  That an agent's possible evidential states form a Boolean algebra (on which
it is natural to define a probability measure) is an assertion that ideally
should be proved, rather than assumed, in justifying rational choice as a
representation of expected utility. A more parsimonious, axiomatic
characterization of evidence is provided here. Two primitive entities are
evidential states and a relation, more specific than, between evidential
states. The axioms specify that more-specific-than is a partial order, there is
a minimally specific e-state, and more-specific-than is a separative order.
Choice alternatives are another primitive entity. A plan is an assignment of a
choice alternative to each evidential state. In general, plans satisfying a
version of the sure-thing principle cannot be rationalized by expected utility.
But there is such a rationalization if the evidential structure is a tree.
