Conversion stories, mystical visions, accounts of saintliness and sick souls fill these Gifford Lectures with the raw testimony of human beings who have been seized by something they cannot name. William James, brother of the novelist and himself a man who had stared into the abyss of despair, approached religion not as a theologian but as a psychologist, asking not whether God exists but what happens to people who believe He does. The result is a masterwork of radical empiricism applied to the inner life, generous to every shade of spiritual experience, dismissive of none. A century after its delivery in Edinburgh, it remains the most honest and capacious book ever written about what it feels like to be claimed by the sacred.