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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: Thu, 20 Feb 2003 17:13:33 -0000
Message-ID: <008d01c2d903$77aa3d50$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: Tuesday, February 18, 2003 12:31 AM
> Subject: Re: Comment on Last Call Working Draft of RDF Semantics document
> concerning treating classes and properties as objects
>

Dear Pat,

> Most of the issues raised by this comment have already been addressed
> in various archived email discussions, many of them concern other
> languages than RDF(S), and many of them are opinions rather than
> facts.

> I will give a summary of my own personal responses in this message. A
> more formal response as Editor may follow later, after consultation
> with the WG.  Right now however I do not see in this message any
> comments which can be responded to editorially, since they do not
> directly impinge on the text of the RDF Semantics document but seem
> to be a much more broad-based commentary on the entire RDF enterprise.
>

First of all, thank you very much for your interesting comments. I appreciate
that.

[...]

> >*Problem 1 (Too few entailments) [1]: In "RDF+ MT", closure rules are
used
> >to represent semantic conditions to facilitate entailment.
>
> Strictly speaking that is not quite accurate. The normative model
> theory itself does not refer to closure rules: they are described in
> informative parts of the RDF semantics document in order to give a
> basic syntactic characterization of the valid inferences sanctioned
> by the MT, as a convenience for implementers of inference engines.

I am not quite sure about this. Without closure rules, how does the normative
 model theory specify that rdfs:subClassOf is transitive?


[...]

> In a semantic extension, such as OWL, which does allow this kind of
> expressivity, the model theory does indeed need to be further
> constrained by imposing appropriate closure conditions. The OWL-RDF
> model theory does this, for example. However, *any* such semantic
> extension will *always* need to have suitable extra constraints
> added, since RDFS itself assigns no meaning to these constructs.

I have to say, well, adding new syntax as well as corresponding semantic
 constraints is ok. However, adding *missing* objects is quite unusual.


[...]

> The extra semantic conditions one requires for OWL-RDF are based not
> on comprehension from assertions, but on a recursive closure
> principle applied to terms,

But according to section 5.2 of [7], the extra semantic conditions rely on the
comprehension axioms (conditions). Or maybe we are not talking about the
same thing?


> >*Problem 3 (Size of the Universe)[3,4]:
[...]
> This is formally correct, and has been noted before, also in archived
> Webont email discussions, but I do not consider it to be a 'problem'.

If this is formally correct, then OWL-RDF MT formally might have such
a problem.

> The RDFS language allows existential quantification over properties
> and classes. This is part of the original design of the language, not
> something imposed on it by the semantics. Given this essentially
> syntactic property of the language, given the existence of deployed
> code which can handle this situation, and given our WG charter to
> clarify rather than change the formalism, it would have been
> irresponsible, in my view, to have insisted on imposing a textbook
> model theory [...]

I totally agree with you here. But on the other hand, it is also unfair to
 force the upper layer FOL languages have unfamiliar atmosphere in
 their semantics just because of the WG charter. Therefore, it would
 also be desirable to have a subset of RDFS (e.g. RDFS(FA)) such
 that when FOL languages are built on top of it, they
have their traditional semantics, and highly optimised implementations of
FOL (or its decidable subsets) can be employed. This is what I meant
by "an alternative approach".


[...]

Thanks again for your reply.

Best regards,
Jeff
--
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
[3]
http://www.cs.man.ac.uk/~horrocks/Publications/download/2003/HorrocksPatelSc
hneider.pdf
[4]
http://www.cs.man.ac.uk/~panz/Zhilin/download/Paper/Pan-Horrocks-rdfsfa-2002
.pdf
[5]
http://www.cs.man.ac.uk/~panz/Zhilin/download/Paper/Pan-Horrocks-rdfsfa-2001
.pdf
[6] http://www.daml.org/2001/03/model-theoretic-semantics.html
[7] http://www-db.research.bell-labs.com/user/pfps/owl/semantics/
[8] http://www.w3.org/2002/06/lbase/20030116Snapshot.html
[9] http://www.w3.org/TR/rdf-mt/#Lbase
Received on Thursday, 20 February 2003 12:11:05 GMT

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