- From: Brian McBride <bwm@hplb.hpl.hp.com>
- Date: Thu, 27 Feb 2003 22:35:17 +0000
- To: Bob MacGregor <macgregor@ISI.EDU>, fmanola@mitre.org
- Cc: www-rdf-comments@w3.org
At 13:31 27/02/2003 -0800, Bob MacGregor wrote: >Hi Frank, > >Currently, only you and Pat Hayes have responded (back to me) to the >comments that I posted earlier to RDF-comments, Bob, I hope you don't think that is a bad thing. The editors of each document are handling the comments on their documents. Please don't expect the entire WG to join you in debate. > and those responses did not >include consideration of some of the issues I raised. That is a matter of concern to me. It is our intent to ensure that each issue is properly addressed. Can I assume that any issues that you feel have not been addressed are captured in this message, or are there others you can refer me to? We are a bit behind the schedule that I hoped we would meet. It was my hope that we would have captured all outstanding comments in our comments tracking document by Friday so I could send out a 'sweeper' message ("We think we've got all the issues recorded- if we missed one, please shout"), but we aren't there yet :( Brian > The >discussions with Pat made it clear that the WG charter imposes >constraints that IMHO preclude a satisfactory resolution of the >reification issue. I don't want to tilt endlessly at windmills >that aren't going to yield, so I've pretty much dropped the >discussion. However, if you indeed can create a new issue out >of this, that would be a good thing. Below, I've summarized my >impression of the body of issues related to reification in RDF. > >Below, I first pose some of the key questions; then I summarize >my take on the status of reification in RDF; then I provide my >answers to each of the questions; and finally I make some >summary recommendations. > >1. Should RDF be able to represent statements about statements? >2. Does the current RDF support statements about statements? > >For each of 1 and 2, what kind of semantics do we have in mind: > 3. Should/can RDF express propositional attitudes? > 4. Should/can RDF express provenance information about statements? > >First, some background remarks: > > Propositional attitudes: > The semantics are very hard to pin down here. It is hard to > imagine the WG reaching agreement on what they are in this > go-round (if ever). So, while I was arguing for their inclusion > in RDF a while back, it seems pretty clear that they are not a part > of RDF now (and perhaps forever). > > Provenance information: > The semantics here are much more tractable, both from what > is meant, and what adjustments are needed in the language > to support them. HOWEVER, there is a big hole in the current > RDF support for provenance information: > > Suppose I wish to make a statement that John is the author of > a statement [S P O]. I can write something like: > > S1 type ReifiedStatement. > S1 subject S. > S1 predicate P. > S1 object O. > S1 dc:creator John. > > Support there are two different graphs G1 and G2 that both contain > statements with values S,P,O. Which one is John the author > of, i.e., which one does S1 refer to? We have no idea, because > RDF makes no provisions for identifying which among a set of > statements a reified statement refers to. > > Is this easy to fix? Unfortunately, in order to select among > a set of graphs, we run into another open RDF issue, which is > roughly phrased as "Does a URI that matches an RDF file URI > denote the document or the graph within it?". Resolving that > that issue might be regarded as a prerequisite to resolving the > provenance issue. Perhaps a resolution of the issue of "contexts" > is also a prerequisite. In any event, there is at present no means > for creating a URI that denotes an RDF graph. > >Back to the original question. My impression is that there is a >(non-explicit) >consensus within the WG that the current RDF cannot represent >propositional attitudes, i.e., the answer to >question 3 above is "No". Can RDF represent provenance information >(question 4)? I claim that RDF provides some very simple (and >non-controversial) hooks (the subject, predicate, object properties) >and omits a key notion (the ability to refer to a graph) that is >needed to make the whole provenance notion workable. So, while >from a mathematical standpoint the answer to question 4 might be >"Yes", from a PRAGMATIC standpoint, the answer is "No". > >If one agrees that the answers to questions 3 and 4 are "No" and >"No", then the answer to question 2 is probably "No" also. > >Hence, one possible recommendation (which I posited in an earlier >e-mail) is to drop the entire notion of reification from RDF. However, >it seems pretty clear that this is a non-starter. Hence, I have >some alternative recommendations. > >Recommendations: > > We probably want RDF to support representation of provenance information. > There ought to be an open issue in one of the RDF documents that states > roughly "RDF currently does NOT provide adequate support for > provenance information, but it may in the future." > > Propositional attitudes are probably out of bounds. In that case, > this should be made clear in the documents. Right now the > "I don't believe that George is a clown" discussion leaves the > impression that this kind of propositional attitude is something > that we can say in RDF. I recommend eliminating this from the > Concepts and Abstract Syntax document. The (non-RDF) example > of a nested statement in the Primer further contributes to such an > impression. I recommend rewording that example (Frank has already > acknowledged this latter comment). > > Asserted and non-asserted forms: The RDF documents do not include > any example of a graph that contains both asserted and non-asserted > RDF statements, unless one counts reified statements as providing > an example of (possily) unasserted statements. If that is what is > meant, then this should be stated plainly. If some other notion > (that I can't guess at) is meant, then that should be stated plainly. > Otherwise, the entire section on asserted and unasserted forms > should be eliminated. > >Cheers, Bob > >Ironic note: Because we need to represent provenance information in some >of our RDF applications, and because its not currently supported, >we had to look for other ways to make things work. We have invented >a variant