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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 20:31:38 +0200
Message-ID: <5251AC8A.3010007@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 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.


> 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 18:32:06 UTC

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