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

From: Frank Manola <fmanola@mitre.org>
Date: Thu, 27 Feb 2003 19:15:07 -0500
Message-ID: <3E5EAA0B.3010503@mitre.org>
To: Bob MacGregor <macgregor@isi.edu>
CC: www-rdf-comments@w3.org, macgreg@isi.edu

Bob--

This last message of yours summarizing the situation helps a lot to 
clarify things, in my opinion.  First of all, you should understand that 
all this messaging is part of a process, which says that either we as 
individual editors decide we can handle an issue as a simple editorial 
change to our individual documents, or we have to refer it to the 
working group as an "issue".  That's what I requested be done with the 
issues you'd raised about reification, for the reasons I mentioned in my 
message.  The process is designed to explicitly respond to comments, and 
make sure to deal with them in some kind of structured manner.  So we 
couldn't really "drop" anything from our end unless you agreed to "drop" 
it too.  In other words, I wasn't trying to raise a new issue, I was 
trying to resolve an old one, by saying the working group needed to 
decide how to handle this.

Speaking for myself, my summary reply to your summary analysis is 
something like this:

1.  I agree 100% with you about the difficulty of the semantics of 
propositional attitudes.  You can, of course, write triples with 
something like ex:believes as the predicate, but that's not the same 
thing.  We're not going to get into those semantics.

2.  I agree with you about the the problem with provenance, i.e., the 
inability (at least, in terms of a built-in RDF facility) to explicitly 
identify specific statements.  Of course, if you (externally to RDF) 
have a way of assigning URIrefs to individual RDF statements, everything 
is cool. (See below for more on this).

3.  We don't explicitly say that we don't support propositional 
attitudes.  To do that, we may have to explain what the problem is 
(e.g., why the semantics of a predicate like ex:believes are hard to pin 
down), which could get a bit complicated.  As far as the Primer is 
concerned, I've thought about adding a short bit about this (basically 
saying these things are propositional attitudes, they're complicated, 
and go off and read the following references to see why).

***Do you think that type of material on propositional attitudes is 
absolutely necessary in the Primer?***  (Alternatively, how bad would it 
be if the Primer didn't cover that?) Note that I'm speaking just for the 
Primer.  If you think this needs to be explicitly mentioned in any other 
document, you probably should explicitly indicate that/those other 
document(s).

4. Regarding provenance, I'm a little confused.  The whole last half of 
the Primer section on reification basically says exactly what you've 
said;  that the reificiation mechanism per se has no way of identifying 
the individual triples it describes.  There is, for example, a paragraph 
starting "This does not mean that such 'provenance' information cannot 
be expressed in RDF, just that it cannot be done using only the meaning 
RDF associates with the reification vocabulary."  It does on to explain 
that if you have a way of assigning URIs to individual statements, you 
can make statements about those statements without needing to use 
reification at all.

The point here is that RDF as we started working on it had a reification 
vocabulary with multiple interpretations.  We decided not to change the 
vocabulary this go-round (too many different views of what was wanted, 
possibly outside our charter as requiring too extensive a fix), and 
we're reporting on the appropriate interpretation of what's there.  Note 
that we can't just drop reification.  My organization, for one, has 
people using it (that is, they defined a way to assign URIs to 
statements, and use the reification vocabulary to describe the 
statements).

   My confusion boils down to this (to try to nail things down *very 
precisely*):  are you saying the Primer:

a.  doesn't explicitly say that you can't really do provenance with only 
the built-in facilities in RDF?  (I sure think it does)

b.  isn't clear enough about this? (I thought at least the paragraph I 
mentioned above, and the surrounding text, was pretty clear)

c.  doesn't say why reification is still in RDF, even though it doesn't 
do provenance the way some people thought it did?  (The Primer doesn't 
talk about that, and I'm not sure it should.  We're trying to describe 
the facilities of a specific language, not why the language isn't some 
other language.)

Given your choice of zero, one, or more of a, b, and c above, what 
specifically would you like changed in the Primer?

Again, thanks for the comments.  This last message was very helpful (at 
least to me).

--Frank

PS:  I'm still going to change the "apparent nested statement" example 
in the Primer as we've discussed already.




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, 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.
> 


-- 
Frank Manola                   The MITRE Corporation
202 Burlington Road, MS A345   Bedford, MA 01730-1420
mailto:fmanola@mitre.org       voice: 781-271-8147   FAX: 781-271-875
Received on Thursday, 27 February 2003 18:55:22 GMT

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