  We introduce a logic for reasoning about evidence, that essentially views
evidence as a function from prior beliefs (before making an observation) to
posterior beliefs (after making the observation). We provide a sound and
complete axiomatization for the logic, and consider the complexity of the
decision problem. Although the reasoning in the logic is mainly propositional,
we allow variables representing numbers and quantification over them. This
expressive power seems necessary to capture important properties of evidence
