  Experimental evidence, the heuristics of indistinguishability, and its
logical inconsistency with quantum formalism all argue against the existence of
a quantum mixture uncorrelated with the exterior, that is, argue for the
postulate "The state of a system uncorrelated with its exterior is pure." This
is shown to be equivalent with "The state of a system describable in terms of
indistinguishable pure states is pure," and with "The state of the universe is
pure"; further, it yields a quantitative expression of the traditional relation
of welcher Weg information to partial coherence. It is concluded that all
mixtures are "improper," the trace-reduction of a composite system's pure
state.
