What, exactly, does it mean to be human? It is an age-old question, one for which theology, philosophy, science, and medicine have all provided different answers. But though a unified response to the question can no longer be taken for granted, how we answer it frames the wide range of different norms, principles, values, and intuitions that characterize today's bioethical discussions. If we don't know what it means to be human, how can we judge whether biomedical sciences threaten or enhance our humanity?

This fundamental question, however, receives little attention in the study of...