- From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
- Date: Sun, 06 Oct 2013 11:44:55 -0700
- 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. 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 18:45:33 UTC