In this paper we make a detailed comparison of the Situation Calculus and the Event Calculus, two logic-based temporal reasoning formalisms. We concentrate on differences between the calculi, considering the similarities sufficiently indicated in the recent literature. We illustrate the inability of Event Calculus to handle counterfactual reasoning problems, and that of Situation Calculus to deal with counterfactual statements in the presence of actions with nondeterministic effects. We present a new calculus which extends both Situation and Event Calculus. In this new calculus we define a natural and clear relation between situations and time points, which differs from those used in previous comparisons. We show the relation of this new calculus to both original calculi. We compare the original calculi using the new calculus as an analysis tool. (C) Elsevier Science Inc., 1997.
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