Thoreau was a philosopher of nature and its relation to the human condition. In his early years, he accepted the ideas of Transcendentalism, an eclectic idealist philosophy that included among its advocates Ralph Waldo Emerson, Margaret Fuller, and Bronson Alcott.
After college, Thoreau taught school, wrote essays and poems for The Dial, and briefly attempted freelance writing in New York City. The death of his brother in 1842 was a profound emotional shock and may have influenced his decision to live with his parents and never to marry.
Thoreau embarked on a two-year experiment in simple living on July 4, 1845 when he moved to a second-growth forest around the shores of beautiful Walden Pond and lived in a self-built house on land owned by Emerson. The house was not in wilderness but at the edge of town, 1.5 miles from his family home. On a trip into town, he ran into the local tax collector who asked him to pay six years of delinquent poll taxes. Thoreau refused because of his opposition to the Mexican-American War and slavery, for which he spent a night in jail. His later essay on this experience, Civil Disobedience, influenced Leo Tolstoy, Mohandas Gandhi, and Martin Luther King, Jr..
In August of 1846, Thoreau briefly left Walden to make a trip to Mount Katahdin in Maine, a journey later recorded in "Ktaadn," the first part of The Maine Woods. At Walden Pond he completed a first draft of A Week on the Concord and Merrimack Rivers, describing his 1839 trip to the White Mountains with his now-deceased brother, John Thoreau Jr. When this book failed to find a publisher, Emerson urged Thoreau to publish at his own expense. He did so with Munroe, Emerson's own publisher, who did little to publicize the book. Its failure put Thoreau into debt that took years to pay off, and Emerson's flawed advice began to cause a schism between the friends that never entirely healed.
Thoreau left Walden Pond on September 6, 1847. Over several years he worked off his debts and also continuously revised his manuscript. In 1854 he published Walden, or Life in the Woods, recounting the two years and two months he had spent at Walden Pond. The book compresses that time into a single calendar year, using the passage of four seasons to symbolize human development. Part memoir and part spiritual quest, Walden at first won few admirers, but today critics regard it as a classic American book that explores natural simplicity, harmony, and beauty as models for just social and cultural conditions.