Cati Martínez wrote: (01)
> I'm new in the Ontology world, and maybe it has been already
> discussed, I'm asking me the question if everything implemented in the
> OWL language can be considered an Ontology. I guess that it's not so,
> but it is difficult for me to say when we can say that it is or not. (02)
I have yet to see a definition of "ontology" that everyone who thinks
s/he is developing one, or tools for one, agrees with. If we take the
loose definition: "a set of terms and definitions using a
machine-interpretable language with a formally defined semantics" or the
even looser definition: "any formal structure of terms and definitions",
one can argue that any OWL model is an "ontology" because it is a
representation of a set of related terms using "a machine-interpretable
language with a formally defined semantics". (03)
My personal preference is a much stronger definition: A formal language
that provides a means of definition of a term using a syntax with a
well-defined semantics that supports (automated) reasoning is an
"ontology language". A model captured in that language is an "ontology"
if and only if most of the terms it introduces are defined in the model
using the definition mechanisms of the formal language (possibly
including references to terms that are formally defined elsewhere). (04)
That definition is fairly strong, and would tend to eliminate most OWL
models, and many research projects that use the "ontology" buzzword.
OWL provides two mechanisms for introducing a term/concept, as a
"primitive" term that has only a natural language definition, or as a
"defined" term that has an OWL definition. As Natasha Noy recently
observed, there are probably 5 people in the world who have actually
published "real ontologies" expressed in OWL. All the others are just
information models -- all, or almost all, the classes they define are
primitive. And with a model like that, a DL reasoner can do almost nothing. (05)
> I'm modelling with OWL some information structure, so OWL is used to
> define the components and relations to these components that compose
> this concrete information structure. Could it be considered an
> Ontology, or only a set of constraints on a data structure? (06)
I personally don't see a problem with an ontology whose domain of
discourse is data structures. That, to me, is not the concern. The
issue is whether your model actually contains formal definitions. (07)
I have argued in this forum that OWL is the next-generation information
modeling language, and I applaud its use for that purpose. But I
distinguish an "information model" from an "ontology" by the language in
which the definition of the terms is written. In information models,
most or all of the definitions are written in natural language. At best
an information model expresses some "necessary characteristics" of a
term, but is silent on the "sufficient characteristics". (08)
Now, given a small set of fundamental "undefined terms", a first-order
language like CLIF is almost always able to express the necessary and
sufficient characteristics that would define a term. And RDF is also
relatively competent in that regard. But OWL/DL is very limited in its
ability to construct such expressions. Ultimately it depends on
definitions that are intersections or unions or depend on simple values
of simple properties. Sometimes that can't be done at all, and
sometimes it can only be done with a significant amount of ingenuity
(i.e., knowledge engineering). The latter often obscures the intent to
the human reader, but it enables the DL engine to do meaningful
reasoning with it. So the plain fact is that, in many cases, OWL/DL is
not an easy language to use for a strong ontology. And the purpose of a
"strong" ontology expressed in OWL/DL is *primarily* to enable reasoning
with a DL engine. (In a sense, that makes it what OMG calls a
"platform-class-specific model" -- a model made for implementation by a
particular kind of software technology.) (09)
So, if your purpose is to use OWL as an information modeling language,
please do. OWL has almost everything you need for that purpose, and it
is better grounded than any other information modeling language (except
possibly for ORM). Whether you can call the result an "ontology"
depends on your circle of friends, and on the necessity of using the
buzzword to obtain management approval or funding for otherwise useful work. (010)
[This is only my view, and it is worth exactly what you paid for it.
You will doubtless get a lot of other opinions, some of them rather
better educated. ;-)] (011)
-Ed Barkmeyer (012)
P.S. For those of you involved in the summit, consider this a stake in
the ground: Use of "formal definition" is one of the primary factors
(and perhaps the most important) in determining the "quality" of an
ontology. (013)
--
Edward J. Barkmeyer Email: edbark@xxxxxxxx
National Institute of Standards & Technology
Manufacturing Systems Integration Division
100 Bureau Drive, Stop 8263 Tel: +1 301-975-3528
Gaithersburg, MD 20899-8263 FAX: +1 301-975-4694 (014)
"The opinions expressed above do not reflect consensus of NIST,
and have not been reviewed by any Government authority." (015)
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