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Re: [issue needed] Re: RDFCore last call WD's: Two comments on the RDF documents

From: Brian McBride <bwm@hplb.hpl.hp.com>
Date: Thu, 27 Feb 2003 22:35:17 +0000
Message-Id: <5.1.0.14.0.20030227222644.09c80d10@localhost>
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 GMT

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