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RE: DTTF: darkest africa

From: Jeremy Carroll <jjc@hplb.hpl.hp.com>
Date: Thu, 30 May 2002 16:25:43 +0100
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
Cc: <www-webont-wg@w3.org>
Message-ID: <JAEBJCLMIFLKLOJGMELDEEBFCEAA.jjc@hplb.hpl.hp.com>


>
> Huh?  You are now saying that the behaviour of OWL graphs that include
>
> 	ex:person owl:sameClassAs owl:ABCDEF
>
> is undefined?  Why?  What is wrong with this?

The most obvious thing wrong with that is there is no such owl property
as owl:ABCDEF. We control the namespace and we haven't made such a
property.

For the names we do define, the only thing wrong with it (IMO) is that
it gives a misleading impression of what an OWL implementation will or
will not do. If I say
 ex:Rest owl:sameClassAs owl:Restriction .

and then try and use ex:Rest just like owl:Restriction then it won't
work because:
i) the OWL model theory
and
ii) OWL implementations
  are looking for syntactic structures with a triple like

_:foo rdf:type owl:Restriction .

rather than doing a least fixed point or something like that on the
interpretation of owl:Restriction.


(I don't particularly like doing it syntactically. However I believe we
are not wanting to support two steps in semantic inference. I think it
is straight forward to simply ban some triples to block off any
misapprehension that it may be possible to have indirect  impact on the
user level ontology using entailments over OWL concepts. Moreover I
believe that such a banning can be done in a way that postpones the
research issues, rather than prematurely closing them in the negative).


>
> > I suspect my list is incomplete, the rules for
> > completing it are:
> > - when in doubt add the triple
> > - the only reason for not adding the triple is
> >   + it seems genuinely useful
> > and
> >   + it expresses something that can be expressed
> >     in mainstream description logic.
> > I am assuming that the List vocabulary is in the
> > owl namespace. Otherwise it would need to be
> > explicitly treated in the black list.
>
> I don't understand this rationale for making triples dark.  I don't
> understand the rationale for identifying triples as dark in
> this way.  I
> don't understand the implications here of making triples dark.
>


I am trying to build a fence. The classes and properties and
restrictions still end up as in the domain of discourse, like in RDFS;
but all the abilities of OWL and RDFS to say anything about them at a
meta level is switched off (other than appropriate use of
rdfs:subClassOf between user level classes etc).

> > Of course, we could address the RDFS layering issue
> > simply by deciding that all the dark graphs identified
> > above are contradictions. Then we would respect all
> > their RDFS entailments (trivially). I wouldn't like
> > that much but could live with it, and would prefer it
> > to making RDF Model Theory non-normative.
>
> Huh?

The suggestion that OWL treat some triples as not having their RDFS
meaning is a suggestion that the RDFS Model Theory is in essence
optional.


>
> > If we wanted to stress conformance with RDFS then
> > we would have slightly different text
> >
> > [[[
> > implementation may treat a dark graph as
> > having its RDFS entailments and any others
> > of the implementation's chosing. The simplest
> > implementation is to treat a dark graph as
> > self-contradictory.
> >
> > ]]]
> >
> > (I doubt there would be group consensus for that).
> >
> > If we wished to stress the dangers of paradox we
> > would have text
> >
> > [[[
> > implementors should note that at least some
> > dark graphs appear self-contradictory in
> > interesting ways e.g.
> >   testA, testB, testC
> > ]]]
>
> > Jeremy
>
> I don't think that this proposal can lead to a solution of
> the layering
> paradoxes.  I certainly don't see any such solution above.
>

No. It doesn't intend to.
It tries to indicate that we may be able to agree on what we agree on,
and identify the areas where we disagree. The areas where we disagree
are less important than the areas where we agree. There may be practical
ways of avoiding paradox by substantially reducing the apparant power of
the language for describing itself. I don't think there are any
implementations yet that would allow us to use this power.
We have no use cases for exploiting that power. Personally I don't want
to rule that power out for ever, but I believe that I agree with you on
the meaning of all RDF graphs that do not contain any of the triples I
identified.

Also, areas where we disagree can be labelled as such. It is not a three
valued logic but simple practical expediency. I am sorry I was less than
precise about interpretations and truth, I got the impression that you
had understood what I was getting at though.

Jeremy
Received on Thursday, 30 May 2002 11:26:31 GMT

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