We attempt to clarify certain puzzles concerning state collapse and
decoherence. In open quantum systems decoherence is shown to be a necessary
consequence of the transfer of information to the outside; we prove an upper
bound for the amount of coherence which can survive such a transfer. We claim
that in large closed systems decoherence has never been observed, but we will
show that it is usually harmless to assume its occurrence. An independent
postulate of state collapse over and above Schroedinger's equation and the
probability interpretation of quantum states, is shown to be redundant.Comment: 13 page