What grounds and justifies conclusions in medical ethics? Is the source external or internal to medicine? Three influential types of answer have appeared in recent literature: an internal account, an external account, and a mixed internal/external account. The first defends an ethic derived from either the ends of medicine or professional practice standards. The second maintains that precepts in medical ethics rely upon and require justification by external standards such as those of public opinion, law, religious ethics, or philosophical ethics. The third claims that distinct medical ethics have emerged from distinct cultural frameworks, each with norms that govern physicians. There is merit in each perspective, but each over reaches its supporting arguments and fails to appreciate what is legitimate in the theses of its competitors. I propose a fourth account that offers a way to escape limitations of the other three, while retaining their most attractive features.