Re: ACTION: task force unasserted triples

From: Guus Schreiber <schreiber@swi.psy.uva.nl>
Subject: ACTION: task force unasserted triples
Date: Tue, 23 Apr 2002 13:35:23 +0200

> PROPOSED ACTION: for the "dark triple task force" (Jeremy Carroll,
> Jonathan Borden, Jos de Roo, Massimo Marchiori, Pat Hayes, Peter
> Patel-Schneider)
>  
> Yesterday at the telecon of the Semantic Web Coordination Group the
> status of the "unasserted/dark triples" request to RDF Core was
> discussed. The CG felt that a  clarification of this request is needed,
> in particular:
> 
> 1. Rationale for the request, including at least one understandable
> example which motivates the request. 

There has already been quite a number of statements of the rationale for
this request and quite a number of examples that motivate the request.
Without a clearer statement from the CG, how can the task force determine
what to do?

> 2. How do "unasserted triples" solve this problem?

Again, there have been quite a number of statments of how dark triples
solve this problem.  In general, there is an argument as to how the problem
comes about, and dark triples destroys one of the premises of this
argument.

> 3. What do we lose (if anything) when adopting unasserted triples?  
> (See, e.g., Problems with dark triples approach, Jeremy Carroll Wed, Apr
> 17 2002
> http://lists.w3.org/Archives/Public/www-webont-wg/2002Apr/0132.html)

Well, a description of the difference between entailment without dark
triples and entailment with dark triples would be quite complex and
difficult to devise.  Why is it not sufficient to state that it is not
possible to proceed without dark triples (or some other way of extending
RDF syntax) and that it is possible to proceed with dark triples?

> The SW CG asks the Webont task force to come up with such a description
> by Monday May 6. Please indicate whether you accept this action and
> whether it is feasible to do this within the stated time frame..
> 
> Thanks, Guus


All that said, here is what I consider to be an adequate response to the
request.

peter



The Web Ontology Working Group is producing a language that can be used to
build ontologies.  This language is supposed to use the syntax of RDF and
to be compatible with the model theory of RDF.  

However, taking all three of these together results in severe problems.


Consider the following RDF graph

  John rdf:type Student .
  John rdf:type Employee .

as the WebOnt language will include an intersection construct, this graph
should entail

  John rdf:type |intersection Student Employee| .

in the WebOnt language (where |intersection Student Employee| is a short
form for a unnamed node that is connected to some other RDF triples that
carry the intersection construct applied to Student and Employee).
However, this cannot be a valid entailment unless there is an object in the
domain of discourse of the initial RDF graph that is this intersection.

Therefore the model theory for the WebOnt language will have to include
comprehension principles for these sort of classes.

Many more such examples can be constructed, together requiring a powerful
theory of classes in the WebOnt language.  This theory is so powerful that
it will contain inconsistencies, leading to a complete breakdown of the
model theory for the WebOnt language.  (Even if the WebOnt language could
be expressively limited to disallow the paradoxical classes, the same
problem will reoccur at higher expressive levels in the Semantic Web
tower.)


If, however, the triples that are used to encode the syntax of the
constructs of the WebOnt language are unasserted, then these constructs do
not need to be part of the domain of discourse.  In this case, there is no
need for comprehension axioms, and a non-trivial theory can be developed for
the WebOnt language.


What is lost?  Well, of course, quite a number of entailments are lost, and
the details depend on which triples are unasserted.  However, if nothing is
done, the empty RDF graph will, in the WebOnt language, entail a
contradiction.

Received on Tuesday, 23 April 2002 09:28:50 UTC