  It is well known, that the causal Schr\"odinger evolution of a quantum state
is not compatible with the reduction postulate, even when decoherence is taken
into account. The violation of the causal evolution, introduced by the standard
reduction postulate distinguishes certain systems (as the measurement devices),
whose states are very close to statistical mixtures (as the ones resulting from
the process of decoherence). In these systems, this violation has not any
observable effect. In arbitrary quantum systems, the transition from the
initial density matrix to a diagonal matrix predicted by the standard reduction
postulate, would lead to a complete breakdown of the Schr\"odinger evolution,
and consequently would destroy all the predictive power of quantum mechanics.
What we show here, is that there is a modified version of the postulate that
allows to treat all the quantum mechanical systems on equal footing. The
standard reduction postulate may be considered as a good approximation, useful
for practical purposes, of this modified version which is consistent with the
Schr\"odinger evolution and via decoherence with the experimental results.
Within this approach, the physical role played by the reduction postulate is as
a tool for the computation of relative probabilities and consequently for the
determination of the probabilities of consistent histories.
