14 July 2005

Components of Ontology: Relationships

Relationships within a formal ontology are the means that allow entities to modify and combine with each other. It is important to realize that the relationship itself does not affect an entity, but that the entities that it relates provide the affects or changes. For instance, relating the entities "truck" and "movement" (truck HAS movement) give the idea of a truck moving somewhere. The relationship HAS does nothing for the truck, but it does allow for the semantic idea of the truck having movement (or "moving", using the correct syntactic form).

Concepts of Relationships

Relationships are comprised of concepts, just as the other compounds (entities and non-atomic concepts) are. These concepts are a little more abstract and difficult to see than they are in the other compounds, but they are present nevertheless. The reason they are difficult to see, is because we are not accustomed to thinking of them consciously. When we communicate, the rules of our language and semantics are definitely bounded by the concepts that comprise relationships. For an example, let us consider the binary relationship "tank has crew". There are two entities, tank (the subject of the relationship) and crew (the object of the relationship), brought together by the relationship HAS. These two entities each have a number of concepts, some of which should be apparent.

Consider the relationship "has". In this case, it is being used to define that an entity has as part of itself a number of other entities. To put it simply, think of it in terms of the taxonomical hypernym classes that exist outside of both "tank" and "crew". Those hypernyms are "vehicle" (in the case of "tank"), and "component" (in the case of "crew"). This then becomes "vehicle has component", for a tank is a vehicle, and crew is a component of a vehicle.

At this point, "has" now has a few interesting concepts. First, it has a time subjectivity concept, by which I mean this - if we say a tank has crew, we mean two things.
  1. A tank has the CAPACITY to contain 4 crew members (and needs 4 to function fully).

  2. A tank has the POTENTIALITY of carrying 0,1,2,3 or 4 crew members subject to it's current state (is it in storage, is it in the field, has it been damaged, etc).

Second, the relationship "has" can imply the concept of specificity, semi-specificity (or class specificity), or non-specificity. By this I men that the tank can have either

  • INSTANCE-SPECIFIC crew (Carol, Bob, Ted, and Alice

  • CLASS-SPECIFIC crew (gunner, loader, driver, commander

  • NON-SPECIFIC crew (4 bodies)

These are all concepts of the relationship, which allow it to be redefined, or to have its properties defined.

When looked at that way, the relationship "has" can be divided up into two more precisely defined relationships of "has capacity of" and "has currently".

Aspects of Relationships

Just as entities can have both a real/non-real aspect as well as a tangible/abstract aspect, so relationships have a number of different aspects, some of which were hinted out in the previous example.

One of the aspects of a relationship is the choice between a relationship showing an actuality or a potentiality. This is often a defining factor as to whether the relationship applies to persistent properties (or property values) of an entity (which are often associated with the class of an entity), or if they relationship applies to the non-persistent properties of an entity (which are often associated with the instance of an entity). A class-entity has the entities affecting its potentiality related to it (which indicates a persistent property), and an instance-entity has the entities affecting its actuality related to it (again, this indicates a non-persistent property).

The nature of relationships (and their application to entities) is sometimes based on the changing state of the entity. This gives a temporal basis to the relationship. As with so many other aspects affecting the other components of our formal ontology, this temporal basis is grounded in the use of the ontological definition. Temporal basis, as it is based on the changing state of entities through time, is very much related to events and phenomena (as events have a time component, and phenomena are concerned with changing state).

Finally, some relationships can be redefined based on qualification, as opposed to quantification. It is fine to say that "tank HAS crew", which is a qualification relationship (giving some definition to the tank entity, by relating it to a defining entity), but it is a quantification relationship to list the number of crew that the tank has (whether potential or actual). Quantification and qualification relationships are often used to define the state (current or otherwise) of the subject entity.

The inherent supporting needs for a relationship to exist between two entities are often very easy to accommodate. For instance, if a relationship exhibiting quantification is to exist, then the only structural (syntax) requirement is that both the object entity and subject entity of the relationship must each support the idea of a number of something being related to something else. Thus, syntactically, it is perfectly fine to say that "tank HAS 2 wings". This is syntactically correct, however it is not semantically correct (unless we are working within a universe of discourse that allows for winged tanks). The semantics of relationships are dealt with in the next system (rules).

Method for Evaluating Relationships

Our method for evaluating relationships should be simple, and easy to define, however it (as with the other methods discussed above) must be based in the intended use of the ontological definition.

The types of relationships (potential/actual, qualification/quantification, temporally based) that exist must each be considered to see whether all of the combinations of entities that are required to satisfy the universe of discourse can be assembled.

The syntactic requirements for the relationships are based on the properties (or concepts) of the entities being related must exist, for all the sorts of relationships that need to be supported for the universe of discourse.

Finally, the relationships must be accessible enough via definition that they can be the objects of relationship-controlling rules.



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