Ralph Waldo Emerson (1803-1882) was an American philosopher essayist, lecturer, and poet, who led the Transcendentalist Movement of the mid-19th century. He was seen as a champion of individualism and critical thinking, as well as a prescient critic of the countervailing pressures of society and conformity. Friedrich Nietzsche considered him «the most gifted of the Americans», and Walt Whitman referred to him as his «master». Emerson wrote on a number of subjects, never espousing fixed philosophical tenets, but developing certain ideas such as individuality, freedom, the ability for mankind to realize almost anything, and the relationship between the soul and the surrounding world. His first two collections of essays, Essays: First Series (1841) and Essays: Second Series (1844), represent the core of his thinking. They include the well-known essays Self-Reliance , Spiritual Laws , Heroism , 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.