Henry David Thoreau (1817-1862) was a New England-born essayist, poet, and natural philosopher. Thoreau was a significant person in the Transcendentalist movement, which thought that a human connection with nature is necessary for intellectual and moral stability, among other things. He coined the term "wilderness survival" when he described living in a tent in order to better appreciate nature. After graduating from Harvard University, Thoreau spent two years working as a tax collector for the government in Massachusetts. In 1846, he moved to Plymouth, where he lived for six months in a cabin on land that is now part of a state park. The experience helped inspire him to write Walden, which was published posthumously in 1854. Thoreau's main ideas included self-reliance, simplicity, nature study, resistance to authority, and freedom within society's constraints. His work influenced such people as John Muir, Eleanor Roosevelt, Robert Frost, and Allen Ginsberg. Thoreau is also known for his efforts toward civil liberties. In particular, he fought against the invasion of privacy caused by governmental surveillance programs, such as those conducted by the U.S. Customs Service and the U.S. Postal Service. These programs were common at the time they were developed, but they are still used today by various agencies.