Over Robert Penn Warrens lifetime, he won almost every prestigious award a writer can win, for novels, volumes of long narrative poetry, volumes of lyric poetry, biography, social criticism, literary criticism, definitive pedagogical texts, drama. His work effected deep and far-ranging influences on American education, American history, and American literature. Yet, despite the fact that Warren declared himself to be a "philosopher-poet," critical scholarship on Warren has not persuasively established what sort of philosophy he embraced. My study argues that Robert Penn Warren was an Existentialist philosopher as well as poet, and close examination of what was in many ways his most important work, Brother to Dragons, yields clear evidence of Warrens truest philosophical affinities. Brother to Dragons depicts the historical incident of Thomas Jeffersons nephew Lilburne Lewis, who was convicted of dismembering one of his slaves in a night of horror. Jefferson never spoke of this incident, and either did not know of it or would not admit to it. In Warrens epic treatment of the long poem, Jefferson, the icon of the triumph of Enlightenment and Romantic Idealism, must confront his nephew Lilburne, who is iconic of Naturalisms most debased possibility in human life. In this confrontation, Warren works through the dialectical problems that obsessed him, and we find them addressed in Existentialist terms. Life and death, existence and nothingness, freedom and finitude, beauty and horror, the light and the darkness, the individual and the community, past and present, truth and lies, the existential abdication and the heroic act, salvation and damnation: Warren uses Existentialist philosophy and conventions to explore and then resolve (insofar as paradox gets resolved) the innate absurdity and holiness of human life. In a close reading of Brother to Dragons, my study centers on the theme of mans tragic nature, or original sin, and applies Existentialist philosophy and its writers to show how Robert Penn Warren developed and created Brother to Dragons as his own philosophical manifesto, declaring his ontology as well.