Chapter 6 - Indirect effects of events

Abstract

The problem of representing and reasoning about the indirect effects of events is known as the ramification problem. This chapter presents several methods for representing indirect effects and dealing with the ramification problem in the event calculus. It also discusses the use of effect axioms, primitive and derived fluent, release axioms and state constraints, effect constraints, causal constraints, and trigger axioms. One way of representing indirect effects is to represent them the same way that direct effects are represented, namely, using positive and negative effect axioms. Furthermore, the advantage of the method of using effect axioms to represent indirect effects is its simplicity. The disadvantage of this method is that more axioms may be required than with other methods. Another way of dealing with the ramification problem is to divide fluent into two categories: primitive fluents, or fluents directly affected by events, and derived fluents, or fluents indirectly affected by events. The truth value of a primitive fluent is determined by effect axioms, whereas the truth value of a derived fluent is determined by state constraints. The derived fluent is released from the commonsense law of inertia at all times, so its truth value is permitted to vary and is determined by state constraints.