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Re: need to determine what RDF is

From: Dan Brickley <danbri@w3.org>
Date: Thu, 30 May 2002 19:11:17 -0400 (EDT)
To: patrick hayes <phayes@ai.uwf.edu>
cc: Dan Connolly <connolly@w3.org>, "McBride, Brian" <bwm@hplb.hpl.hp.com>, <www-rdf-comments@w3.org>, <em@w3.org>, <pfps@research.bell-labs.com>, <guha@guha.com>
Message-ID: <Pine.LNX.4.30.0205301846280.16322-100000@tux.w3.org>


+cc: guha

On Thu, 30 May 2002, patrick hayes wrote:

> >On Thu, 2002-05-30 at 11:10, patrick hayes wrote:
> >>  >On Thu, 2002-05-30 at 10:26, Peter F. Patel-Schneider wrote:
> >>  >[...]
> >>  >>  I'm only interested in relationships between RDF graphs.  Which such
> >>  >>  relationships are RDF relationships?
> >>  >>
> >>  >>  My view is that the only such relationships are RDF entailment and RDFS
> >>  >>  entailment.  Any agent that computes any other relationship between RDF
> >>  >>  graphs is not doing RDF.
> >>  >
> >>  >Why is RDFS special? It's just the first of many RDF vocabularies,
> >>  >no?
> >>
> >>  Its more than just an RDF vocabulary because it has some extra
> >>  semantic conditions.
> >
> >
> >That doesn't look special, to me; I expect each vocabulary
> >to come with some extra semantic conditions.
>
> Wait. Each vocabulary comes with some content that is expressed in
> the RDF graph(s) that use that vocabulary, sure. But that doesn't
> extent the model theory; it doesn't add any *extra* semantic
> conditions, it just leverages the semantic conditions that the
> language spec provides.
>
> >  > For example, if RDFS is considered purely as an
> >>  RDF vocabulary, then rdfs:subClassOf is not required to be
> >>  transitive;
> >
> >???
>
> Why are you puzzled? There isn't anything in the RDF spec (here
> considered as separate from the RDFS spec) which says that it is
> transitive, and one cannot express transitivity in an RDF graph, so
> how could it possible be required to be transitive?

'required' is perhaps unhelpful.

It just *is* a thing that is a transitive relation, just like
phone:+44-117-907wxyz is a thing that is (nearly) my home phone number and
java:org.example.rdf.xyz is a thing that is a Java class and
mailto:danbri@w3.org is my mailbox and isbn:0-88730-824-4 is a book on
marketing and z3950://my.desire.org:2001/ is a Z39.50 bibliographic
database that's currently offline and irc://irc.openprojects.net/rdfig is
my favourite IRC channel.

(Picking another transitive relation that isn't introduced by the RDF
specs might help make this point in a less tangled way. eg:
http://example.com/t1.)

This is what we get for putting vocabulary constructs (relations,
classes etc) in the domain  of discourse. They take on a life of their
own, over and above the descriptions made about them in our various Web
data languages.

RDF's (possibly contentious) simplifying claim is that some things just
are phones, java classes, mailboxes, books, databases, irc channels,
people, ideas, events, documents, classes, relationship types... And that
for our purposes we can use similar conventions for making propositions
about all these kinds of things.  Just as the limitations of our
descriptive machinery don't affect the actual characteristics of phones,
events, books and databases, they don't affect binary relationship types.
Some are transitive; some aren't.  Whether we can express this in some
deep way in machine language is another matter. Most things are unaffected
by their RDF and/or XML descriptions (fortunately!). RDFS can be read as
taking view that RDF classes and properties are unaffected by their
description in RDF Core Schema language 1.0. Properties that are
transitive are still transitive even if RDF Core's Schema language doesn't
provide any way of noting, indicating, flagging this.

Dan


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Received on Thursday, 30 May 2002 19:12:08 GMT

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