Justifying Actions by Accruing Arguments

This paper offers a logical formalisation of an argument-based account of reasoning about action, taking seriously the abductive nature of this form of reasoning. The particular question addressed is what is the best way to achieve a specified goal? Given a set of final goals and a set of rules on the effects of actions, the formation of subgoals for a goal is formalised as the application of an inference rule corresponding to the practical syllogism well-known from practical philosophy. Positive and negative applications of the practical syllogism are then accrued as a way to capture the positive and negative side effects of an action. Positive accruals can be attacked by negative accruals and by arguments for alternative ways to achieve the same goal. Defeat relations between accrued action arguments are determined in terms of the values promoted and demoted by the actions considered in the arguments. Applying preferred semantics to the result then yields the admissible ways to achieve the desired goal.