Science and religion have long been thought incompatible. But nowhere has this apparent contradiction been more fully resolved than in the figure of A. S. Eddington (1882-1944), a pioneer in astrophysics, relativity, and the popularization of science, and a devout Quaker. Practical Mystic uses the figure of Eddington to show how religious and scientific values can interact and overlap without compromising the integrity of either. Eddington was a world-class scientist who not only maintained his religious belief throughout his scientific career but also defended the interrelation of science and religion while drawing inspiration from both for his practices. For instance, at a time when a strict adherence to deductive principles of physics had proved fruitless for understanding the nature of stars, insights from Quaker mysticism led Eddington to argue that an outlook less concerned with certainty and more concerned with further exploration was necessary to overcome the obstacles of incomplete and uncertain knowledge. By examining this intersection between liberal religion and astrophysics, Practical Mystic questions many common assumptions about the relationship between science and spirituality. Matthew Stanley's analysis of Eddington's personal convictions also reveals much about the practice, production, and dissemination of scientific knowledge at the beginning of the twentieth century.