INTRODUCTION

Since the beginning of artificial intelligence research on action, researchers have been concerned with reasoning about actions with preconditions and postconditions. Through the work of Moore (1980), Pratt's (1980) dynamic semantics soon established itself in artificial intelligence as the appropriate semantics for action. Mysteriously, however, actions with preconditions and postconditions were not given a proper treatment within the modal framework of dynamic logic. This paper offers such an analysis. Things are complicated by the need to deal at the same time with the notion of competence, or an actor's ability. Below, a logic of actions with preconditions and postconditions is given a sound and complete syntactic characterization, in a logical formalism in which it is possible to express actor competence, and the utility of this formalism is demonstrated in the generation and evaluation of plans.

The notion of actions with pre- and postconditions arose in artificial intelligence in the field of planning. In formulating a plan to reach some particular goal, there are a number of things which a planning agent must take into account. First, he will have to decide which actions can and may be undertaken in order to reach the goal. The physical, legal, financial and other constraints under which an actor must act will be lumped together below, since we will be interested in what is common to them all, namely that they restrict available options.