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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: Fri, 21 Feb 2003 15:25:03 -0000
Message-ID: <008301c2d9bd$79c894f0$6bc65882@cs.man.ac.uk>
To: "pat hayes" <phayes@ai.uwf.edu>
Cc: <www-rdf-comments@w3.org>, <bwm@hplb.hpl.hp.com>

> ----- Original Message -----
> From: "pat hayes" <phayes@ai.uwf.edu>
> To: "Jeff Z. Pan" <pan@cs.man.ac.uk>
> Cc: <www-rdf-comments@w3.org>; <bwm@hplb.hpl.hp.com>
> Sent: Thursday, February 20, 2003 6:43 PM
> Subject: Re: Comment on Last Call Working Draft of RDF Semantics document
> concerning treating classes and properties as objects
>
>
> >  > ----- 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.

[...]

> >I am not quite sure about this. Without closure rules, how does the normative
> >  model theory specify that rdfs:subClassOf is transitive?
> >
>
> Er...by saying that it is transitive? But in fact, section 3 says:

Yes, it does.


> <x,y> is in IEXT(I(rdfs:subClassOf)) if and only if x and y are in IC
> and ICEXT(x) is a subset of ICEXT(y)
>
> and the transitivity follows from the transitivity of the subset relation.
>
>
> >[...]
> >
> >>  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.
>
> Not sure if I follow you here. There are many constructs in OWL that
> are not in RDFS; are those considered 'missing'?

Let me put it this way. Because classes and properties are treated as objects,
all the objects of the expected classes should be added (i.e. by explicitly
stating
that they exist) to the universe before  the extra semantic constraints are added
for the extended languages. Otherwise, these expected objects might be
 missed. See [1,2,4] for more details.


> >
> >[...]
> >
> >>  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?
>
> I am not responsible for the wording used in [7], but I stand by my
> distinction (using the language of set theory).
>
> The term 'comprehension principle' is traditionally used to refer to
> an inference principle which asserts that sets exist corresponding to
> open sentences, of the form 'the set of x such that P' where P is the
> open sentence. In other words, it requires sets to exist which
> satisfy syntactic expressions. Closure conditions, in contrast,
> require that sets exist on the basis of the existence of other sets,
> in the same way that almost all axiomatic set theories are stated. As
> is well known, comprehension principles are dangerous (Russell
> paradox) but these dangers are not inherited by the use of closure
> conditions, which provide the solidest foundations of consistent
> mathematics yet known. That is why we used closure conditions rather
> than comprehension principles in the OWL-RDF semantics.

Very interesting. Thank you. By the way, OWL-RDF semantics [5] calls
them comprehension conditions, maybe you call them closure conditions.
That's fine. Now my point is that then we've got to make sure we have
considered all the needed objects exist, favourably by providing a formal
proof.


>
> >  > >*Problem 3 (Size of the Universe):
> >[...]
> >>  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.
>
> Or might not. Please point to the problem.

As pointed out in problem 3 of [6], such an interpretation is possible in FOL,
but
not in OWL Full.


>I am confident that there
> is no mathematical problem, and unless you can find any error in
> Peter's proofs in the OWL semantics document,

In my impression, OWL tries to avoid some expressive power from
 RDFS by considering only ontologies in abstract syntax in the theorems [5].
Therefore, I think Peter's proof won't guarantee that there is no problem
 with OWL-RDF MT.


> I see no reason to
> think there might be. And in any case this issue should be addressed
> to Webont.
>
> >  > 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.
>
> I do not think I can respond to the term 'unfamiliar atmosphere'.

I stole a similar phrase from what a CL expert said - "I agree that
things happen in CL that have an air of unfamiliarity." - in the following
discussion:

http://philebus.tamu.edu/pipermail/kif/2002-September/001268.html

In other words, CL has some features that other FOL languages
don't have. So it is desirable to have both styles being able to be
built on top of RDFS.


> Personally I have been breathing this kind of air for many years and
> find it highly congenial. It has been used by working KIF code which
> has been available for over a decade. It is used by CYC and by
> OntologyWorks. The CL draft ISO standard is going to be constructed
> this way. Why should I not respond that it is your responsibility to
> get used to the air at this height? :-)

I am already getting used of it :)


> >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,
>
> If you build them properly, this is true already.

I am afraid I don't quite follow you at this point.


> But read on....
>
> >and highly optimised implementations of
> >FOL (or its decidable subsets) can be employed. This is what I meant
> >by "an alternative approach".
>
> Since several comments have made a similar suggestion, and in view of
> the need to give a clean account of the relationship between the
> various OWL sub-languages and RDFS, I am sympathetic to the idea of
> defining what might be called a layered subset of RDFS, intended to
> be forward-compatible with OWL-DL in a way that parallels the
> relationship between 'full' RDFS and OWL-Full.
>
> There is a logged comment
> http://www.w3.org/2001/sw/RDFCore/20030123-issues/#qu-03
> from Qu Yuzhong which I think makes the same point.
>
> Would it be OK to interpret your comment as an endorsement of #qu-03 ?

They are similar, although RDFS(FA) is not only an "ontology
 language", but also a "meta-language" (in Qu's term).


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/HorrocksPatelSchneid
er.pdf
[4]
http://www.cs.man.ac.uk/~panz/Zhilin/download/Paper/Pan-Horrocks-rdfsfa-2002.pdf
[5] http://www-db.research.bell-labs.com/user/pfps/owl/semantics/
[6] http://lists.w3.org/Archives/Public/www-rdf-comments/2003JanMar/0266.html

> Pat
>
>
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Received on Friday, 21 February 2003 10:22:39 GMT

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