On Liberty (1859) is John Stuart Mill's defense of individual freedom against both state power and social conformity. It advances the "harm principle": that power may rightly be used over a member of society, against their will, only to prevent harm to others. The excerpt here is Chapter II, "Of the Liberty of Thought and Discussion," where Mill argues that silencing any opinion presumes our own infallibility, and that even a true belief, left uncontested, hardens into dead dogma.
Original summary for this showcase, informed by public sources including Wikipedia.
Read more: On Liberty on Wikipedia · Full text: On Liberty at Wikisource