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Re: Existentially Quantified Variables (bNodes) in RDF-like-languages

From: Graham Klyne <GK@NineByNine.org>
Date: Sat, 06 Apr 2002 07:20:10 +0100
Message-Id: <5.1.0.14.2.20020406065425.04426280@joy.songbird.com>
To: Sandro Hawke <sandro@w3.org>
Cc: www-rdf-logic@w3.org
Sandro,

Thanks for the clarifications.

I think others have posted responses more compelling than I ever could.

One additional response I'll make:
>The fuzziness in #4 is about how a shared naming is established among
>parties communicating via the Semantic Web.  I understand this is
>something philosphers have been debating for a long time.  I have my
>own theory of how it works, but until I convince the world of it, I'll
>still say it's a gray area.  (My theory is you constrain the models
>until you're sufficiently convinced no "reasonable" person could have
>an interpretation materially different from yours.  Dialog makes this
>much easier.  The W3C spec-writing process is all about this, as far
>as I can tell.)

If I'm understanding this correctly, I thought briefly about something 
similar.  Essentially, I think it amounts to having *every* node 
representing something existentially quantified.  Which from a practical 
perspective means that we have no clues about where to start trying to find 
separate graph nodes that denote the same thing.  Having an explicit idea 
of a variable distinct from a named constant at least provides a starting 
point for trying to find additional bindings between nodes.

And there's a point I picked up from your message to Dave Reynolds:
>I have a different Response to Reason 3, which covers your use.  I
>think the Semantic Web consists of (or should consist of) an
>accumulation of assertions.  If you're using RDF for something other
>than asserting facts, then you're using it for an application which is
>out-of-scope to my interests.   (Perhaps I should have defined that
>scope in my original message.)

I think this may be the crux of differing viewpoints.  Speaking for myself, 
when RDFcore were discussing this I started in adherence to the view you 
offer about Skolemization.  And I think it's accepted that if *all* you use 
RDF for is asserting facts then the Skolemization approach is equivalently 
expressive.  But it seemed clear from our discussions that this wasn't the 
whole scope of RDF usage, so we really did need to be able to distinguish 
between "ground" assertions and assertions involving variables in some way 
(otherwise we never would be able to get off the ground).

#g
--


At 06:22 PM 4/4/02 -0500, Sandro Hawke wrote:
> > I think that the existential-conjunctive (EC) subset of FOL that is
> > expressible in RDF is a very interesting waypoint from simple 
> propositional
> > assertions (e.g. a relational database) to full FOL.  It is simple enough
> > to be easily processed, yet rich enough to capture ideas with a degree of
> > abstraction.  A striking example from John Sowa's book Knowledge
> > Representation is the mapping of musical notation to EC logic.
>
>My question is what does EC give you that propositional assertions
>dont?   I know in a formal sense, but I can't think of a semantic-web
>use case.    That may have to do with my vision of how the semantic
>web works -- perhaps this discussion will flush that out.
>
> > I'm interested in your claim that RDF query languages don't use this
> > approach.  To the extent that they are expressible in basic RDF, I cannot
> > see how this can be.
>
>An RDF query language certainly involves some kind of variables --
>whether existentially or universally quantified is largely a matter of
>perspective -- but that language is not itself RDF, and those features
>don't need to be in RDF.   The RDF query languages I've seen do NOT
>use bNodes as their query variables; they use their own sort of query
>variables.
>
> > Your point 4 doesn't make sense to me.  Using existentials is not a matter
> > of being fuzzy;  it seems to me a case of being very precise that the
> > particular identity or instance of some resource is unknown.
>
>The fuzziness in #4 is about how a shared naming is established among
>parties communicating via the Semantic Web.  I understand this is
>something philosphers have been debating for a long time.  I have my
>own theory of how it works, but until I convince the world of it, I'll
>still say it's a gray area.  (My theory is you constrain the models
>until you're sufficiently convinced no "reasonable" person could have
>an interpretation materially different from yours.  Dialog makes this
>much easier.  The W3C spec-writing process is all about this, as far
>as I can tell.)
>
>So it's quite clear, yes, that using an existential means you are not
>using a name for some object, but it's *not* perfectly clear what
>using a name for an object means.  It means at least that if the same
>name is used by two parties, they are by definition talking about the
>same object.  But since we can use UUIDs to prevent that from
>happening accidentally, I'm at a loss to find an application scenario
>where existentials are particularly useful.
>
>I should add Reason #5, paraphrase from Dave Beckett on IRC: in some
>computing environments (handhelds) it may be impossible to generate
>good UUIDs.  My response is that it's okay to have existentials in
>some serialization languages, for these cases and for human authors,
>but to have the model encourage Skolemization as soon as it's
>practical.
>
>Reason #6, reading between the lines of something else Dave said, is
>that it's harder to debug parsers which are doing Skolemization,
>because the output is harder to compare.   I don't have an answer to
>that one right now, but I don't think it's a compelling reason in and
>of itself.   We can help parser writers without complicating the model
>theory.
>
>     -- sandro

-------------------
Graham Klyne
<GK@NineByNine.org>
Received on Saturday, 6 April 2002 04:15:49 GMT

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