Norman McCain and Hudson Turner
This paper is concerned with the problem of determining the indirect effects or ramifications of actions. The problem is usually investigated, as in [Kartha and Lifschitz, 1994], in a framework in which action domains are described in part by state constraints. (Informally, a state constraint is a formula that says of a proposition that it is true in every possible state of the world.) Our main objective is to argue that an adequate theory of ramifications requires the representation of information of a kind that is not conveyed by state constraints. In particular, what is required is the representation of the causal relations (or, more generally, the determination relations) that hold between states of affairs. It turns out that this is also the information that is needed for an adequate theory of derived action preconditions or qualifications. Previous approaches to the problem of ramifications have assumed a definition of the following kind: A ramification, roughly speaking, is a change (not explicitly described) that is implied by the performance of an action. In our approach, we substitute the word "caused" for the word "implied". In determining the ramifications of actions, it is not enough, we say, to infer that a change must occur when an action is performed; it is necessary to infer that the action causes the change to occur. As we will see, this stronger requirement makes it possible to avoid unintended ramifications and to infer derived qualifications. (The need for the latter is argued in [Ginsberg and Smith, 1988] and [Lin and Reiter, 1994].) Again roughly speaking, our theory of qualifications is this: An action cannot be performed if the performance of the action implies a change that it does not cause.