Ralph Waldo Emerson 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. His first two collections of essays – Essays: First Series and Essays: Second Series, published respectively in 1841 and 1844 – represent the core of his thinking, and include such well-known essays as Self-Reliance, The Over-Soul, Circles, The Poet and Experience. Together with Nature, these essays made the decade from the mid-1830s to the mid-1840s Emerson's most fertile period.