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

From: Graham Klyne <GK@ninebynine.org>
Date: Thu, 04 Apr 2002 20:28:54 +0100
Message-Id: <>
To: Sandro Hawke <sandro@w3.org>
Cc: www-rdf-logic@w3.org
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.

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.

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.


At 12:52 PM 4/4/02 -0500, Sandro Hawke wrote:
>This is an old issue.  Pat Hayes explained very nicely a year ago [1] that
>RDF M&S has always had existentially-quantified variables (in the subject
>role), and to take them out would be a change.  Many others (including myself
>at the time) supported this position, including the RDF Core WG.  Some
>agreed that's what M&S said, others simply thought existentials would
>be good to have.
>But I want to revisit the question more fundamentally.  What good are
>existentially quantified variables in a language like RDF? [2] Why would we
>possibly want them?  I no longer know a good reason.
>Here are four reasons I have heard for them, and my counter-arguments.
>Reason 1:  Because people often express themselves without giving URIs for
>            things.  Using pronouns like "something" feels like a very natural
>            and comfortable way to communicate.
>            Response: While we may not explicitely have names for things, the
>            ability of natural language to "go meta" means you can always talk
>            about "the thing you meant when you said '...'".   So we can still
>            identify the denotations of terms used in conversations, even 
> if no
>            explicit identification was given at the time.
>            Alternatively, who cares.  This is a formal system, and any 
> appeals to
>            human qualities are likely to be misleading anyway.
>Reason 2:  We were getting a lot of noisy URIs in our knowledge bases.
>            Response: This isn't really a model-theory issue.  A
>            best-practices approach might be to suggest that urn:uuid's be
>            used for any identifiers which are both not meant for humans and
>            not tied to any on-the-web knowledge base.  User interfaces
>            shouldn't be showing URIs anyway, but they could hide these even
>            more.
>            Another approach is to have meta-data, with triples saying that
>            the URI "urn:uuid:57c259ca-47ed-11d6-8002-0050ba4812a6" is not
>            meant for humans, or something.   (Note that such an assertion
>            must be about the above string, not the object denoted by that
>            string.   That object may be denoted by many URIs, some of which
>            are prettier than others.)
>Reason 3:  Because we can use existential variables for querying RDF.
>            (Pat suggests this in [1].  It was my reason a year ago.)
>            Response: I've seen a lot of RDF query languages now, and this
>            approach is not being used.  In my own query language designs,
>            I've found them unsatisfactory.
>Reason 4:  One should be able to commit to a statement like "Someone on E-Bay
>            is selling a signed copy of 'Weaving the Web'."  That statement is
>            different from "Someone on E-Bay is selling the signed copy of 
> 'Weaving
>            the Web' which has the URI http://example.com/copy7".  The second
>            one entails the first, of course.
>            Response: Why do you want to make the first statement?  What bad
>            thing will happen if you call it
>            <urn:uuid:57c259ca-47ed-11d6-8002-0050ba4812a6> ?
>            Are you worried that someone else will say something about
>            <urn:uuid:57c259ca-47ed-11d6-8002-0050ba4812a6>?  Like "it's
>            missing page 7?"  But they could do that anyway (with 
> existentials)
>            by using various more-expressive ontologies, perhaps talking about
>            all the books on E-Bay, or talking about the terms in your
>            original statement.
>            When you used a URI for E-Bay, were you committing to any
>            facts?  Which ones?     I think that's the real issue here.
>            I think a URI enables a client to learn more about a thing,
>            but the use of a URI in one document cannot be taken to
>            mean that someone asserting the document believes any facts
>            outside the document.
>            ... but then how do you know what was meant?  If I write
>            you a check for $100, how do you know if I meant $US or
>            $Canadian?  The fuzzy vision of the Semantic Web involves
>            the meaning of URIs being established by some kind of
>            consensus reality, I think.  But how does one commit to a
>            statement in such fuzzy terms?
>            Anyway, maybe existentials will be useful as being free
>            from consensus reality issues, but I don't see how.  I want
>            to see the argument you present to a judge when you're
>            suing someone for breech-of-RDF-assertion.  Maybe it comes
>            down to the natural-language comments in the ontology.  Do
>            you need to explicitely commit to an ontology, or just use
>            terms from it.  What if two ontologies exist with the same
>            URIs?  Which one are you committing to?  I think the only
>            solid ground is to explicitely import facts (including
>            natural-language documentation) from some URI, giving a
>            secure hash of the contents if you're worried it might
>            change.   (but that's not the way RDF is being used today...)
>The reason to leave them out is that they complicate things.
>Its worth noting, too, that if you really want to make an existential
>statement, you can do it in some higher-layer logic language.  Of course, it
>wont be understood by as wide an audience.  But that brings us back to
>original question: why would you want someone to understand it?
>     -- sandro
>[1] http://lists.w3.org/Archives/Public/w3c-rdfcore-wg/2002Apr/0014.html
>[2] I don't want to get too into what a language-like-RDF might mean, so
>     let's just say RDF/XML without anonymous resources, or N-Triples without
>     namedNodes, and hope that gets the point across.

Graham Klyne
Received on Thursday, 4 April 2002 15:04:21 UTC

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