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

From: Peter Patel-Schneider <pfpschneider@gmail.com>
Date: Thu, 3 Oct 2013 09:32:25 -0700
Message-ID: <CAMpDgVwCxx_eXXuQnrQe6HdVqK_43APX5WuSPuwcc5Vb+iLTJA@mail.gmail.com>
To: Pat Hayes <phayes@ihmc.us>
Cc: RDF Working Group <public-rdf-wg@w3.org>
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
> >
> >
> >
> >
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Received on Thursday, 3 October 2013 16:32:53 UTC

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