State defaults and ramifications in the unifying action calculus

We present a framework for reasoning about actions that not only solves the frame and ramification problems, but also the state default problem-the problem to determine what normally holds at a given time point. Yet, the framework is general enough not to be tied to a specific time structure. This is achieved as follows: We use effect axioms that draw ideas both from Reiter's successor state axioms and the nonmonotonic causal theories by Giunchiglia et al. These axioms are formulated in a recently proposed unifying action calculus to guarantee independence of a specific underlying notion of time. Reiter's default logic is then wrapped around the resulting calculus and plays a key role in solving the ramification as well as the state default problem.