AbstractStarting in the middle decades of the nineteenth century, a group of American Christians rejected their parents’ Calvinism and fashioned new views of sin, the self, and spiritual growth. These believers were aided in this process by new, psychological sciences such as phrenology, sciences that pointed to the existence of powerful spiritual faculties in the self and new ways of using and measuring them. Especially for those who felt paralyzed by sensibilities of sinfulness and moral impotence, phrenology was a liberation. But phrenology appealed to Americans for other reasons as well. By linking mental and spiritual states to physiological structures, phrenology brought the mysterious emotions and dispositions of faith to the surfaces of the self, where they could be more easily understood and reflected upon. Inner conditions could be discerned in bumps and contours of the head and body or even in one's characteristic postures and gestures. In short, the new science made confounding inner spaces visible again. This article explores the spiritual struggles of a wide range of believers who used phrenology to develop more sober and measured, and therefore more certain, forms of spiritual assurance. It argues that, beginning in the early nineteenth century, a broad coalition of religious liberals used these new, scientific psychologies such as phrenology to find in external, especially bodily, conditions signs of inner spiritual states.