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A recent article by Ronald Brachman (Brachman, 1985) points out some philosophical or semantic problems in using the notion of a prototype, which is described by using default properties. The problem arises since default properties can be overridden or cancelled in representing particular instances, and therefore lack definitional power: i.e., they are not really essential to the concept being represented. As an example, Brachman presents an elephant joke: Q: What's big and gray, has a trunk, and lives in the trees? A: An elephant-I lied about the trees. Before discussing a solution to this dilemma, consider the following modified version of the elephant joke, perhaps not quite as funny: Q: What's big and gray, has a trunk, and lives in the trees?