  We study conditioning on null events, or surprises, and behaviorally
characterize the Ordered Surprises (OS) representation of beliefs. For feasible
events, our Decision Maker (DM) is Bayesian. For null events, our DM considers
a hierarchy of beliefs until one is consistent with the surprise. The DM adopts
this prior and applies Bayes' rule. Unlike Bayesian updating, OS is a complete
updating rule: conditional beliefs are well-defined for any event. OS is
(behaviorally) equivalent to the Conditional Probability System (Myerson,
1986b) and is a special case of Hypothesis Testing (Ortoleva, 2012), clarifying
the relationships between the various approaches to null events.
