  Apparently, Priorian tense logic had no influence on  Amarel 1968.  But there is no important difference between Amarel’s world-states and those of Priorian tense logic. The “situations” of the Situation Calculus are these same world-states, under a new  name.[23]  They resemble possible worlds in modal logic in providing abstract locations that support a consistent and complete collection of truths. As in tense logic, these locations are ordered, and change is represented by the variation in truths from one location to another. The crucial difference between the Situation Calculus and tense logic is that change in the situation is dynamic—changes do not merely occur, but occur for a reason.