Ralph Waldo Emerson (May 25, 1803 – April 27, 1882) was an American essayist, lecturer, and poet, who led the Transcendentalist movement of the mid-19th century. He was seen as a champion of individualism and a prescient critic of the countervailing pressures of society, and he disseminated his thoughts through dozens of published essays and more than 1,500 public lectures across the United States. Emerson gradually moved away from the religious and social beliefs of his contemporaries, formulating and expressing the philosophy of Transcendentalism in his 1836 essay, Nature. Following this ground-breaking work, he gave a speech entitled The American Scholar in 1837, which Oliver Wendell Holmes, Sr. considered to be America's "Intellectual Declaration of Independence". Emerson wrote most of his important essays as lectures first, then revised them for print. (Courtesy of Wikipedia) 

Henry David Thoreau (1817-1862) was a writer and philosopher as well as a naturalist. Walden is considered his masterpiece.