  We give a simple and intuitive proof that the only states which are
completely passive, i.e. those states from which work cannot be extracted even
with infinitely many copies, are Gibbs states at positive temperatures. The
proof makes use of the idea of virtual temperatures, i.e. the association of
temperatures to pairs of energy levels (transitions). We show that (i) passive
states are those where every transition is at a positive temperature, and (ii)
completely passive states are those where every transition is at the same
positive temperature.
