Tom Beauchamp and James Childress‘s revolutionary textbook, Principles of Biomedical Ethics, shaped the field of bioethics in America and around the world. Midway through the Principle’s eight editions, however, the authors jettisoned their attempt to justify the four principles of bioethics —autonomy, beneficence, nonmaleficence, justice—in terms of ethical theory, replacing it with the idea that these principles are part of a common morality shared by all rational persons committed to morality, at all times, and in all places. Other commentators contend that their theory has never been empirically confirmed and is unfalsifiable, since counterexamples can be deemed irrational, or as held by those living lives not committed to morality. The thesis of this paper is that common morality theory is the artifact of a category mistake—conflating common areas regulated by moral norms with common norms regulating moral conduct—that accords mid-twentieth century American liberal morality the status of transcultural, transtemporal, eternal moral truths. Such a conception offers bioethicists no tools for analyzing moral change—moral progress, regress, reform, evolution, devolution, or revolution—no theoretical basis for deconstructing structural classicism, racism, and sexism, or for facilitating international cooperation on ethical issues in the context of culturally based moral differences.