Interesting citation, I had heard of John Bell's theorem, which disproves local hidden variable theories. But I did not know that von Neumann had his own attempt at disproving these theories. Researching this a little further, I also found people arguing that von Neumanns arguments were misconstrued by Bell. https://arxiv.org/abs/1006.0499 I wonder how Grete Hermann's arguments compare to the arguments from Bell and what the real truth is.