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Re: log:forSome/#rdfms-identity-anon-resources

From: Dan Connolly <connolly@w3.org>
Date: Thu, 28 Jun 2001 12:35:41 -0500
Message-ID: <3B3B6AED.A9865D01@w3.org>
To: pat hayes <phayes@ai.uwf.edu>
CC: w3c-rdfcore-wg@w3.org
pat hayes wrote:
> 
> >Graham Klyne wrote:
> >[...]
> > > (1) Unknown binding...
> >[...]
> > > Following this line, any pair of names can bind to the same object in the
> > > domain of interpretation that matches what we know about them, so from your
> > > examples:
> > >
> > > [[[
> > > <http://skolem.example#432oj34oij2o3ijo23j>
> > > <http://purl.org/dc/elements/1.1/title> "Fractals Everywhere" .
> > >   :
> > > ]]]
> > >
> > > and
> > >
> > > [[[
> > > <http://booksRus.example/inv2001-06-25#item342323>
> > > <http://purl.org/dc/elements/1.1/title> "Fractals Everywhere" .
> > >   :
> > > ]]]
> > >
> > > Can be matched by an interpretation in which
> > > <http://skolem.example#432oj34oij2o3ijo23j> and
> > > <http://booksRus.example/inv2001-06-25#item342323> indicate the same value.
> >
> >But that's the case only for *some* interpretations;
> >using the formula in existentially-quantified
> >form, before skolemiziation, *all* interpretations
> >that satisfy the bookseller's database satisfy
> >the query; i.e. going from one to the other
> >is a valid inference.
> >
> > > The problem here seems to be one of computation rather than logic:  if
> > > *any* pair of constants must be tested to see if they match the same
> > > conditions, then the search space becomes impossibly large for a practical
> > > problem.
> >
> >No, it's a problem of logic: going from
> >       (title item34 "f a")
> >to
> >       (title sk1 "f a")
> >is not a valid inference; but going from
> >
> >       (title item34 "f a")
> >to
> >       (exists (?x0) (title ?x0 "f a"))
> >is.
> 
> True, but Graham has a point.

Please let's not go into a tangent about tractable reasoning.
My point is that this very common usage of RDF, describing
something you want, is not possible if RDF parsers skolemize
away all the existentials; i.e. if the abstract syntax
of RDF doens't have existentially quanitified terms in it.

When you say "True", it seems that you are agreeing
with the my position on this issue,
namely, from my message of Mon, 25 Jun 2001 02:44:47 -0500 :

| So an RDF document with anonymous nodes is *not*
| interchangeable with a skolemized version of that document.

It also seems that you agree with me on this issue
when, in your message
of Thu, 28 Jun 2001 10:56:44 -0500, you write
| >In the current RDF specification (that we are chartered to clarify), 
| >is this a purely syntactic issue, or does the "model" also allow for 
| >anonymous resources?
| If "model" means the set-of-triples abstract graph syntax, the answer 
| is surely yes, in the m&s. 

At least a few people in the WG are of the opinion
(when last I heard from them) that the parsers can
just make up constant names/URIs
for all the anonymous nodes in an RDF document without
substantially reducing the utility of RDF; i.e. that
we don't need the _:xyz construct in n-triples at all.

> If the logic has equality then it is
> logically easy to express what you want, ie (= item34 ski),

No, not in this use case:
	* I, book buyer, can't state that, because I
	do not know that the book that I want is
	known, to the bookseller, as item34, when
	I make my query.

	* the book seller can't validly infer that
	if the query comes to him/her in skolemized form.

> but there
> is a computational cost to allowing equality in the logic which is
> exactly what Graham says: the reasoner has be constantly checking to
> make sure that one name isnt equal to every other name, and it gets
> expensive. Classical resolution equality reasoners generate subgoals
> of the form (not (= a b)) like kudzu, which is what led to the desire
> for general principles for rapidly deciding that a wasn't = to b, eg
> the unique-names assumption.

I don't care how much computational horsepower the bookseller
brings to bear on this problem; their inference engine
had better not deduce from

       (title item34 "f a")
and
       (title sk1 "f a")
that
	(= item34 ski)

because it's not a valid inference.

I'm saying that RDF must be able to express

	(exists (?x) (title ?x "f a"))
so that booksellers can deduce from
	(title item34 "f a")
that they have a book that meets the description of the buyer.

-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Thursday, 28 June 2001 13:35:44 EDT

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