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summary of current position with respect to semantics proposals (was Re: WOWG: agenda Aug 15 telecon)

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Wed, 14 Aug 2002 14:17:39 -0400
To: schreiber@swi.psy.uva.nl
Cc: www-webont-wg@w3.org
Message-Id: <20020814141739Q.pfps@research.bell-labs.com>


We would like to summarise (our understanding of) the current situation
with respect to OWL semantics ahead of tomorrow's discussion.  There are
two proto-proposals [1] [2] currently available for the semantics of OWL.
These proposals differ in a number of areas, but we feel that the most
significant difference is how certain entailments come out.  In proposal
[1] there are entailments like

Entailment 1:
	John in the intersection of Student and Employee
	entails
	John in the intersection of Employee and Student

This is the standard way of treating syntactic constructs like
intersection, union, conjunction, and disjunction.  Proposal [2]
necessarily has weaker entailment, with the result that the above is no
longer an entailment.

Extending the approaches in the two proposals to the logic layer would
result in Proposal [1] supporting and Proposal [2] not supporting

Entailment 2:
	`it is raining'  or  `today is Tuesday'
	entails
	`today is Tuesday'  or  `it is raining'

We feel that any formalism that does not support entailments like 1
and 2 would manifestly fail to promote interoperability and to support
the development of "applications that depend on an understanding of
logical content" [3].

The two proposals differ on these entailments because Proposal [2] includes
syntax in its domain of discourse.
 When syntax is in the domain of
discourse the above entailments require comprehension principles which lead
to paradoxes when the formalism has sufficient expressive power.  OWL has
sufficient expressive power for this purpose, e.g., see [4].

There are other differences between the two proposals, which are summarized
below:

Proposal [2]:
 
  - start with RDF
    - all RDF graphs are OWL KBs
  - OWL model theory is an augmentation of RDF model theory
    - everything is in the uniform domain of discourse
      - including syntax, so entailment is weak to avoid paradoxes
      - including classes, so classes are also instances

Proposal [1]:

  - start with abstract syntax
    - only RDF graphs that correspond to the abstract syntax are OWL KBs
  - provide model theory for abstract syntax
    - entailment is standard
  - translate abstract syntax to RDF graphs
    - only RDF graphs that are the result of this translation are OWL KBs

  - Pat Hayes is providing an RDF-like model theory for those RDF graphs
      that correspond to the abstract syntax
    - translating the model theory into one based on triples
    - adding necessary syntactic constraints and "darkening"


Ian Horrocks
Peter F. Patel-Schneider



[1] http://lists.w3.org/Archives/Public/www-webont-wg/2002Jun/att-0082/01-semantics.html
[2] http://lists.w3.org/Archives/Public/www-webont-wg/2002Jun/0208.html
[3] http://www.w3.org/2001/sw/WebOnt/charter
[4] http://lists.w3.org/Archives/Public/www-webont-wg/2002Jan/0099.html
Received on Wednesday, 14 August 2002 14:17:49 GMT

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