  Aharonov and Anandan's claim that the notion of ``proper mixture'' is
improper is shown to be unjustified. The point is made that if a purely
operationalist standpoint is taken the three difficulties these authors
describe relatively to the conventional interpretation of density matrices in
fact vanish. It is noted that nevertheless it is very difficult for us to do
without any form of realism, in particular when the quantum measurement problem
is considered, and it is stressed that the proper mixture notion comes in
precisely at this level. The more general question of the real bearing of
Aharonov and Anandan's ideas on the interpretation of quantum mechanics problem
is considered.
