CIRC(KB; P) ? KB + P’s circumscription axiom schema Schema imposes additional constraint on KB: P’s extension is minimal. I.e., P is true only when KB forces it to be! P’s circumscription axiom schema: (?P*. ( KB(P*) ? (?x.P*(x) ? P(x)) ) ? (?x.P(x) ? P*(x)) ) Tweety Ralph P* P ? P P* Ralph Tweety P* satisfies the same constraints as P in KB Those x’s that satisfy P* are a subset of those that satisfy P Those x’s that satisfy P are a subset of those that satisfy P* I.e., if P* is any predicate satisfying P’s axioms, then P*’s extensions cannot be smaller than P’s.