Assume (for simplicity) that S0 is completely specified, so there is one model wrt S0 E.g., on(Pump,S0) ?broke(Boiler, S0 ) on(Boiler, S0) ?broken(Pump, S0 ) Suppose the action turnOff(Pump) is performed, transitioning the system to a new situation, do(turnOff(Pump, S0), which we will abbreviate to S1. From the effect axiom Poss(a,s) ? a=turnOff(Pump) ? ? on(Pump,do(a,s)) we know that the pump will be ?on in S1, but what is the truth value of other fluents? Intended Model ? on(Pump, S1) ?broken(Boiler, S1 ) on(Boiler, S1) ?broken(Pump, S1) Numerous Unintended Models ? on(Pump, S1) broken(Boiler, S1 ) on(Boiler, S1) ?broken(Pump, S1) ? on(Pump, S1) broken(Boiler, S1 ) ? on(Boiler, S1) ?broken(Pump, S1) (etc.) ? ? ?