A man spends one night in the Concord jail for refusing to pay a poll tax, and from that refusal builds an argument that will cross oceans. Henry David Thoreau published this essay in 1849, protesting both slavery and the Mexican-American War, and its central claim is elemental: that the individual conscience is a higher authority than any statute, and that obedience to an unjust law is itself injustice. The prose has the compressed force of a man who has thought long and spoken little. Thoreau's jail stay was brief, his bail paid against his wishes, but the essay proved inexhaustible. Gandhi carried it to South Africa; King carried it to Birmingham. It endures as proof that one person standing still can move the world.