This dissertation explores, primarily, the implications of a properly constructed anthropology for ethics, and secondarily, the implications of ethics for properly constructing anthropology, by placing the work of Reinhold Niebuhr and Jean-Paul Sartre in critical dialogue. Contemporary ethical debates hinge on anthropological claims about human nature, but ethicists often fail to explicitly acknowledge, much less critically develop and defend, these claims, which tend to overemphasize either human freedom or human limitation. In theory, Niebuhr appropriately acknowledges both finitude and transcendence; as he develops his anthropology, however, especially in his doctrine of sin, he fails to maintain this tension. This failure produces various weaknesses in his anthropologically based personal and social ethics, which risk alienating Christians who would otherwise find Niebuhr an attractive source of theological and ethical guidance. At one level, the dissertation argues that Sartre, whose similar anthropology is myopic in the opposite direction, can render Niebuhr's anthropology symmetrical, thereby salvaging his practical ethics. At a broader level, the dissertation argues that attending to a properly constructed anthropology is essential to furthering debates in practical ethics, and that these debates in turn test, and thus contribute to, increased understandings of anthropology, including revised views of human freedom and its limits.