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

From: Bob MacGregor <macgregor@ISI.EDU>
Date: Thu, 27 Feb 2003 13:31:47 -0800
Message-Id: <5.1.1.6.0.20030227132048.00b81360@tnt.isi.edu>
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 GMT

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