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A semantic layering proposal

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Fri, 27 Sep 2002 17:06:00 +0100
Message-ID: <15764.33256.249590.936191@galahad.cs.man.ac.uk>
To: www-webont-wg@w3.org

The WG has for some time been wrestling with difficult problems
surrounding the issue of semantic layering. The basic problem arises
from the observation that layering OWL on top of RDF in the DAML+OIL
style leads to an incorrect set of entailments.  Proposed solutions
have included asking RDF core to provide a mechanism for switching off
the RDF meaning of some triples (dark triples) and weakening the
semantics of OWL such that many intuitive entailments are lost (e.g.,
(A or B) not entailing (B or A)).

More recently solutions have involved restricting the set of RDF
graphs that are considered to be valid OWL, and only providing an OWL
semantics for these graphs. This proposal follows the same line,
restricting the set of OWL/RDF graphs to those that can be generated
from the OWL abstract syntax.

The proposal is as follows:

* The abstract syntax document provides the basic definition of OWL
  and specifies the subset of possible RDF graphs that are valid OWL
  graphs (i.e., those that can be derived via a mapping from this
  syntax) [1].

* An XML presentation syntax can be developed from the RDF triple

* RDF triples conforming to mappings from the abstract syntax are the
  normative exchange format for OWL ontologies [2].

* The normative semantics for OWL is given by [3], which defines the
  semantics in terms of the abstract semantics. A definition of the
  semantics w.r.t. the resulting RDF triples is also available [4],
  but should be considered non-normative.

* The link between OWL and RDF semantics is defined by [4]. What this
  basically says is that *IF* RDF graphs are constrained to those
  generated by the abstract syntax and *IF* extra semantic conditions
  are added to the RDF MT, *THEN* entailment in this extended RDF is
  the same as OWL entailment. (This is roughly what Pat has been
  calling fast OWL.) If the extra semantic conditions are not
  considered, then RDF entailment is still consistent with OWL
  entailment, but it becomes a subset of the entailments justified by
  the OWL semantics.

* A form of "classes as instances" can be achieved by a naming
  convention or external mapping that identifies pairs of classes and
  instances. This is similar to techniques already being used in
  implemented RDF engines. It does not support all the entailments
  that would be possible by eliminating the distinction completely,
  but it does allow for a layered architecture with complete OWL
  entailment within each layer.

* A subset of OWL (OWL Lite) will also be defined [1,5].

The proposal provides for a complete language specification, almost
all of the elements of which are already available. It addresses all
of the following issues:

5.2 Language Compliance Levels
5.3 Semantic Layering
5.10 DAML+OIL semantics is too weak
5.17 XML presentation syntax

and proposes solutions that, it is hoped, will be acceptable to
most/all of the WG.


Peter Patel-Schneider
Ian Horrocks

[1] http://www.w3.org/TR/2002/WD-owl-absyn-20020729/
[2] http://www.w3.org/TR/2002/WD-owl-ref-20020729/
[3] http://www-db.research.bell-labs.com/user/pfps/owl/semantics.html
[4] http://www-db.research.bell-labs.com/user/pfps/owl/embed.html
[5] http://www.w3.org/TR/2002/WD-owl-features-20020729/
Received on Friday, 27 September 2002 11:58:00 UTC

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