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=== Von Neumann's proof ===
[[John von Neumann]] in his 1932 book ''[[Mathematical Foundations of Quantum Mechanics]]'' had presented a [[Mathematical Foundations of Quantum Mechanics#No hidden variables proof|proof]] that there could be no "hidden parameters" in quantum mechanics. The validity of von Neumann's proof was questioned by [[Grete Hermann]] in 1935, who found a flaw in the proof. The critical issue concerned averages over ensembles. Von Neumann assumed that a relation between the [[expected value]]s of different observable quantities holds for each possible value of the "hidden parameters", rather than only for a statistical average over them.<ref>{{cite book|first=Max |last=Jammer |author-link=Max Jammer |title=The Philosophy of Quantum Mechanics |pages=265–274 |year=1974 |publisher=John Wiley and Sons |isbn=0-471-43958-4}}</ref><ref>{{Cite journal |last1=Mermin |first1=N. David |author-link1=N. David Mermin |last2=Schack |first2=Rüdiger |date=September 2018 |title=Homer Nodded: Von Neumann's Surprising Oversight |journal=Foundations of Physics |language=en |volume=48 |issue=9 |pages=1007–1020 |arxiv=1805.10311 |bibcode=2018FoPh...48.1007M |doi=10.1007/s10701-018-0197-5 |issn=0015-9018 |doi-access=free}}</ref> However Hermann's work went mostly unnoticed until its rediscovery by John Stewart Bell more than 30 years later.<ref>Hermann, G.: Die naturphilosophischen Grundlagen der Quantenmechanik (Auszug). Abhandlungen
der Fries’schen Schule 6, 75–152 (1935). English translation: Chapter 15 of “Grete Hermann —
Between physics and philosophy”, Elise Crull and Guido Bacciagaluppi, eds., Springer, 2016, 239-
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