Definability and Commonsense Reasoning

The deenition of concepts is a central problem in commonsense reasoning. Many themes in nonmonotonic reasoning concern implicit and explicit deenability. Implicit deenability in nonmonotonic logic is always relative to the context-the current theory of the world. We show that xed point equations provide a generalization of explicit deenability, which correctly captures the relativized context. Theories expressed within this logical framework provide implicit deenitions of concepts. Moreover, it is possible to derive these xed points entirely within the logic.