This article explores the dispute between the philosopher Immanuel Kant and the physician Johann Daniel Metzger over the moral autonomy of individuals with mental illness. Situating the debate within the broader context of the evolving philosophical and medical professions in eighteenth-century Germany, the article examines how a professional conflict emerged over who – the physician or the philosopher – should serve as the legal authority in cases where moral responsibility was in question. The analysis shows that this was not merely a theoretical issue for Kant, but a practical one, brought to the fore by the infanticide trial of Margarethe Kaveczynska, in which Kant’s friend, Theodor Gottlieb Hippel, presided as judge. The article argues that while Kant’s vision for the practical application of his anthropology influenced his conception of moral autonomy, he ultimately lost ground to the rising authority of the medical profession.