  We consider state changes in quantum theory due to "conditional action" and
relate these to the discussion of entropy decrease due to interventions of
"intelligent beings" and the principles of Szilard and Landauer/Bennett. The
mathematical theory of conditional actions is a special case of the theory of
"instruments" which describes changes of state due to general measurements and
will therefore be briefly outlined in the present paper. As a detailed example
we consider the imperfect erasure of a qubit that can also be viewed as a
conditional action and will be realized by the coupling of a spin to another
small spin system in its ground state.
