Actions in Situation Theory
I Kovacs Alexander · 2003
We propose a new formalism, AST, to reason about actions and change that is based on Barwisean situation theory. Known facts are expressed by infons while events are pairs of sets of infons describing the change of facts. Actions are discriminated from events; they are equated with the events they cause, or bring about. We define the notion of a scenario, which is a tuple (EV,AC,C,O,Q) where EV is the set of events, AC a set of actions, and C a set of constraints. Constraints are also events, but are interpreted differently. The set O contains known information about facts and event occurrences, Q is a set of queries. The interpretation of a scenario is a set of infons and event occurrences, which contains at least the facts in O and Q, and fills gaps in O by events accounting for the change of properties. Preferred models are those interpretations that contain the least number of actions. AST contains a novel solution to the frame problem, a graph constructed from information about properties obtaining at certain times and about event occurrences changing those properties. Besides deciding the persistence of properties, the nexus also has the function of detecting missing events and to find out whether the occurrence of events is relevant. Every model has an associated Barwisean situation. We postulate six foundational axioms to characterize the supports relation, the relation between situations and infons, such that if the relation obtains the situation is said to support the fact expressed by an infon. With the help of the nexus, these axioms are supplemented by non-axiomatic criteria to determine whether a situation supports certain facts, mainly whether facts persist between timepoints at which they are explicitly known.