Actions and Other Events in Situation Calculus

This article presents a situation calculus formalism featuring events as primary and the usual actions as a special case. Events that are not actions are called internal events and actions are called external events. The effects of both kinds of events are given by effect axioms of the usual kind. The actions are assumed to be performed by an agent as is usual in situation calculus. An internal event e occurs in situations satisfying the occurrence axiom for that event. A formalism involving actions and internal events describes what happens in the world more naturally than the usual formulations involving only actions supplemented by domain constraints. Ours uses only ordinary logic without special causal implications. The first example is the buzzer with only internal events and which cannot be treated at all with domain constraints, because the system never settles down. Our second example is the stuffy room scenario. One occurrence axiom states that when both vents are blocked and the room isn’t stuffy, the event Getstuffy occurs. Domain constraints are unneeded.