Karl Rahner's primary contribution to moral theology lies in his development of an anthropology able to serve as a foundation for a rehabilitated concept of natural law. Human persons in Rahner's theology are characterized by certain existentials with logical consequences for ethics. From human transcendence and freedom follows the concept of a fundamental option for or against God. From human materiality, historicity, guilt, and individuality follow the methodological necessity of distinguishing between principles and prescriptions and the unavoidable need to take risks. In contrast to unchanging metaphysical human nature, the historicity of concrete human nature and therefore of natural law requires that all material norms be recognized as conditioned by history and culture and hence limited, not universal or absolute.