In their work on Turner's formative period, Ray A. Billington and Fulmer Mood have shown that the Frontier Thesis, formulated in 1893 in “The Significance of the Frontier in American History,” is not so much a brilliant early effort by a young scholar as a mature study in which Turner gave his ideas an organization that proved to be final. During the rest of his life he developed but never disclaimed or modified them. Billington and Mood also add that the Frontier Thesis is meant to test a new approach to history that Turner had been developing since the beginning of his academic career. We can fully understand it, then, only by setting it within the framework of the assumptions and goals of his 1891 essay, “The Significance of History,” Turner's only attempt to sketch a philosophy of history.