  We review two strands of conceptual approaches to the formal representation
of a decision maker's non-knowledge at the initial stage of a static
one-person, one-shot decision problem in economic theory. One focuses on
representations of non-knowledge in terms of probability measures over sets of
mutually exclusive and exhaustive consequence-relevant states of Nature, the
other deals with unawareness of potentially important events by means of sets
of states that are less complete than the full set of consequence-relevant
states of Nature. We supplement our review with a brief discussion of
unresolved matters in both approaches.
