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

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Thu, 23 May 2002 20:14:13 -0400
To: connolly@w3.org
Cc: phayes@ai.uwf.edu, danbri@w3.org, RDF Logic <www-rdf-logic@w3.org>
Message-Id: <20020523201413Q.pfps@research.bell-labs.com>
[Moved from WebOnt to RDF Logic per the direction of the WebOnt chair.]

From: Dan Connolly <connolly@w3.org>
Subject: Re: MISC: Internet Media Type registration: proposed TAG finding
Date: 23 May 2002 17:31:20 -0500

> On Thu, 2002-05-23 at 15:25, pat hayes wrote:
> > On 22 May 2002, Dan Brickley wrote:
> > >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.
> 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.

I fail to see how the intended meaning of dc:title can be considered to be
part of RDF, except for that portion of the intended meaning that is
captured by RDF triples that are available on the web and are readily
retrieved from a well-known place.  To make it otherwise would turn RDF
into an undefined thing.  

For example, I could claim that pfps:Truth is a part of RDF, whose intended
meaning is the type of all true propositions in first-order logic.  Does
mean that RDF captures first-order truth?  Not at all!

> 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.

Sure, but this informal part is *not* part of RDF.

> 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.

If so, then they are seriously deluding themselves that they are doing
RDF.  They are instead doing RDF + what I mean, or RWIM.

> Now likewise, the DAML+OIL spec divides interpretations
> between those that are consistent with it and those
> that are not.

Well, aside from the fact that DAML+OIL interpretations are not RDF
interpretations, sure.

> 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.

Who discovers this, and how?  

> 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.

Sure, but what you have done is *not* RDF, it is RDF plus DAML+OIL.

> That's how DAML+OIL fits into the Resource Description Framework,
> and how I hope/expect OIL will too.

That is, DAML+OIL does not fit into the Resource Description Framework,
instead it extends the Resource Description Framework (or, more accurately,
DAML+OIL could extend the Resource Description Framework if the natural
extension didn't give rise to paradoxes).

> [...]
> > >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.

Sure, an RDFS tool can handle a DAML+OIL document - by treating it as an
RDFS document and completely ignoring the DAML+OIL part.

> 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.

Well, maybe, but such partial understanding is prone to misunderstandings.
(Think of propositional negation.)  I don't see how it could be more gain
than loss, in general.

> [...]
> > >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.

Sure, but the non-understood part may completely change the meaning of the
other part, so the partial understanding may not be related to the ``real''
meaning in any worthwhile fashion.  If that is what you want then go right
ahead, but don't expect me to send any business to a system that acts in
this fashion.

> > 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.

Sure, sometimes you win.  However, you can also lose.

> [...]
> > 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...)
> vocabulary.

Well, maybe, but expecting there to be a useful view is, in my opinion, not
something to expect.

> -- 
> Dan Connolly, W3C http://www.w3.org/People/Connolly/

Peter F. Patel-Schneider
Received on Thursday, 23 May 2002 20:14:23 UTC

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