Actions, interactions and narratives

Sammanfattning: The area of reasoning about action and change is concerned with the formalization of actions and their effects as well as other aspects of inhabited dynamical systems. The representation is typically done in some logical language. Although there has been substantial progress recently regarding the frame problem and the ramification problem, many problems still remain. One of these problems is the representation of concurrent actions and their effects. In particular, the effects of two or more actions executed concurrently may be different from the union of the effects of the individual actions had they been executed in isolation. This thesis presents a language, TAL-C, which supports detailed and flexible yet modular descriptions of concurrent interactions. Two related topics, which both require a solution to the concurrency problem, are also addressed: the representation of effects of actions that occur with some delay, and the representation of actions that are caused by other actions. Another aspect of reasoning about action and change is how to describe higher-level reasoning tasks such as planning and explanation. In such cases, it is important not to just be able to reason about a specific narrative (course of action), but to reason about alternative narratives and their properties, to compare and manipulate narratives, and to reason about alternative results of a specific narrative. This subject is addressed in the context of the situation calculus, where it is shown how the standard version provides insufficient support for reasoning about alternative results, and an alternative version is proposed. The narrative logic NL is also presented; it is based on the temporal action logic TAL, where narratives are represented as first-order terms. NL supports reasoning about (I) metric time, (II) alternative ways the world can develop relative to a specific choice of actions, and (III) alternative choices of actions.