? Sorted first-order language for representing changing worlds. ? fluents -- predicates whose truth value changes from situation to situation. E.g., on(Pump,s), steam(s), AB(boiler,s) ? world dynamics specified by effect axioms. E.g., Poss(a,s) ? a=turnOn(Pump) ? on(Pump,do(a,s)) ? preconditions for actions specified by necessary conditions for actions. E.g., Poss(fix(Pump),s) ? off(Pump,s)