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Re: MISC: Internet Media Type registration: proposed TAG finding

From: Jim Hendler <hendler@cs.umd.edu>
Date: Thu, 23 May 2002 21:22:26 -0400
Message-Id: <p05111707b913463c3afa@[]>
To: Dan Connolly <connolly@w3.org>, pat hayes <phayes@ai.uwf.edu>
Cc: Dan Brickley <danbri@w3.org>, www-webont-wg@w3.org
Please take this to RDF-logic, the issue is not open.

At 5:31 PM -0500 5/23/02, Dan Connolly wrote:
>On Thu, 2002-05-23 at 15:25, pat hayes wrote:
>>  On 22 May 2002, Dan Brickley wrote:
>>  >On 22 May 2002, Dan Connolly wrote:
>>  >
>>  >>  >  I ask
>>  >>  > that the WebOnt WG discuss whether to send a polite note 
>>back rejecting
>>  >>  > this interpretation of our work.
>>  >>
>>  >>  I don't think we should.
>>  >
>>  >FWIW, Peter's dissatisfaction with my note (which wasn't addressed here)
>>  >is noted.
>>  >
>>  >I continue to regard the WebOnt language (and the RDF 1.0 syntax, and it's
>>  >MT, and RDFS) as a component of the wider Resource Description Framework,
>>  What "wider RDF"?? I've heard phrases like this before, but they seem
>>  to refer to a secret W3.org ritual, because nobody is able to tell
>>  the rest of us what they are supposed to mean.
>I suppose he meant RDF plus all the applications of it: RDFS, RSS,
>PRISM, XMP, dublin core, etc. Nothing secret about it:
>"The Resource Description Framework (RDF) integrates a variety of
>applications from library catalogs and world-wide directories to
>syndication and aggregation of news, software, and content to personal
>collections of music, photos, and events using XML as an interchange
>syntax. "
>   -- http://www.w3.org/RDF/
>>  I take the phrase 'Resource Development Framework' to refer to a
>>  rather limited database language based on triples, as defined in the
>>  documents being produced now by the RDF Core WG. If it means
>>  something else, will someone PLEASE say CLEARLY what that other thing
>>  is?  I would like to know in case I'm supposed to be writing a model
>>  theory for it.
>'Clearly' is in the eye of the beholder, but I'll try...
>The RDF model theory is an important part of the semantics
>of RDF document but it's not the whole thing. The
>important parts from the RDF MT:
>   * it explains to the community how formal languages work;
>   i.e. that each RDF document divides the possible
>   worlds/interpretations into those
>   interpretations that agree with it and those that don't.
>   * it licenses erasure and existential introduction
>   as inference rules for the whole framework.
>so take something like...
>	<rdf:Description rdf:about="http://www.w3.org/">
>		<dc:title>bananas and pears</dc:title>
>		<dc:date>2002-05-23</dc:date>
>	<rdf:Description>
>The RDF MT says that anybody who agrees/commits to
>that also agrees to stuff derived by erasure, e.g.
>	<rdf:Description rdf:about="http://www.w3.org/">
>		<dc:date>2002-05-23</dc:date>
>	<rdf:Description>
>and stuff derived by existential introduction; e.g.
>	<rdf:Description>
>		<dc:date>2002-05-23</dc:date>
>	<rdf:Description>
>But that's not all there is to it. dc:title is a term
>with widely deployed semantics/meaning/definition/specification.
>The dublin core folks have some reasonably clear notion
>of which interpretations are consistent with its
>intended use and which are not.
>In particular, in my interpretation of the world I currently
>live in, it's false that "bananas and pears"
>is a dc:title of http://www.w3.org/. i.e.
>the pair (http://www.w3.org/, "bananas and pears")
>isn't in the extension of the dc:title property
>in any of the specified interpretations.
>I think most folk would agree with me, especially if
>they had read the dublin core title spec (i.e. the document
>you get by dereferencing the full URI of the term,
>http://purl.org/dc/elements/1.1/title) and the W3C
>home page.
>So there's some sense of "meaning of a document"
>which is: it limits interpretations to the
>intersection of the interpretations
>that are RDF-core-MT-consistent with that document,
>and consistent with all documents
