Increasingly, contemporary medical ethicists have become aware of the need to explicate a foundation for their various models of applied ethics. Many of these theories are inspired by the apparent incompatibility of patient autonomy and provider beneficence. The principle of patient autonomy derives its current primacy to a large extent from its legal origins. However, this principle seems at odds with the clinical reality. In the bioethical literature, the notion of authenticity has been proposed as an alternative foundational principle to autonomy. This article examines this proposal in reference to various existentialist philosophers (Heidegger, Sartre, Camus and Marcel). It is concluded that the principle of autonomy fails to do what it is commonly supposed to do: provide a criterion of distinction that can be invoked to settle moral controversies between patients and providers. The existentialist concept of authenticity is more promising in at least one crucial respect: It acknowledges that the essence of human life disappears from sight if life's temporal character is reduced to a series of present decisions and actions. This also implies that the very quest for a criterion that allows physicians to distinguish between sudden, unexpected decisions of their patients to be or not to be respected, without recourse to the patient's past or future, is erroneous.