  We propose a new way of looking at the quantum Maxwell's demon problem in
terms of conditional action. A "conditional action" on a system is a unitary
time evolution, selected according to the result of a previous measurement,
which can reduce the entropy of the system. However, any conditional action can
be realized by an (unconditional) unitary time evolution of a larger system and
a subsequent L\"uders measurement, whereby the entropy of the entire system is
either increased or remains constant. We give some examples that illustrate and
confirm our proposal, including the erasure of $N$ qubits and the Szilard
engine, thus relating the present approach to the Szilard principle and the
Landauer principle that have been discussed as possible solutions of the
Maxwell's demon problem.
