I generally agree with Chris on technical issues,
but there are many unresolved practical issues
about how to develop and use the technology. (01)
So I'll add more qualifications and comments
to some of the points in this thread: (02)
CM> I prefer a nice, simple definition:
>
> An ontology is a set of sentences in a formal language. (03)
Yes, but. That says what it is, but it doesn't explain
why anybody would want one or what they'd do with it.
I would therefore add the following clause: (04)
"that is designed to characterize the entities of interest
in some domain for the purpose of representing, storing,
and communicating information about them and performing
deductions and computations with that information." (05)
>> Is "an ontology" and "a theory" the same? (06)
Again, I don't disagree with Chris, but I would qualify
his answer with the the same clause above. (07)
>> What is the scope of an ontology?
>
CM> I don't know what "scope" means. (08)
I would say that it characterizes some domain of interest
for some purpose, such as developing and supporting
applications that can successfully use a significant
amount of detailed information about the entities in
that domain. (09)
>> Do they change over time?
>
CM> No. Though of course one can trace change through an
> evolving series of ontologies. (010)
That is like asking whether a program changes over time.
The answer may be "no", but version 1.0 is typically
replaced by v. 1.1, 1.2, ..., 2.0, 2.1,.... (011)
And by the way, when we're talking about migration paths
from legacy systems to new ontology-based systems, we
must also recognize that migration *never* stops -- there
will *always* be newer versions coming along. One of the
most important issues for us to address is the question
of how different versions can coexist during the inevitable
transition periods. (012)
>> How big is an ontology? How small can it be? (013)
I agree with Chris that the size is somewhere between 0 and
infinity, but for reasonable estimates, we should look at
Cyc, which had about 2 million axioms about 2 years ago,
and may have acquired a few more since then. Most current
ontologies are much smaller, but future versions are likely
to be as big or bigger. (014)
If they become as big as Cyc, they're not likely to consist
of a single theory, but of a core theory together with a
collection of modules (which are called "microtheories" in Cyc). (015)
>> If it is a lattice, what are the relations between the nodes? (016)
I agree with Chris that an ontology is a theory, not a lattice.
But it is possible to relate all the theories expressed in a
particular version of logic in a lattice. Each node of the
lattice would be a particular theory, and the theories could
be related as generalizations, specializations, siblings, or
distant cousins in the lattice. For more info about those
issues, see (017)
http://www.jfsowa.com/logic/theories.htm (018)
John Sowa (019)
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