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RE: DDTF/layering: weak class theory seems good enough (5.3, 5.10)

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Wed, 29 May 2002 12:47:28 -0400
To: hendler@cs.umd.edu
Cc: jjc@hplb.hpl.hp.com, connolly@w3.org, www-webont-wg@w3.org
Message-Id: <20020529124728Z.pfps@research.bell-labs.com>

From: Jim Hendler <hendler@cs.umd.edu>
Subject: RE: DDTF/layering: weak class theory seems good enough (5.3, 5.10)
Date: Wed, 29 May 2002 12:24:24 -0400

> At 8:29 AM -0400 5/29/02, Peter F. Patel-Schneider wrote:
> >From: "Jeremy Carroll" <jjc@hplb.hpl.hp.com>
> >Subject: RE: DDTF/layering: weak class theory seems good enough (5.3, 5.10)
> >Date: Wed, 29 May 2002 13:06:07 +0100

[...]

> >The question is what is the correct layering.  If you mean a same-syntax
> >extension, then, again, that is impossible for the uppper layers without
> >going to a very non-standard logic.  If, however, you allow syntax
> >extensions, and I see absolutely no reason not to allow syntax extensions,
> >then the situation changes completely.  (Of course, you are still left with
> >dealing with the odd parts of RDF.)
> >
> >peter
>
> Peter - I must admit that I am confused, and I'd like to be straight 
> on this.  

In my response above, ``upper layers'' doesn't really refer to the ontology
layer, but to the rule and logic layers, and above.  However, my comments
do, in fact, also apply to the ontology layer.

> As I understand the world, the current DAML+OIL semantics 
> is an option, but only if we are willing to live without entailments 
> that many of us think should be there.  I base this on the wording in 
> Issue 5.10 which you authored, to wit:
> 
> >DAML+OIL semantics (both the model theory and the axiomatization) 
> >are too weak. For example, it does not allow the inference of 
> >membership in any restrictions that are not present in the knowledge 
> >base, even though many of these are desirable consequences. For 
> >example, if John is an instance of both Person and Employee, 
> >DAML+OIL does not sanction the conclusion that John is an instance 
> >of an intersection of Person and Employee.

> I took this to mean that one option we have is to live with these 
> weak semantics by  saying we give up on desirable consequences (that 
> is - if we rule this issue as POSTPONED we would tacitly be accepting 
> this solution).

This would, in effect, end up with a very non-standard logic for the
ontology level.  We would also be doing a grave disservice to the upper
levels of the layer cake.  We would, in effect, be taking some of the bad
decisions made in RDF and reusing them, making it even harder to get to the
rule or logic level.

The only difference between the ontology layer and the upper upper layers,
is that some of the work has already been done in producing this
non-standard logic at the ontology layer.  For the logic layer, one way of
proceeding would be to produce a logic that didn't have entailments like
(OR p q) entails (OR q p), a very strange logic indeed, and probably one
that has not been studied.  (Well, at least not as part of the mainstream
development of mathematatical logic.  There may be a philosopher who has
investigated the properties of these kinds of logics.)

> This is also how I read your earlier document, the layering proposal, 
> and is what you presented at the first f2f -- this is all summed up 
> in note you produced at the last f2f which stated:
> 
> >  >If the WebOnt language is to
> >>  1/ use RDF syntax, and
> >>  2/ be an extension of RDF, and
> >>  3/ to be able to entail appropriate class memberships,
> >>then the WebOnt language needs to have a facility for having RDF triples
> >>that do not denote in the RDF model theory (a.k.a. dark triples) and URI
> >>references that do not denote in the RDF model theory (e.g., URI references
> >>that only exist in dark triples).
> >
> 
> which I take to mean that we could have 1 and/or 2, but not 3.
> 
> In the message above, you claim we cannot do it by accepting these 
> weak semantics, which seems to contradict my previous understanding.

The weak semantics is one of these non-standard logics.  

> PLEASE NOTE - I AM NOT EXPRESSING AN OPINION ON THIS - I just want to 
> make sure that I (and the WG) have a handle on this --
> 
>   -JH

peter
Received on Wednesday, 29 May 2002 12:49:32 GMT

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