[topicmapmail] Getty Art and Architecture Thesaurus On Line

[Thomas B. Passin]

[Murray Altheim]
> I think in this instance the answer comes from Peirce: class-instance
> is a dyadic relation, and what we need is Thirdness, triadic relations.
> I.e., class-instance relations should [usually] be stated within a
> specific context. Something can be considered an instance of a class
> in one context, but may be used as a class in another. In Topic Maps
> one could scope the associations.
>
> So in the example provided, "Homo sapiens" is an instance of the
> class of "species" (and would appear as such in a Topic Map describing
> the zoological taxonomy),

The terminology used by rdfs and OWL is sometimes very useful.  I'm thinking
here of domain/range.  Say the relationship between "species" and "Homo
sapiens" is class-subclass.  Then you could say

subclassOf domain Class
subclassOf range Class
"Homo sapiens" subclassOf species

No ambiguity here. you could also invent a relationship instanceClass, say,
to indicate that you want to consider a class to be an instance of another
class, and you would give that relationship the same domain and range as
subclassOf.  In fact, it is not clear to me why the two would be different
relationships.

With an instance that is supposed to be an actual individual thing, not a
class, you could make up a new relationship, say individualInstanceOf.  Then
you could have something like this -

individualInstanceOf domain Class
individualInstanceOf range IndividualThing

My point here is that I think of these two usages of "instance" as different
relationships, since they have different ranges.  Of course, it is hard to
be certain without a good definition of "Class" and "Instance", which we do
not have.  In OWL, a class is not just a set of individuals.  Instead, a
class somehow is associated with a collection of individuals, but is
distinct from that set.  Apparently this bit of trickery has made OWL
mathematically tractable.

Notice that this approach is different from saying that a class is a set of
individuals that is ussed for the purposes of classification, which is
another definition that is sometimes used (not is OWL).

Maybe we should define what a class is, for the purposes of being more
precise about themeaning of superclass-subclass and instanceOf.

Cheers,

Tom P