- From: Bob MacGregor <macgregor@ISI.EDU>
- Date: Thu, 27 Feb 2003 13:31:47 -0800
- To: fmanola@mitre.org
- Cc: www-rdf-comments@w3.org, macgreg@ISI.EDU
Hi Frank, Currently, only you and Pat Hayes have responded (back to me) to the comments that I posted earlier to RDF-comments, and those responses did not include consideration of some of the issues I raised. 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 16:33:27 UTC