Those who think that all virtue is to be found in their own party principles push matters to extremes
 Aristotle, Politics (4th century BC). copy citation

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Author Aristotle
Source Politics
Topic virtue principles
Date 4th century BC
Language English
Reference
Note Translated by Benjamin Jowett
Weblink http://classics.mit.edu/Aristotle/politics.html

Context

“Neither should we forget the mean, which at the present day is lost sight of in perverted forms of government; for many practices which appear to be democratical are the ruin of democracies, and many which appear to be oligarchical are the ruin of oligarchies. Those who think that all virtue is to be found in their own party principles push matters to extremes; they do not consider that disproportion destroys a state. A nose which varies from the ideal of straightness to a hook or snub may still be of good shape and agreeable to the eye; but if the excess be very great, all symmetry is lost, and the nose at last ceases to be a nose at all on account of some excess in one direction or defect in the other;” source