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

From: Guus Schreiber <guus.schreiber@vu.nl>
Date: Sun, 6 Oct 2013 21:01:02 +0200
Message-ID: <5251B36E.60707@vu.nl>
To: "Peter F. Patel-Schneider" <pfpschneider@gmail.com>
CC: Pat Hayes <phayes@ihmc.us>, RDF Working Group <public-rdf-wg@w3.org>


On 06-10-13 20:44, Peter F. Patel-Schneider wrote:
> 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.

Response has been sent.
Guus

>
> peter
>
> 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 19:01:30 UTC

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