A Constructive Approach to the Ramification Problem

In the state of the art on the ramification problem, it is acknowledged that ramifications of actions are not logical consequences of state constraints and require explicit representation through causal laws. Yet, causal laws are still tightly coupled with state constraints in that their syntactic form is derived from state constraints or in that they merely serve to restore integrity of state constraints. In this paper, we cut this strong connection. Our focus is not on state constraints but on the propagation of physical or logical forces and effects through the dynamic system. Causal laws are considered representations of how these forces and effects propagate. As such, causal laws may be but do not need to be related to state constraints. We define a semantics for causal laws which allows for representing cyclic dependencies between effects and combined effects on complex fluent formulae. The effect propagation process is constructive. To adequately model this process, we propose a semantics for causal laws based on the main mathematical constructive principle: inductive definitions. Our approach is language-independent and can be embedded in Situation Calculus, Event Calculus, A-type or STRIPS-type languages. We discuss the relation with other recent approaches based on a restricted notion of nondeterminism. Supported by the Vlaams Instituut voor de Bevordering van het WetenschappelijkTechnologisch Onderzoek in de Industrie (IWT) Supported by GOA 93/97-03