REASONING ABOUT ACTION .1. A POSSIBLE WORLDS APPROACH

Reasoning about change is an important aspect of commonsense reasoning and planning. In this paper we describe an approach to reasoning about change for rich domains where it is not possible to anticipate all situations that might occur. The approach provides a solution to the frame problem, and to the related problem that it is not always reasonable to explicitly specify all of the consequences of actions. The approach involves keeping a single model of the world that is updated when actions are performed. The update procedure involves constructing the nearest world to the current one in which the consequences of the actions under consideration hold. Computationally, this construction procedure appears to be tractable for worlds like our own where few things tend to change with each action, or where change is regular.