On the Existence and Formalization of Natural Events

In logical theories of change it is common to consider two types of objects that provoke change: 1) actions that are carried out by agents, and 2) events which occur externally, without agent intervention. In this paper we argue for the existence of a special type of event, which we call natural events. An event is called natural if its occurrence is the result of a natural process. For example, if a ball is dropped it is natural that it hits the ground. Thus, the event of the ball hitting the ground is considered natural. A natural event can be prevented by actions or events that intervene and change the nature of the process leading to the event. If no such intervening events occur, then the natural event has to occur. Hence, this third type of \change provoking" entity, the natural event, may be the result of some agent's prior action(s). Thus, in general, natural events need not be external. In this paper, we discuss the need to introduce explicitly the notion of natural event in a logic of change and present a formalization based on the situation calculus.