form of context that solves our problem, AND, we like >that solution much better than the reified statement solution. So, >if an RDF WG ever fixes reification, we probably won't use it anyway. > > >At 04:10 PM 2/26/2003 -0500, Frank Manola wrote: >>Brian-- >> >>Can I have an issue for this please? Basically I'm raising this (or at >>least the part about the Primer; the message also has a comment about >>Concepts) to the WG level because I need input from the WG (especially >>Pat, but others may have opinions as well) on how we should handle these >>comments about reification. Pat and I need to be in synch on this in >>order to also deal with Bob's comments on the Semantics document in >>http://lists.w3.org/Archives/Public/www-rdf-comments/2003JanMar/0211.html, >>and we also have issue danc-03 raised in >>http://lists.w3.org/Archives/Public/www-rdf-comments/2003JanMar/0218.html >>that suggests deleting the Primer reification section entirely. And I >>recall some messages from Pat about "propositional attitudes" (I can't >>recall the thread right now) that are pertinent to Bob's comments. >> >>--Frank >> >>Bob MacGregor wrote: >> > >> > Frank, >> > >> > At 03:04 PM 2/13/2003 -0500, Frank Manola wrote: >> > > > >> > >snip >> > > > >> > > > So, how am I recommending that you fix things? Unfortunately, I'm >> mostly >> > > > stating what you should NOT do. I'm claiming that >> > > > using nested syntax will convey the wrong impression to many >> readers (e.g., >> > > > those that model belief they way I did above), so something like an >> > > > EXPLICIT quotation needs to be included. To me, the use of double >> brackets >> > > > didn't adequately convey the notion of quotation. >> > > > >> > >snip >> > > >> > >I will try using a diagram, rather than what appears to be nested >> > >syntax, since we don't support nested syntax (and I didn't intend for >> > >what the Primer uses to be interpreted as nested syntax). However, we >> > >don't really support explicit quotation either, so we can't really >> > >substitute that. Whatever is said in the Primer on reification will >> > >have to be consistent with what is said in the Semantics document, which >> > >means this is related to your message >> > >http://lists.w3.org/Archives/Public/www-rdf-comments/2003JanMar/0211.html >> > >"Statings -- Much ado about nothing". >> > >> > I agree that figuring out a representation is tough. I'm used to KIF, >> where >> > I can say pretty much anything that I want to. Its hard even to have an >> > e-mail conversation about certain aspects of RDF, since the vocabulary >> just >> > isn't available. >> > >> > > > I consider introducing "what we would *like* to be able to do" very >> > > > dangerous. It >> > > > gives the impression that RDF might be used to represent propositional >> > > > attitudes, >> > > > when in fact it can't. I would prefer that the WG be as up front as >> > > > possible about >> > > > stating the limitations it has placed on RDF. >> > > >> > >I don't think the Primer actually conveys this impression, and I think >> > >it tries to be up front about the limitations of RDF reification (it >> > >certainly spends a lot of space talking about them at any rate). >> > >However, I could see adding a caveat at the beginning along the lines of >> > >"you might think you're going to be able to do foo, but watch carefully, >> > >because you can't". Does this make sense? >> > >> > Yes. Again, we have a vocabulary problem. I've seen the phrase >> "propositional >> > attitude" pop up now and again, but the notion of "proposition" is quite >> > difficult >> > to pin down, so that makes it hard to discuss. But, I will try. >> > >> > I think of propositions as being the proper objects of belief. So, >> one doesn't >> > believe in a *sentence* "George is a clown". Rather, one believes in the >> > proposition >> > that that sentence is true. In KIF, I'm used to seeing nested syntax >> > employed when >> > representing statements about belief. Thus, if RDF allowed nested >> > statements, then >> > I would use them to represent belief. But it doesn't, and hence I'm >> thinking >> > that an example that *does* employ nesting gives the wrong impression. >> > >> > However, the following text appears in the Concepts and >> > Abstract Syntax document: >> > >> > > Not every RDF/XML expression is asserted. Some may convey meaning >> > that is partly >> > > determined by the circumstances in which they are used. For example, >> > in English, a >> > > statement "I don't believe that George is a clown" contains the >> words >> > "George is a clown", >> > > which, considered in isolation, has the form of an assertion that >> > George exhibits certain >> > > comic qualities. However, considering the whole sentence, no such >> > assertion is considered >> > >to be made. >> > >> > I'm still waiting to see an example of an RDF statement that is not >> > asserted. If someone >> > could illustrate one, that would be a big help. If, in fact this passage >> > is meant to >> > refer to reified statements, then please write down how this would look in >> > RDF using reified >> > statements. >> > >> > Note: The text above actually uses term "expression" rather than >> > "statement" when it >> > talks about things being asserted. RDF terms are expressions, and >> terms cannot >> > be asserted, so that makes the >> > first sentence trivially true. But I assume that that was not the >> authors' >> > intent. >> > Suggestion: Replace "expression" by "statement" in the first sentence, if >> > that's what's >> > meant. >> > >> > My impression is that its impossible to express the sentence >> > "I don't believe that George is a clown" >> > in RDF. In the OLD RDF, I would have said yes, but not now that we have >> > switched >> > to "statings". If it is acceptable for a stating to serve as the >> object of >> > a statement >> > about belief, then I would have to withdraw my objection. So, can we >> represent >> > beliefs in RDF? >> > >> > Cheers, Bob >> >>-- >>Frank Manola The MITRE Corporation >>202 Burlington Road, MS A345 Bedford, MA 01730-1420 >>mailto:fmanola@mitre.org voice: 781-271-8147 FAX: 781-271-8752 > >Robert MacGregor >Project Leader >USC Information Sciences Institute >4676 Admiralty Way, Marina del Rey, CA 90292 >macgregor@isi.edu >Phone: 310/448-8423, Fax: 310/822-6592 >Mobile: 310/251-8488
Received on Thursday, 27 February 2003 17:34:19 UTC