  We examine carefully the rationale underlying the approaches to belief change
taken in the literature, and highlight what we view as methodological problems.
We argue that to study belief change carefully, we must be quite explicit about
the ``ontology'' or scenario underlying the belief change process. This is
something that has been missing in previous work, with its focus on postulates.
Our analysis shows that we must pay particular attention to two issues that
have often been taken for granted: The first is how we model the agent's
epistemic state. (Do we use a set of beliefs, or a richer structure, such as an
ordering on worlds? And if we use a set of beliefs, in what language are these
beliefs are expressed?) We show that even postulates that have been called
``beyond controversy'' are unreasonable when the agent's beliefs include
beliefs about her own epistemic state as well as the external world. The second
is the status of observations. (Are observations known to be true, or just
believed? In the latter case, how firm is the belief?) Issues regarding the
status of observations arise particularly when we consider iterated belief
revision, and we must confront the possibility of revising by p and then by
not-p.
