  The study of belief change has been an active area in philosophy and AI. In
recent years two special cases of belief change, belief revision and belief
update, have been studied in detail. Roughly, revision treats a surprising
observation as a sign that previous beliefs were wrong, while update treats a
surprising observation as an indication that the world has changed. In general,
we would expect that an agent making an observation may both want to revise
some earlier beliefs and assume that some change has occurred in the world. We
define a novel approach to belief change that allows us to do this, by applying
ideas from probability theory in a qualitative setting. The key idea is to use
a qualitative Markov assumption, which says that state transitions are
independent. We show that a recent approach to modeling qualitative uncertainty
using plausibility measures allows us to make such a qualitative Markov
assumption in a relatively straightforward way, and show how the Markov
assumption can be used to provide an attractive belief-change model.
