Qualification and Causality

In formal theories for reasoning about actions, the qualification problem denotes the problem to account for the many conditions which, albeit being unlikely to occur, may prevent the successful execution of an action. While a solution to this problem must involve the ability to assume away by default these abnormal disqualifications of actions, the common straightforward approach of globally minimizing them is inadequate as it lacks an appropriate notion of causality. This is shown by a simple counter-example closely related to the well-known Yale Shooting scenario. To overcome this difficulty, we propose to incorporate causality by treating the fact that an action is qualified as ordinary fluent, i.e., a proposition which may change its truth value in the course of time by potentially being (indirectly) affected by the execution of actions. Abnormal disqualifications then are initially assumed away, unless there is evidence to the contrary. Our formal account of the qualification problem includes the proliferation of explanations for surprising disqualifications and also accommodates so-called miraculous disqualifications, which go beyond the agent's explanation capacity. In the second part, we develop a fluent calculus-based encoding of domains that require a proper treatment of abnormal disqualifications. In particular, default rules are employed to account for the intrinsic nonmonotonicity of the qualification problem. The resulting action calculus is proved correct wrt. our formal characterization of the qualification problem.

ICSI Technical Report TR-96-026

Technical Report