Frederick Jackson Turner was an extremely influential U.S. historian, who is best known for his "frontier thesis", which proposed that American democracy was profoundly shaped by the existence of an undeveloped frontier area from the founding through the 1880s. He originally put forth his idea in the essay "The Significance of the Frontier in American History", published just three years after the frontier was "officially closed" in 1890. He continued to elaborate on the frontier thesis as well as the influence of sectionalism, and the unique contributions of the Midwest to American democracy. His theory and other writings are still debated by modern day historians. This work, The Frontier in American History, collects a number of Turner's essays and speeches, including "The Significance of the Frontier in American History."