Recently, Lakemeyer and Levesque proposed a logic of onlyknowing
which precisely captures three forms of nonmonotonic
reasoning: Moore’s Autoepistemic Logic, Konolige’s
variant based on moderately grounded expansions, and Reiter’s
default logic. Defaults have a uniform representation
under all three interpretations in the new logic. Moreover, the
logic itself is monotonic, that is, nonmonotonic reasoning is
cast in terms of validity in the classical sense. While Lakemeyer
and Levesque gave a model-theoretic account of their
logic, a proof-theoretic characterization remained open. This
paper fills that gap for the propositional subset: a sound and
complete axiom system in the new logic for all three varieties
of default reasoning. We also present formal derivations for
some examples of default reasoning. Finally we present evidence
that it is unlikely that a complete axiom system exists
in the first-order case, even when restricted to the simplest
forms of default reasoning.