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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: Thu, 5 Dec 2013 12:04:06 +0100
Message-ID: <52A05DA6.8000604@vu.nl>
To: David Booth <david@dbooth.org>
CC: public-rdf-comments <public-rdf-comments@w3.org>
David,

As far as we can see we did not receive a reply from you on this 
response from the RDF WG on ISSUE-148. Could you please indicate whether 
you can live with this?

Best,
Guus


On 06-10-13 20:59, Guus Schreiber wrote:
> 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 Thursday, 5 December 2013 11:04:38 UTC

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