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Re: [closed] re: Pan-01

From: Jeff Z. Pan <pan@cs.man.ac.uk>
Date: Thu, 17 Jul 2003 23:08:59 +0100
Message-ID: <007601c34cb0$05e8be00$6401a8c0@percival>
To: "pat hayes" <phayes@ihmc.us>
Cc: <www-rdf-comments@w3.org>

Dear Pat,

> with respect to your comment
> http://lists.w3.org/Archives/Public/www-rdf-comments/2003JanMar/0266.html
> archived as pan-01, the working group has decided not to accept your
comment.

Thank you for your messages,

[...]

> We will also note a possible way to reconcile your position
> with the current RDF design.

and your efforts.


> Your message makes a number of points, but they can be summarized by
> the complaint that the RDF (and RDFS) model theory (MT) is
> "non-standard".
[...]
>In order to avoid getting into details, let me summarize
> these various points of view as the 'layered' vision of a
> metamodelling architecture.

First of all, I would like to make clear that I didn't suggest replacing
RDFS with RDFS(FA) in my last call comments. Instead, since there are
problems
when layering FOL on top of RDFS, I suggested identifying a proper
sub-language
of RDFS, RDFS(FA), so that we can layer FOL languages on top of it within
a layered metamodeling architecture. That is what I meant by "an alternative
approach". See

http://lists.w3.org/Archives/Public/www-rdf-comments/2003JanMar/0330.html


[...]

>Aczel's results have been the source of much
> recent work in the foundations of mathematics and philosophy of
> language and meaning, by the way, and as he is a professor at
> Manchester

I talked with Professor Aczel about RDF MT and RDFS(FA) again the other day.
He still has the impression that the non-well-founded set theory isn't
relevant
to RDF.


[...]

> I mention
> all this only as background to the more essential point, however.

Thanks, but isn't the fact that the non-well-founded set theory and RDF MT
are two separate things? The success of the non-well-founded set theory
doesn't
guarantee that there are no problems layering FOL on top of RDF MT.


> The key point about the RDF design is that it *allows* the use of
> layering but does not *require* it.

I think the point is that the users should be informed the existence of the
mentioned problems when they choose to use the non-layering style of RDFS.
Recognising RDFS(FA) as a proper sub-language of RDFS can make people
aware of the two different styles of using RDFS. This can also motivate
various tool supports such as checking which style the users are using in
their ontologies.


>This is one aspect of a critical
> design decision taken early in the RDF project: that RDF should
> impose as few restrictions as possible on what can be said in it or
> how it can be used; c.f
>
http://www.w3.org/2001/sw/RDFCore/TR/WD-rdf-concepts-20030117/#section-anyon
e.

Introducing  RDFS(FA) as a sub-language of RDFS allows users to be more
explicit about the style of modelling they intend. They will be able to
choose
if they are willing to use the more free style of RDFS, or if they are
willing
to constrain the kinds of model that can be represented (in RDFS(FA))
in order to facilitate the extension of the language with more expressive
FOL modelling primitives, towards e.g. OWL DL.


[...]

>since the technical
> work mentioned above has shown that there is no *technical* need or
> requirement to adopt the layering assumptions.

Again, the fact that non-layering works with set theory doesn't imply that
non-layering will work with RDF and OWL. As previous discussions
 have shown, there are  problems layering FOL on top of RDF MT,
 therefore there *are* technical needs to adopt the layering
assumptions if we are to layer FOL on top of (a sub-language
of) RDFS. The way OWL DL treats RDF is a good example.


> To turn to the particular points you raise in your message: all of
> the numbered problems you mention seem to refer to issues which have
> arisen in the layering of OWL onto RDF. As I expect you know, these
> issues have all been resolved by the Webont working group.

I am afraid I disagree. As for OWL Full, no proofs have been provided to
show that the  three problems mentioned have been solved in OWL Full,
and in fact several problems have arisen since the
OWL layering proposal first emerged (e.g., the failure of the claimed
equivalence between OWL DL and OWL Full entailment within the OWL DL
subset). *No* evidence  has been provided to show that no new problems will
arise in the future. In OWL DL, RDFS is cut into a very primitive language,
so that a lot of interesting things (like the use of meta-classes) can't be
represented in such a language. We can't just assume that all these issues
have
been resolved by the Webont WG.

As for RDFS(FA), on the one hand, there are no problems layering FOL
 languages on top of it; on the other hand, you still can say a lot, even
though not all, of interesting things that can be said in RDFS. So I think
 it is a pretty good trade-off.


[...]

> ----
>
> The discussions surrounding the relationship between OWL-DL (which is
> layered) and OWL-Full (which is not) have led to the Webont WG
> formulating a set of conditions on an OWL/RDF graph which suffice for
> it to be a legal OWL-DL document, see for example
> http://www.daml.org/2002/06/webont/owl-ref-proposed#app-DLinRDF .

These rules give only an *informal* characterization of the conditions for
an
RDF graph to be an OWL DL ontology. The reason that they can
somehow help is that there is a formal OWL DL semantics behind
it (in OWL there are two different semantics, one for DL and the
other for Full).


> These involve providing a labelling of the non-RDFS vocabulary into
> individual, class and property URIrefs and some conditions on the
> occurrence of blank nodes in various contexts. It has occurred to
> several of us that these similar structural conditions could be
> applied to RDFS graphs and might provide a useful way to recognize
> that an RDF graph was 'layered' in the sense that you might accept as
> more "standard".

Similarly, we can have RDFS(FA) as a sub-language of RDFS, where
these two (sub-)languages have different semantics and RDFS(FA)
has a few extra syntax to support layering. Then we can have rules
of thumb for RDFS(FA) ontologies, providing an *informal*
characterization of the conditions for an RDF graph to be an RDFS(FA)
ontology. Note that such rules won't work without a formal RDFS(FA)
semantics.

Best regards,
Jeff
--
Jeff Z. Pan  ( http://DL-Web.man.ac.uk/ )
Computer Science Dept., The University of Manchester


>We have not checked what relationship, if any ,
> there might be between these criteria and the design of RDFS(FA), but
> mention this work only to suggest that there might be a useful
> embedding of your preferred architectural style into the RDFS design;
> for if so, it would seem that RDF is able to serve its primary design
> aim of being a universal medium of data interchange even for users
> who prefer a layered architecture.
>
> ----------
>
> Please reply to this message, copying www-rdf-comments@w3.org,
> indicating whether this response adequately addresses your comment.
>
> Sincerely
>
> Pat
> --
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Received on Thursday, 17 July 2003 17:59:06 GMT

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