A Formal Model for Classical Planning

In this paper, we describe a formal semantic model that applies to many "classical" planning systems. This gives a unifying framework in which to study diverse planners, and motivates formal logics that can be used to study their properties. As an example of the model's util­ ity, we present a general truth criterion which tests for the necessary truth of a proposition at arbitrary points in the planning process. 1 Introduction In this paper we investigate a formal model for a broad class of A.I. planning systems. We define the range of applicability of our model later; here, we simply note that it is relevant to such well-known "classical" planners Each of these planners has its own very distinctive features. But there are also many similarities, and we can define a useful formal model of the concept of "plan" used by all of them. The model can be used to provide semantics for much of the planning process. That is, we can develop a logic which is interpreted as making assertions about plans. Relative to the model and the logic, claims about particular plans or planning systems can be proven true or false. Section 3 gives a concrete application of this: we develop a truth criterion for a broad class of planning systems. The concept of a truth criterion (TC) is due to Chapman [Chapman, 1987]. In fact, many of the ideas in this paper were inspired by Chapman's work and so we spend the rest of this introduction contrasting the goals of his work and ours. In general, a planner's truth criterion is the test it uses to check whether a particular proposition holds at some point in a proposed plan. It is easy to see how impor­ tant this test is. For example, we are always interested in whether the goal propositions will be true after execu­ tion of a proposed plan. A truth criterion tells the plan­ ner when to stop planning for the current goal. Aside from computational efficiency, there are three desirable properties a truth criterion can have. First, it should be correct: when it tells us that a goal is achieved, we can safely stop planning (for this goal). Second, a TC can be sufficient: if the goal is achieved, it will tell us. Sufficiency makes planning faster, because we can avoid unnecessary work. Finally, and somewhat more vaguely, a truth criterion may be …