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Re: Issue #pan-01 Re: Comment on Last Call Working Draft of RDF Semantics document concerning treating classes and properties as objects

From: Jeff Z. Pan <pan@cs.man.ac.uk>
Date: Fri, 21 Feb 2003 18:12:21 -0000
Message-ID: <00fb01c2d9d4$d8b88260$6bc65882@cs.man.ac.uk>
To: "pat hayes" <phayes@ai.uwf.edu>
Cc: <www-rdf-comments@w3.org>

> ----- Original Message -----
> From: "pat hayes" <phayes@ai.uwf.edu>
> To: "Jeff Z. Pan" <pan@cs.man.ac.uk>
> Cc: <www-rdf-comments@w3.org>
> Sent: Thursday, February 20, 2003 7:24 PM
> Subject: Re: Issue #pan-01 Re: Comment on Last Call Working Draft of RDF
> Semantics document concerning treating classes and properties as objects
> >----- Original Message -----
> >From: "Brian McBride" <bwm@hplb.hpl.hp.com>
> >To: "Jeff Z Pan" <pan@cs.man.ac.uk>; <www-rdf-comments@w3.org>
> >Sent: Tuesday, February 18, 2003 11:35 AM
> >Subject: Issue #pan-01 Re: Comment on Last Call Working Draft of RDF Semantics
> >document concerning treating classes and properties as objects
> >
> >
> >>  Jeff,
> >>
> >>  With Pat responding that your comments are not editorial, I'm going to try
> >>  to identify the comments you are making for consideration by the WG.
> >>
> >>
> >>
> >>  At 21:15 14/02/2003 -0800, Jeff Z Pan wrote:
> >>
> >>
> >>  >Recent research (http://dl-web.man.ac.uk/rdfsfa/paper.htm) has shown that
> >>  >RDF Schema (RDFS) has a non-standard metamodeling architecture, which
> >>  >some elements in the model have dual roles in the RDFS specification.  As
> >>  >result, this can be confusing and difficult to understand and,  more
> >>  >importantly, the specification of its semantics requires a non-standard
> >>  >model theory - RDF MT. RDF MT justifies dual roles by "treating classes
> >>  >properties as objects", which seems to be ok within RDFS. However, this
> >>  >leads to semantic problems when we extend RDFS with more expressive FOL
> >>  >constructs, and extend RDF MT to the so called ``RDF+ MT'' to give meaning
> >>  >to this extended language:
> >>
> >
> >Brian,
> >
> >>  Ok - I think you are saying you are trying to extend RDF(S) to some
> >>  (proper?) subset of FOL and are having problems.  Yes?  The rest of this
> >>  message is an explanation of the difficulties you are experiencing.
> >
> >It also suggests that it might be a good idea to also locate a sub-language of
> >(e.g. RDFS(FA)) such that when FOL languages are built on it, they can have
> >their traditional semantics, and the existing highly optimised
> >implementations of
> >FOL (or its decidable subsets) can be employed.
> It would be very helpful if you could be more precise. My
> understanding is that RDFS already has this character,

I hope so, but currently I am not sure (see below).

>if one
> interprets the phrases 'built on' and 'can be deployed'
> appropriately, and that the existence of OWL illustrates this.
> Your comment can be interpreted, therefore, in several ways.
> (1) You do not agree with this understanding - in which case, it
> would be helpful if you could document the problems in some detail,
> or give examples. (All the issues you mentioned in your first comment
> have been dealt with already by the Webont WG, as noted previously.)

Well, I am afraid I don't. In my opinion, "treating classes and properties
as objects"  brings some unclearness when people layer FOL on top of RDFS.
Three problems were discussed in earlier papers [1,2,3,4] and also in Webont
WG (see below):

- Problem 1 indicates that we should prove that all the expected (classes and
objects are explicitly required in the universe.

- Problem 2 indicates that we should also prove that all the contradiction class
won't bring any semantic problems for the extended language (e.g. OWL Full).

- Problem 3 indicates that we should also prove that the individual objects,
class objects
and property objects are handled well enough such that no more semantic problems
the "size of Universe" problem) for the extended language. However, in RDFS an
individual object can easily and possibly become a class object and/or a property
which makes things really complicated.

Without these proofs, I don't believe all these problems have been solved by
Webont WG
in OWL Full. On the contrary, OWL tries to avoid some expressive power from
 RDFS by considering only ontologies in abstract syntax in the theorems [5], i.e.
restricting the possible form of ontologies. Therefore their proofs by no means
cover all the
above three problems.

So my view is that the FOLs with free syntax [6], e.g. SKIF, can be built on top
of RDFS, while
the FOLs without such features can be built on top of a sub-language of RDFS,
such as RDFS(FA),
without any unclearness.

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

[1] http://www-db.research.bell-labs.com/user/pfps/papers/dl-2002.ps
[2] http://www-db.research.bell-labs.com/user/pfps/papers/layering.ps
[5] http://www-db.research.bell-labs.com/user/pfps/owl/semantics/
[6] http://reliant.teknowledge.com/IJCAI01/HayesMenzel-SKIF-IJCAI2001.pdf

> (2) You feel that the way that OWL was 'built on' RDFS is unsuitable
> or inappropriate in some way - in which case, it would help if you
> could sketch how you feel it would have been done better on a
> sublanguage, ie what you mean by 'built on' which is different from
> the relationship between OWL and RDF;
> (3) What sense of 'can be deployed' makes OWL-DL not an illustration
> of how this is already  possible?
> It would also help if you could say more exactly what you mean by a
> "FOL language". In my understanding, Lbase, CL and KIF are all FOL
> languages, but I take it that this does not satisfy you.
> Pat
> --
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Received on Friday, 21 February 2003 13:09:56 UTC

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