  For a macroscopic, isolated quantum system in an unknown pure state, the
expectation value of any given observable is shown to hardly deviate from the
ensemble average with extremely high probability under generic equilibrium and
nonequilibrium conditions. Special care is devoted to the uncontrollable
microscopic details of the system state. For a subsystem weakly coupled to a
large heat bath, the canonical ensemble is recovered under much more general
and realistic assumptions than those implicit in the usual microcanonical
description of the composite system at equilibrium.
