  A formal framework is given for the characterizability of a class of belief
revision operators, defined using minimization over a class of partial
preorders, by postulates. It is shown that for partial orders
characterizability implies a definability property of the class of partial
orders in monadic second-order logic. Based on a non-definability result for a
class of partial orders, an example is given of a non-characterizable class of
revision operators. This appears to be the first non-characterizability result
in belief revision.
