ISSUE-148: RDF Concepts - IRIs do *not* always denote the same resource

From: Guus Schreiber <guus.schreiber@vu.nl>
Date: Sun, 6 Oct 2013 20:59:40 +0200
Message-ID: <5251B31C.8030705@vu.nl>
To: David Booth <david@dbooth.org>
CC: public-rdf-comments <public-rdf-comments@w3.org>
David,

The Working Group thanks again you for your concerns on this important 
aspect of the RDF recommendations, which have been tracked as ISSUE 148.

The wording that you mention "IRIs have global scope: Two different 
appearances of an IRI denote the same resource." is part of the 
introduction to IRIs in RDF.  Even though this introduction is informal 
and non-normative and has to be short, it is in fact very important as 
it sets the tone for the rest of the discussion on IRIs in both Concepts 
and Semantics. The wording is trying bring forward the idea that every 
occurrence of an IRI is the *same* identifier, i.e., IRIs are global 
identifiers.

The first part of the wording says this explicitly, but it was felt that 
some amplification of the point was desirable hence the second part of 
the wording, emphasizing that different occurrences of IRIs are treated 
the same in any formal context.  Your concerns have illustrated that 
this part is not achieving its desired purpose.

The working group has two proposals that might address your concerns:
1/ Remove the second part, and make the first part carry the entire load.
2/ Replace the second part with "Two different appearances of an IRI 
identify the same resource.", which appeals to the non-formal notion of 
identification instead of the formal notion of denotation.

Could you please respond to public-rdf-comments@w3.org as to whether 
either of these changes is satisfactory, and whether you have any 
preferences between them?

Best,
Guus Schreiber
co-chair RDF WG

On 02-10-13 13:23, Guus Schreiber wrote:
> Dear David,
>
> Thanks for your comment. We have raised an issue for tracking your
> comment [1]. We will get back to you on this.
>
> Best,
> Guus, on behalf of the RDF WG
>
> [1] https://www.w3.org/2011/rdf-wg/track/issues/148
>
> On 02-10-13 07:05, David Booth wrote:
>> In https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-concepts/index.html
>> I see this statement:
>>
>>    "IRIs have global scope: Two different appearances of an IRI
>>    denote the same resource."
>>
>> This is wrong.  If it were true then there could never be a URI Collision
>> http://www.w3.org/TR/webarch/#URI-collision
>> and there would be no point in the AWWW discussing it or admonishing
>> against it.
>>
>> An IRI can and often does denote different resources in different
>> *interpretations*.  And this, in practice, means that an IRI often
>> denotes different resources in different *graphs*, because any graph has
>> a set of satisfying interpretations, and different graphs may have
>> different sets of satisfying interpretations.  For example, suppose
>> graphs g1 and g2 have sets of satisfying interpretations s1 and s2,
>> respectively, and those sets may be disjoint.  Then colloquially (and
>> technically) we can say that an IRI may map to one resource in g1 (i.e.,
>> in some interpretation in s1) and a different resource in g2 (i.e., in
>> some interpretation in s2).
>>
>> This requires thinking about graphs in terms of sets of satisfying
>> interpretations -- an important and valid perspective -- rather than
>> assuming that one looks at them only through the lens of a single
>> interpretation.
>>
>> As a simple example of how a URI can denote different things in
>> different graphs, suppose Alice sends this graph G1 from her smart phone
>> to her home computer to turn *on* her porch light (assuming the usual
>> URI prefix definitions):
>>
>> G1: {  @prefix db: <http://dbooth.org/>
>>         ex:alicePorchLight rdf:value db:x .
>>         db:x owl:sameAs ex:on .
>>         ex:on owl:differentFrom ex:off . }
>>
>> and her light turns on.
>>
>> In contrast, Bob sends this graph G2 from his smart phone to his home
>> computer to turn *off* his oven:
>>
>> G2: {  ex:bobOven rdf:value db:x .
>>         db:x owl:sameAs ex:off .
>>         ex:on owl:differentFrom ex:off . }
>>
>> and his oven turns off.
>>
>> It is perfectly reasonable and natural to ask "What resource does db:x
>> denote in G1?", and it is reasonable and natural to ask the same of G2.
>>   The RDF Semantics (along with OWL) tells us that in G1 db:x denotes
>> whatever ex:on denotes, whereas in G2 db:x denotes whatever ex:off
>> denotes.   That is useful!  Furthermore, the semantics tells us that if
>> we merge those graphs then we have a contradiction -- there are no
>> satisfying interpretations for the merge -- and that is useful to know
>> also, because it means that Alice and Bob's graphs **cannot be used
>> together**.
>>
>> Furthermore, the RDF Semantics notion of an interpretation maps well to
>> real life applications: in effect, an application chooses a particular
>> interpretation when it processes RDF data.  This is a very useful aspect
>> of the model theoretic style of the semantics.  In this example, Alice's
>> home control app interpreted db:x to denote "on" and Bob's home control
>> app interpreted it to denote "off".  And *both* were correct (in
>> isolation): they both did The Right Thing.
>>
>> In short, I think the above statement needs to be qualified somehow,
>> such as:
>>
>>    "IRIs are *intended* to have global scope: Two different
>>    appearances of an IRI are *intended* to denote the same resource."
>>    (However, the RDF Semantics explains how an IRI may denote
>>    different resources in different interpretations.)
>>
>> David
>>
>
Received on Sunday, 6 October 2013 19:00:09 UTC

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