  We propose a new approach to justify the use of the microcanonical ensemble
for isolated macroscopic quantum systems. Since there are huge number of
independent observables in a macroscopic system, we cannot see all of them.
Actually what we observe can be written in a rather simple combination of local
observables. Considering this limitation, we show that almost all states in a
energy shell are practically indistinguishable from one another, and hence from
the microcanonical ensemble. In particular, the expectation value of a
macroscopic observable is very close to its microcanonical ensemble average for
almost all states.
