A book by American philosopher and psychologist William James, published in 1902. It is a classic work in the psychology of religion, examining various forms of religious experience and their psychological significance, including conversion, mysticism, and saintliness.
William James famously tried to identify the characteristic aspects of religious experience and one of the most important was the ineffability.
""William James was mentioned by name in the context of identifying the 'ineffable quality' of religious experience. This book is his most famous and influential work on the subject, directly addressing the characteristics of such experiences."