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How to layer the semantic web properly?

From: Dieter Fensel <dieter@cs.vu.nl>
Date: Fri, 04 Jan 2002 02:41:26 +0100
Message-Id: <>
To: www-webont-wg@w3.org
Cc: Dieter Fensel <dieter@cs.vu.nl>
How to layer the semantic web properly?

We had recently a large amount of discussions on the right
layering of OWL and RDF. This is not really surprisingly
because it touches a key issue of the semantic web. How
to understand the layering of the various elements that
Tim put in his visionary slide on top of each other (see
[1]). I would like to provide the following intuitions:

1. This layering cannot be interpreted in model-theoretic
terms. That is, we cannot expect that each set of inferences
at a higher level is a super set of all inferences at a
lower level. Each level will have its specific style on
how to draw inferences. Therefore:

	- we should not be surprised if the rule language
	will choose some kind of minimal model semantics
	that differs from our OWL semantics.

	- we should not worry too much that a Pat Hayes
	RDF agent draws some conclusions an OWL agent
	would never dare to take.

Instead of viewing this as a bug we should view it as a
nice feature that provides the world with different reasoning

2. I would strongly recommend to keep the layers syntactically
as close as possible. Even if a Pat Hayes aware RDF agent draws
some strange conclusions in the average we will win a lot
if an RDF agent can process the syntax of OWL statements
and if it draws conclusions which are reasonable for its
level and even right in the average for an OWL agent.

3. It is highly desirable that OWL has a sound model theory. This
can easily be achieved. Only if one uses RDF as a syntax AND
as a semantics (i.e., if somebody makes the mistake to define
the OWL semantics as an extension of the Pat Hayes RDF semantics)
then he runs into problems. My personal conclusion: Lets
define a simple semantics of OWL without reference to the
model theories of other working groups. Or spoken as a
sociologist: I see some changes that we agree on syntax between
different working groups but we will run in endless war or
non-compatibility if we try to agree on the basis of model theory
between different working groups.

4. The question whether to base OWL syntactically on XML or
RDF is a question of high strategic impact. Such a
question can neither be decided on the background of problems
in expressing DAML+OIL lists in RDF nor based on problems
in monotonically extending Pat Hayes RDF model theory. There
need to be much more serious arguments to justify such a schism
of the semantic web.

Dieter Fensel
Division of Mathematics & Computer Science,
Vrije Universiteit Amsterdam,
De Boelelaan 1081a, 1081 HV Amsterdam, NL
The Netherlands
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Received on Thursday, 3 January 2002 20:43:32 GMT

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