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Henry David Thoreau (see name pronunciation; July 12, 1817 - May 6, 1862) was an American essayist, poet, philosopher, abolitionist, naturalist, tax resister, development critic, surveyor, and historian. A leading transcendentalist, Thoreau is best known for his book Walden, a reflection upon simple living in natural surroundings, and his essay "Civil Disobedience" (originally published as "Resistance to Civil Government"), an argument for disobedience to an unjust state. Thoreau's books, articles, essays, journals, and poetry amount to more than 20 volumes. Among his lasting contributions are his writings on natural history and philosophy, in which he anticipated the methods and findings of ecology and environmental history, two sources of modern-day environmentalism.

Thoreau contracted tuberculosis in 1835 and suffered from it sporadically afterwards. In 1860, following a late-night excursion to count the rings of tree stumps during a rainstorm, he became ill with bronchitis. His health declined, with brief periods of remission, and he eventually became bedridden. Recognizing the terminal nature of his disease, Thoreau spent his last years revising and editing his unpublished works, particularly The Maine Woods and Excursions, and petitioning publishers to print revised editions of A Week and Walden. He wrote letters and journal entries until he became too weak to continue. His friends were alarmed at his diminished appearance and were fascinated by his tranquil acceptance of death. When his aunt Louisa asked him in his last weeks if he had made his peace with God, Thoreau responded, "I did not know we had ever quarreled."  Aware he was dying, Thoreau's last words were "Now comes good sailing", followed by two lone words, "moose" and "Indian". He died on May 6, 1862, at age 44. Amos Bronson Alcott planned the service and read selections from Thoreau's works, and Channing presented a hymn. Emerson wrote the eulogy spoken at the funeral. Thoreau was buried in the Dunbar family plot; his remains and those of members of his immediate family were eventually moved to Sleepy Hollow Cemetery (42deg27'53''N 71deg20'32''W) in Concord, Massachusetts.  Thoreau's friend William Ellery Channing published his first biography, Thoreau the Poet-Naturalist, in 1873. Channing and another friend, Harrison Blake, edited some poems, essays, and journal entries for posthumous publication in the 1890s. Thoreau's journals, which he often mined for his published works but which remained largely unpublished at his death, were first published in 1906 and helped to build his modern reputation. A new, expanded edition of the journals is under way, published by Princeton University Press. Today, Thoreau is regarded as one of the foremost American writers, both for the modern clarity of his prose style and the prescience of his views on nature and politics. His memory is honored by the international Thoreau Society and his legacy honored by the Thoreau Institute at Walden Woods, established in 1998 in Lincoln, Massachusetts.

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Aware he was dying, Thoreau's last words were "Now comes good sailing", followed by two lone words, "moose" and "Indian".