Hi Bill, (01)
Quoting Bill Andersen <andersen@xxxxxxxxxxxxxxxxx>: (02)
> Hi Leo...
>
> I have some comments about some of these proposed dimensions..
>
> On Jan 28, 2007, at 14:40 , Obrst, Leo J. wrote:
>
> > Dimensions of Ontology Types:
> >
> > 1) Formality: Informal (Formality = 0) vs. Formal (Formality = 1)
>
> I don't quite get what this means. If we're not talking about
> artifacts that are somehow used to influence software to do things we
> want, I don't see the point. So, for some thing O, if Formal(O)
> means that O is a logical theory, then, following ChrisM, I don't see
> what anything not formal should even be considered, since otherwise,
> it would be pretty close to impossible to say how it could be used
> for some computational end. (03)
The ecumenical definition of "ontology" that Mike Uschold and I have used is:
"An ontology includes a vocabulary of terms together with a specification of
the intended meaning of the terms." (04)
Different approaches to ontologies are distinguished by the latter condition,
that is, in the way that they specify the intended meanings. (05)
Leo is pointing to a basic partitioning of these approaches. (06)
A formal ontology is a set of sentences in a language that has a model theory,
that is, a notion of interpretation that supports truth assignments,
satisfiability,
and entailment. (07)
(As observed by recent discussions, this is a necessary but not sufficient
condition,
since there are sets of sentences that not everyone would consider to be an
ontology,
but this is not my point here). (08)
On the other hand, the specification of intended meanings in an informal
ontology relies on
extralogical mechanisms (natural language, diagrams, canonical software
implementations). (09)
The line does become a little blurry when the expressiveness of the underlying
logical language
for an ontology is insufficient to axiomatize the intended interpretations of
the terms.
For example, in OWL-S (the OWL ontology for web services), different classes of
processes
such as Unordered,Sequence,etc are axiomatized in OWL.
Nevertheless, OWL is not expressive enough to capture the full intended
interpretations
of these classes, which is specified in natural language as documentation. (010)
>
> > 2) Expressivity: Expressivity of the semantic model (i.e., underlying
> > knowledge
> > representation language or logic) [No scale determined yet]
>
> This is a property of a logical system, independent of the instances
> of which we wish to regard as "ontology", so I'd exclude this dimension. (011)
It is a property of a class of structures AND the logical language.
There are classes of structures that are not first-order definable,
and classes of structures that are not definable in DL.
This is an important issue because we need to be able to determine
whether the intended interpretations of the terms in the ontology
can be defined within the advertised formal language. (012)
- michael (013)
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