Bill, Leo, and Chris M & W, (01)
I mostly agree with Chris M, but I'd like to add a condition
that distinguishes an ontology from an arbitrary theory: (02)
A theory O expressed in a language L is an ontology iff O
defines one or more types of entities, and for each defined
type T, O shall determine whether any individual x is or
is not an instance of T. (03)
If L is a formally defined version of logic, then O is called
a _formal ontology_; otherwise, O is an _informal ontology_. (04)
Note that this definition allows an ontology to be distinguished
from an arbitrary theory. For example, the following statements
constitute a theory, but not an ontology, because they do not
state conditions for defining any types: (05)
There is a man named Bob who is taller than a man named Joe. (06)
Every individual named Tiny Tim is shorter than any
individual named Big Bad Bob. (07)
No letters in circular envelopes are delivered on Tuesdays. (08)
Note that this theory could be converted into an ontology if
it included a definition of the type LoopyLetter as a letter
enclosed in a circular envelope. That single definition would
be sufficient to make it an ontology, but not a good one, since
it contains statements that are irrelevant to the defined type. (09)
I certainly agree that some ontologies are better than others
and that we should bestow honors and shame where appropriate.
But that can be done by putting little words like "good" or
"bad" in front of the word "ontology". (010)
The conditions of goodness and badness are independent of whether
the ontology is formal or informal. We could have some carefully
written good informal ontologies, and some bad formal ontologies.
But to be interpretable by a computer, the ontology must be formal. (011)
John Sowa (012)
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