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DTTF: another summary

From: Jeremy Carroll <jjc@hplb.hpl.hp.com>
Date: Fri, 3 May 2002 10:45:34 +0100
To: <www-webont-wg@w3.org>
Message-ID: <CEECKEAMDAJDDEDGJNBEMECICAAA.jjc@hpl.hp.com>

It seems symptomatic of being difficult to refuse to even agree on where we
are at ...

I am difficult.


I believe we have three live proposals:

- comprehensive entailments: Jeremy
  [1]
- dark owl: Peter
  [2], [3]
- dark lists: Pat
  [4]

We are agreed that the first does not use dark triples
and that the second and third do.

Pat and Peter have asserted that comprehensive entailments
involves significantly more risk and research than the
other two proposals. I have not conceded this.

I believe that DanC has understood my proposal.

There has been no evidence that anyone has understood either
of the other proposals. I have claimed to have understood
both, but seem to have been mistaken vis-a-vis Pat's.
I have also proved unable to articulate Peter's.

In particular there does not appear
to be any evidence that Pat and Peter agree on anything other
than that the right proposal involves dark triples. Since the
"right proposal" is an unbound variable in that agreement it
seems to me to be high risk to build on that agreement.


In my view, we have three distinct issues:

  comprehension
  paradox
  layering


  comprehension
  =============

How does the semantics of OWL account for the entailment

_:x <rdf:type> <student> .
_:x <rdf:type> <employee> .

entails

_:x <rdf:type> _:inter .
_:inter <rdf:type> <owl:class> .
_:inter <owl:intersectionOf> _:list .
_:list <rdf:type> <owl:List> .
_:list <owl:first> <student> .
_:list <owl:rest> _:tail .
_:tail <rdf:type> <owl:List> .
_:tail <owl:first> <employee> .
_:tail <owl:rest> <owl:nil> .


(Perhaps we don't really need _:x here at all).

  paradox
  =======

How are paradoxes like the Patel-Schneider paradox avoided.

  layering
  ========

What is the relationship between RDF entailment,
RDFS entailment and OWL entailment.

This is explored by Mike Smith [5].
There is a particularly pertinent form if the lower
layers of OWL reuse the terms from rdfs.

For a document that uses rdf and rdfs vocabulary and
no OWL vocabulary is OWL entailment monotonically
layered on top of RDFS entailment.


******************************

On the relationship between the proposals and the problems.

Comprehension + Paradox
=======================

The comprehension rules are shared between the
comprehensive entailments and the dark lists proposal [6].

These rules are the way both these proposals
address the comprehension issue without falling into
the paradox problem.

In particular, my attempt to articulate to Pat how
I understood Peter's proposal as addressing Peter's
paradox [7] fizzled out.

I don't feel we have seen from Peter an account of how
either issue is addressed.


Layering
========

Peter and Mike see the impossibility of layering as
self-evident.

I have disputed this, seeing an ill-formed list as a semantic
rather than a syntactic problem.

I believe that a clean monotonic layering of OWL on top of RDFS
is highly desirable, and that we need to have good evidence of
its impossibility before not attempting it.

A monotonic layering would be one in which

if

  a rdfs-entails b

then

  a owl-entails b

and both rdfs-entails and owl-entails are monotonic.
This is Mike's SEMANTIC PROPERTY 1.
I drop Mike's T and TINV by noting that we have agreed
that the syntax for OWL is RDF graphs which is also the
syntax for RDFS.

My belief that this is an important goal is based on the
following assumptions
- the semantic web project is fairly high risk
- the key risks are in terms of semantics and scale
- monotonicity is a key distinctive feature of the
  semantic web that addresses these risks
  and makes it a plausible bet
  (one that I am prepared to spend a chunk of my
  life on).

I wonder whether Pat and Peter see strict layering as
advocating that the object denoted by a URI in the RDFS layer
is the same as that in the OWL layer. I see that as an
irrelevance. Layering expressed using Mike semantic property
1 does not reuse an RDFS interpretation as an OWL interpretation.

I note that this monotonic layering is less important
than the monotonicity of the individual layers, and I
am certainly open to Pat's fig-leaf position, given
a reasoned argument as to why some aspect of a monotonic
layering either won't work or has excessive cost on some
other dimension.

No one has yet put such an argument.

My proposal does respect this constraint.


*****************************

An important aspect of the difference between the
proposals is syntax vs semantics.

The dark stuff is seen as syntactic, the not dark stuff
is seen as semantic. Hence whenever we are talking about X
there is often miscommunication because the advocates of
dark X think we are discussing the syntax and its semantics,
whereas the opponenets assume we are talking at only a semantic
level. Quite who is on which side varies with X.



References:

[1] Jeremy: Comprehensive Entailments
http://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0155.html

[2] Peter: one *quick* proposal
http://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0229.html


[3] Peter: longer proposal
http://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0241.html

[4] Pat: proposal
http://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0254.html

[5] Mike: layering
http://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0200.html

[6] Pat (in reply to Jeremy): comprehension
http://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0338.html
[[[ [Pat]
In RDF it doesnt. In OWL it would come from an OWL semantic
constraint of the form that if JOhn is in A and JOhn is in B then
JOhn is in (A intersect B).
]]]

[7] Jeremy & Pat: Peter & paradox
http://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0343.html

[[[
[ jeremy on Peter's approach ]
> >>  >Potential paradoxes, like the Patel-Schneider paradox (Test
> Case C) are
> >>  >resolved outside of DAML+OIL.
> >Pat:
> >>  'Outside' means what?
> >
> >
[ jeremy again ]
> >In the model, because of set theoretic constraints on the model.
> >But an axiomatisation of the DAML+OIL model theory
>
[ pat ]
> Please don't use meaningless phrases.
[ jeremy ]
I am sorry, I clearly didn't use the right language.
I will not try and indicate how darkening all of owl solves the
Patel-Scheider paradox, suffice to say I think it does. Peter can give a
fuller account if he wants.
]]]
Received on Friday, 3 May 2002 05:45:48 GMT

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