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There are only four ways in which a ruling group can fall from power. Either it is conquered from without, or it governs so inefficiently that the masses are stirred to revolt, or it allows a strong and discontented Middle group to come into being, or it loses its own self-confidence and willingness to govern. These causes do not operate singly, and as a rule all four of them are present in some degree. A ruling class which could guard against all of them would remain in power permanently. Ultimately the determining factor is the mental attitude of the ruling class itself.
-- George Orwell, Nineteen Eighty-Four