in this thread and in the "Ontology and methodology" thread the category
ROLE was mentioned several times.
I am wondering what a role is ontologically? What are the
super-categories of ROLE? In one of the extensions of DOLCE, a role is
considered a non-physical social object.
Roles seem helpful in terms of achieving taxonomically "clean"
ontologies in the sense of OntoClean (Welty, Guarino).
Yet employing the category ROLE in ontology engineering seems to be
problematic since many (most) categories used in domain ontologies would
suddenly have ROLE as super-category. What do I get wrong here?
Take for example an object that is identified as being a chair. I guess
in many ontologies, CHAIR would be classified as sub-category of
PHYSICAL OBJECT. This is, any entity that is identified to be a chair is
also a physical object.
However, in the context of the previous mails, one could claim that a
chair is identified being a chair by affording sitting on it; by the
role it plays. If a certain entity which was previously identified as
chair, is never used again for sitting but only for, say, a step for
changing light bulbs, it stops playing the role of being a chair and is
hence only playing the role of a step for changing light bulbs (?).
Compare this to an employee being fired. The person does not loose its
ability of playing the role of an employee, yet the certain entity is
not classified as employee anymore.
Why should a physical object, that is never used for sitting be
considered a chair?
Why should employee be a role while chair is not? (01)
John F. Sowa wrote:
> Dear Matthew,
> I agree that the distinction is essential, but that approach
> makes it difficult to express all necessary constraints:
>> MW: Actually it was a modelling principle, that entity types should be
>> based on the underlying nature of a thing rather than a role that it
>> MW: Following this approach relations were also modelled as entity types
>> and roles were modelled on the relationships (E-R sense) linking the
>> relations to the domains of the things they related.
> But I put all types in a single hierarchy, in which all roles are
> under the type Role. Since the hierarchy is a lattice, it allows
> all possible combinations, such as MaleEmployee or PregnantEmployee.
> This approach makes it possible to state constraints on
> permissible relationships very conveniently. For example,
> a relationship that is restricted to PregnantEngineerEmployee
> can be hard to represent in a typical E-R diagram.
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