  Environment induced decoherence, and other quantum processes, have been
proposed in the literature to explain the apparent spontaneous selection - out
of the many mathematically eligible bases - of a privileged measurement basis
that corresponds to what we actually observe. This paper describes such
processes, and demonstrates that - contrary to common belief - no such process
can actually lead to a preferred basis in general. The key observation is that
environment induced decoherence implicitly assumes a prior independence of the
observed system, the observer and the environment. However, such independence
cannot be guaranteed, and we show that environment induced decoherence does not
succeed in establishing a preferred measurement basis in general. We conclude
that the existence of the preferred basis must be postulated in quantum
mechanics, and that changing the basis for a measurement is, and must be,
described as an actual physical process.
