3 The Instance Inference

KR systems and Object-Oriented systems define another language construct for establishing a subclass relationship between two classes. The interpretation of this link can vary in subtle ways [Brachman83], but is widely understood to have special implications with respect to the instance relationship. In particular, we define the instance inference:

I2 If in some universe of discourse U, object A is an instance of object B, and B is a subclass of object C, then A is an instance of C.

In examples like that shown in Figure 4, it is generally accepted that Harry is an instance of bird by virtue of eagle being a subclass of bird and Harry being an instance of eagle.

I2 is an important part of the pure semantics (which describes the rules of a language [Carnap61]) of Object Oriented and most KR languages. This inference is commonly exploited to establish set membership and inherit properties. A procedure e.g. to determine the cardinality of the set of all birds would use I2 to count Harry as a member of that set. A property of birds, for example that they have beaks, would be considered true of all the instances of bird, and would, by I2, therefore be true of Harry.

What's in an Instance? - 01 MAY 95

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