  According to what has become a standard history of quantum mechanics, in 1932
von Neumann persuaded the physics community that hidden variables are
impossible as a matter of principle, after which leading proponents of the
Copenhagen interpretation put the situation to good use by arguing that the
completeness of quantum mechanics was undeniable. This state of affairs lasted,
so the story continues, until Bell in 1966 exposed von Neumann's proof as
obviously wrong. The realization that von Neumann's proof was fallacious then
rehabilitated hidden variables and made serious foundational research possible
again. It is often added in recent accounts that von Neumann's error had been
spotted almost immediately by Grete Hermann, but that her discovery was of no
effect due to the dominant Copenhagen Zeitgeist. We shall attempt to tell a
story that is more historically accurate and less ideologically charged. Most
importantly, von Neumann never claimed to have shown the impossibility of
hidden variables tout court, but argued that hidden-variable theories must
possess a structure that deviates fundamentally from that of quantum mechanics.
Both Hermann and Bell appear to have missed this point, moreover, both raised
unjustified technical objections to the proof. Von Neumann's argument was
basically that hidden-variables schemes must violate the "quantum principle"
that physical quantities are to be represented by operators in a Hilbert space.
As a consequence, hidden-variables schemes, though possible in principle,
necessarily exhibit a certain kind of contextuality.
