Underhill's greatest book, Mysticism: A Study of the Nature and Development of Man's Spiritual Consciousness, was published in 1911, and is distinguished by the very qualities which make it inappropriate as a straightforward textbook. The spirit of the book is romantic, engaged, and theoretical rather than historical or scientific. Underhill has little use for theoretical explanations and the traditional religious experience, formal classifications or analysis. Underhill substituted (1) mysticism is practical, not theoretical, (2) mysticism is an entirely spiritual activity, (3) The business and method of mysticism is love. (4) mysticism entails a definite psychological experience. Her insistence on the psychological approach was that it was the glamorous science of the pre-war period, offering the potential key to the secrets of human advances in intelligence, creativity, and genius, and already psychological findings were being applied in theology (i.e., William Sanday's Christologies Ancient and Modern).