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

On 12/13/2013 02:23 PM, David Booth wrote:
> Hi Guus,
> I'm sorry, I was holding off on replying to that previous email, pending
> a long conversation with Pat Hayes, in which I hoped to reach a common
> view on this issue.

I mean to include a link, as the discussion was in the www-archive list;

> But apparently I have failed.  :(
> No, I cannot live with this.  The current draft of the RDF Concepts says:
>    "IRIs have global scope: Two different appearances of an
>    IRI denote the same resource.
> and that is simply misleading and false, as explained here:
> Please let me know what I can do as a next step toward resolving this
> satisfactorily.  Or, alternately, let me know what I should do to
> present this as a formal objection.
> Thanks,
> David
> On 12/05/2013 06:04 AM, Guus Schreiber wrote:
>> 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 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]
>>>> On 02-10-13 07:05, David Booth wrote:
>>>>> In
>>>>> 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
>>>>> 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: <>
>>>>>         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 Friday, 13 December 2013 19:29:43 UTC