Event Calculus

The event calculus [45, 66, 74, 98, 100] is a formalism for reasoning about action and change. Like the situation calculus, the event calculus has actions, which are called events, and time-varying properties or fluents. In the situation calculus, performing an action in a situation gives rise to a successor situation. Situation calculus actions are hypothetical, and time is tree-like. In the event calculus, there is a single time line on which actual events occur. A narrative is a possibly incomplete specification of a set of actual event occurrences [63, 98]. The event calculus is narrative-based, unlike the standard situation calculus in which an exact sequence of hypothetical actions is represented. Like the situation calculus, the event calculus supports context-sensitive effects of events, indirect effects, action preconditions, and the commonsense law of inertia. Certain phenomena are addressed more naturally in the event calculus, including concurrent events, continuous time, continuous change, events with duration, nondeterministic effects, partially ordered events, and triggered events. We use a simple example to illustrate what the event calculus does. Suppose we wish to reason about turning on and off a light. We start by representing general knowledge about the effects of events: