In order to solve a planning problem, STRIPS constructs some datastructures from the representation of the problem, and manipulates such datastructures until it finds a solution or it concludes that one cannot be found [11]. We describe each configuration of those datastructures as a mental situation, and the manipulations that STRIPS can perform on them as mental actions. Notice that mental situations and mental actions describe the internal state of a system as it reasons about a problem, whereas situations and actions describe the particular problem the system reasons about. In order to specify the reasoning strategy of a system, we need to describe: (1) the mental situations it constructs to reason about a problem; (2) the mental actions it can perform to transform those mental situations; and (3) the heuristics it uses to select the mental action to perform next during the process of searching for a solution.