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

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Sun, 06 Oct 2013 11:44:55 -0700
Message-ID: <5251AFA7.1000004@gmail.com>
To: Guus Schreiber <guus.schreiber@vu.nl>
CC: Pat Hayes <phayes@ihmc.us>, RDF Working Group <public-rdf-wg@w3.org>
I suppose that I could send out the message, as I haven't been significantly 
involved, but having a chair send it would be fine by me as well.


On 10/06/2013 11:31 AM, Guus Schreiber wrote:
> On 04-10-13 16:37, Peter F. Patel-Schneider wrote:
>> I believe that this should get some more buy-in from the WG before it is
>> sent out, but I think that someone not involved in the back-and-forth
>> should be sending it out.
> The proposal seems perfectly sensible to me. If you need a neutral person to 
> send it out, I'm happy to volunteer.
> Guus
>> peter
>> On 10/03/2013 07:49 PM, Pat Hayes wrote:
>>> I like this approach and agree that this is a good way to respond.
>>> Which of us should send the official response?
>>> Pat
>>> On Oct 3, 2013, at 11:32 AM, Peter Patel-Schneider wrote:
>>>> Here is my proposal for a response on this topic, sidestepping all
>>>> the technical details, which I believe are not relevant to the
>>>> discussion.
>>>> Greetings David:
>>>> The Working Group thanks 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?
>>>> Peter F. Patel-Schneider
>>>> for the W3C RDF WG
>>>> On Thu, Oct 3, 2013 at 1:36 AM, Pat Hayes <phayes@ihmc.us> wrote:
>>>> This issue requires a more careful response than the others. Here is
>>>> my initial 2c to what is likely to be a slightly extended process.
>>>> On Oct 2, 2013, at 5:43 AM, RDF Working Group Issue Tracker wrote:
>>>>> RDF-ISSUE-148: LC Comment: IRIs do *not* always denote the same
>>>>> resource [RDF Concepts]
>>>>> http://www.w3.org/2011/rdf-wg/track/issues/148
>>>>> Raised by: Guus Schreiber
>>>>> On product: RDF Concepts
>>>>> LC Comment  by David Booth
>>>>> http://lists.w3.org/Archives/Public/public-rdf-comments/2013Oct/0008.html
>>>>> 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.
>>>> First, the AWWW is talking about identification rather than
>>>> denotation, although it does use the "denote" language at one point.
>>>> But I don't expect David to buy this, given his other comments. So,
>>>> second, the point is that even if this can happen in the real world,
>>>> it is a pathological situation, and produces errors and confusions,
>>>> and standards are designed to prevent it happening. So for example,
>>>> one way one might detect a URI collision might be to discover a
>>>> formal inconsistency between two pieces of RDF, each using the same
>>>> IRI to mean different things and therefore making mutually
>>>> inconsistent statements. BUT....
>>>>> An IRI can and often does denote different resources in different
>>>>> *interpretations*.
>>>> THAT is irrelevant to the point being made just previously.
>>>>>   And this, in practice, means that an IRI often
>>>>> denotes different resources in different *graphs*
>>>> No, it does not mean that. In fact, that usage "denote...in different
>>>> graphs" is meaningless. The phrase "denote in a graph" is not used
>>>> anywhere in the RDF specification nor in any other literature on
>>>> related topics, such as logical textbooks on semantics. Denotation is
>>>> defined relative to an interpretation, not relative to a graph or a
>>>> sentence.
>>>>> , because any graph has
>>>>> a set of satisfying interpretations, and different graphs may have
>>>>> different sets of satisfying interpretations.
>>>> That is true, of course, but it does not have the consequence that
>>>> David seems to be claiming that it has.
>>>>>   For example, suppose
>>>>> graphs g1 and g2 have sets of satisfying interpretations s1 and s2,
>>>>> respectively, and those sets may be disjoint.
>>>> So that g1 and g2 are together unsatisfiable, ie no interpretation
>>>> makes them both true.
>>>>>   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).
>>>> No, we may not. Technically, the IRI will typically map to a
>>>> different resource in each different interpretation (not each graph),
>>>> and may map to different resources in the various interpretations
>>>> which satisfy the graph, so the locution "map to a resource in a
>>>> graph" is meaningless. Colloquially, the only way to make sense of
>>>> this kind of a case is to speak of what we can infer from assuming
>>>> that a graph is true, ie that the actual world or situation is one of
>>>> those which satisfy the graph.
>>>>> 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.
>>>> This is just confused. Of course we think about graphs in terms of
>>>> the sets of interpretations which satisfy them. That is what
>>>> model-theoretic semantics is about.
>>>>> 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
>>>> First, note in passing that this is not a typical or intended use of
>>>> RDF, to be a command or scripting language for devices. But in any case:
>>>>> (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.
>>>> Why would that be? According to the RDF semantics, this is an
>>>> assertion about the state of the light. Why would that make the light
>>>> come on? (Does the light have a desire to make any RDF it sees be
>>>> true, maybe?)
>>>> But in case, we can say that in all interpretations which satisfy G1,
>>>> db:x and ex:on co-refer but ex:on and ex:off don't. Which I think is
>>>> the point.
>>>>> 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.
>>>> In all interpretations which satisfy G2, db:x and ex:off co-refer,
>>>> but ex:on and ex:off don't.
>>>>> 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.
>>>> It is incorrect (and meaningless) to ask this question as posed. The
>>>> way it is worded here is confused, and invites a confused answer.
>>>>>   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.
>>>> No, it does not. What RDF and OWL tell us is, that any interpretation
>>>> which satisfies G1 will not satisfy G2, and vice versa. So no
>>>> interpretation satisfies both of them. So they are inconsistent.
>>>>>    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
>>>> Yes, it tells us that, indeed.
>>>>> -- and that is useful to know
>>>>> also, because it means that Alice and Bob's graphs **cannot be used
>>>>> together**.
>>>> Well, they are inconsistent. Maybe you have some way to handle
>>>> inconsistency. But OK, the point is taken.
>>>>> 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.
>>>> I have no idea what this even means. I suspect it is meaningless.
>>>>>   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.
>>>> No, they were not both correct. Their doing the "right thing" in this
>>>> inappropriate scenario is irrelevant. If they were both published as
>>>> data on the open web, then the inconsistency would be rapidly clear
>>>> to basic RDF/OWL engines. And such open publication is the intended
>>>> use case for RDF, not private, idiosyncratic uses in point-to-point
>>>> command transactions.
>>>> OK, thats my initial 2c.
>>>> Pat
>>>>> 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
>>>> ------------------------------------------------------------
>>>> IHMC                                     (850)434 8903 home
>>>> 40 South Alcaniz St.            (850)202 4416   office
>>>> Pensacola                            (850)202 4440   fax
>>>> FL 32502                              (850)291 0667   mobile (preferred)
>>>> phayes@ihmc.us       http://www.ihmc.us/users/phayes
>>> ------------------------------------------------------------
>>> IHMC                                     (850)434 8903 home
>>> 40 South Alcaniz St.            (850)202 4416   office
>>> Pensacola                            (850)202 4440   fax
>>> FL 32502                              (850)291 0667   mobile (preferred)
>>> phayes@ihmc.us       http://www.ihmc.us/users/phayes
Received on Sunday, 6 October 2013 18:45:33 UTC

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