W3C home > Mailing lists > Public > www-webont-wg@w3.org > May 2002

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

From: Dan Connolly <connolly@w3.org>
Date: 23 May 2002 17:31:20 -0500
To: pat hayes <phayes@ai.uwf.edu>
Cc: Dan Brickley <danbri@w3.org>, www-webont-wg@w3.org
Message-Id: <1022193085.14014.185.camel@dirk>
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...)
vocabulary.


-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
Received on Thursday, 23 May 2002 18:31:19 GMT

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