>that you get by looking up the terms used
>(formally, as properties) in the document, and looking
>up the terms used in those documents, and so on, until
>you ground out in informal/prose documents. These
>informal prose documents, e.g. the dublin core spec,
>still have semantics: they still divide interpretations
>into true and false.
>I don't think that notion of "meaning of a document"
>is specified very well, but I think
>it's what most RDF authors/implementors have
>in mind.
>Now likewise, the DAML+OIL spec divides interpretations
>between those that are consistent with it and those
>that are not.
>If I say
>	:age rdf:type ont:UniqueProperty.
>	:bob :age "10".
>	:bob :age "20".
>and I investigate its meaning in the Resource Description
>Framework, I start with the conjunction of the three
>facts there, and then I'll look up rdf:type; its
>spec tells me the extension of rdf:type is computed
>from the class extension of its object; so I go
>and look up ont:UniqueProperty, and I discover
>that its class extension is properties whose
>subjects determine their objects uniquely.
>But the RDF MT tells me that "10" and "20"
>denote distinct things, so there isn't any
>way to satisfy the combination of this
>document, the rdf:type spec, and the
>ont:UnambiguousProperty spec, no matter
>what specification for :age I might find.
>i.e. this document, combined with the
>specifications for the terms it uses,
>is false.
>That's how DAML+OIL fits into the Resource Description Framework,
>and how I hope/expect OIL will too.
>>  >How about we try to think about this issue in forward-looking rather than
>>  >backward-looking terms?
>>  >
>>  >Given RDFS and WebOnt, we're looking at partial understanding in terms of
>>  >RDFS-aware tools dealing with with WebOnt-enriched RDF Schemas (er,
>>  >Ontologies).
>>  I'm not sure what that means, but I think it is wrong. That is, I
>>  would not expect an RDFS-aware tool (which, by the by, is more than
>>  an RDF-aware tool) to be able to handle WebOnt. (If it could, why are
>>  we bothering to develop WebOnt? We could just all use RDFS.) So an
>>  RDFS-aware tool will NOT be able to handle WebOnt-enriched RDF
>>  Schemas, even if (as seems highly unlikely) WebOnt could even be
>>  expressed as 'enriched' RDF Schemas.
>An RDFS tool can handle a document that uses WebOnt terms
>much more gracefully than a version 2 word processor
>usually handles version 3 documemtns: halt and catch
>fire totally.
>e.g. given the age 10/20 example above, an RDFS-capapble
>tool might not detect the inconsistency by reducing
>the possible interpretations to none, but it can
>tell that in all satisfying interpretations,
>:age has rdf:type rdf:Property; it can derive
>conclusions by erasure and existential introduction,
>by subPropertyOf and subClassOf rules, etc.
>That's partial understanding.
>>  >At the instance data level, all this shouldn't matter. (Thankfully, for
>>  >the poor end users...)
>>  It has to matter. If someone marks up their webpage using WebOnt,
>>  then an RDF engine isn't going to be able to understand it, right?
>It will understand it partially.
>>  This isn't rocket science: all of computation is like this.
>No, some computation degrades gracefully. Try looking
>a the W3C home page, written in XHTML 1.0 circa 1999, with
>a web browser written in 1994. I think you'll find
>a remarkable degree of fidelity.
>>  Well, if you use WebOnt then it will be a WebOnt document, rather
>>  than an RDFS document. (Why do I even need to say things like this,
>>  for God's sake?)
>But it won't stop being an RDF schema just because
>you use WebOnt (or RSS, or dublin core, or XMP or prism...)
>Dan Connolly, W3C http://www.w3.org/People/Connolly/

Professor James Hendler				  hendler@cs.umd.edu
Director, Semantic Web and Agent Technologies	  301-405-2696
Maryland Information and Network Dynamics Lab.	  301-405-6707 (Fax)
AV Williams Building, Univ of Maryland		  College Park, MD 20742
Received on Thursday, 23 May 2002 21:22:40 UTC

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