William James delivered these lectures at Edinburgh in 1901-1902, surveying mysticism, conversion, saintliness, and the psychology of religion with a philosopher's rigor and a novelist's sympathy. James refused to reduce religion to pathology or to accept uncritically any doctrine. What remained was the function of belief, its fruits in human life, the pragmatic test. The lectures remain essential for anyone interested in religion, psychology, or the peculiar genius of American thought.