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On a non-philosophical and academic level, Popper was always an outsider, a maverick and a reactionary in that he was constantly rebelling against what he regarded as mainstream, Marxist bias in academia (post-revolutionary Russia). Hence his rejection of the Vienna Circle (Wittgenstein, etc) but probably more accurately, they didn’t want him in. His reactionary side is apparent in The Open Society and Its Enemies (against Plato and Marx, for example)