"The Varieties of Religious Experience: A Study in Human Nature" by William James is a book published in 1902, based on lectures delivered at the University of Edinburgh. This pioneering work in psychology explores individual religious experiences and mysticism across different traditions, examining what James calls "healthy-minded" and "sick soul" religion. Drawing on extensive personal testimonies, James identifies common markers of mystical experience while arguing pragmatically for religion's value through its transformative effects on individuals, regardless of theological truth claims.