The practice or tendency to rigidly adhere to doctrines, theories, or principles without regard for practical experience or real-world results.
From doctrinaire (French, from doctrine + -aire 'one who') + -ism (from Greek -ismos 'practice or system'). This was borrowed from French political discourse to describe ideological inflexibility.
French revolutionaries coined 'doctrinaire' to mock politicians who stuck to abstract theories despite evidence they weren't working—it's a warning about the danger of letting ideology override reality that we still need today.
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