AbstractThis article explores arguments among medico-legal experts (including Amariah Brigham, Isaac Ray, John P. Gray, and W. A. Hammond) about the social and moral ramifications of expanding the definition of insanity to include moral insanity. This is an important corrective to a standard view that sees the movement to transform the insanity defense in nineteenth-century America as a rejection of the explanatory power of human depravity in the face of an optimistic understanding of human nature as found in Scottish common sense philosophy. The debate, especially between Ray and Brigham, on the one hand, and Gray, on the other, finally led to a situation in which religious discussions of sin, the will, grace, and the shape of the human self were legally sidelined.