8 Conclusion

The instance restriction is important to the unambiguous interpretation of represented knowledge; when the same construct in a modeling language is used to represent instances and classes, undesirable ambiguities result. The constructs for instance and class must be kept distinct. Certain objects, however, act as classes and instances in different contexts, and these special objects must be recognized by the modeling language (and the modeler) as existing in multiple universes of discourse. In each universe, the object is interpreted as either a class or an instance. This approach also avoids undesirable inferences when classes appear in taxonomies.

We believe that clarifying an ambiguity in the use of instances and classes is important for domain modeling and knowledge representation in general. While supporting the notion of spanning objects and multiple universes of discourse in modeling languages is a necessary part of this clarification, it does give rise to some interesting questions that must still be addressed. The most prominent question is on the relationship between the universes in which a spanning object exists. What relationships can be expressed between objects in different universes? Can inferences made about an spanning object in one universe be used in another universe in which the object exists? If such mapping across universes is allowed, it may affect the completeness and tractability of the inference mechanisms, since the essential nature of spanning objects is second-order.

What's in an Instance? - 01 MAY 95

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