Circumscription is based on the notion of minimal models. E.g., KB= bird(Tweety) ? bird(Ralph) Assume our universe only contains Tweety and Ralph. Then there are 3 models: bird(Tweety) ? bird(Tweety) bird(Tweety) ?bird(Ralph) bird(Ralph) bird(Ralph) There are 2 minimal models. This model is not minimal in predicate bird. Intuitive description: Circumscription transforms KB into a stronger sentence, CIRC(KB;P) such that the models of CIRC(KB;P) are precisely the desired minimal models. As with predicate completion, circumscription ? minimizes occurrences of one or more predicates. ? computes a formula that captures our assumptions and adds it to KB. So what formula are we adding? We’re adding a circumscription axiom schema for P.

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