The mystical experience usually presents itself as an intense experience that implies personal unification with the Divine or Ultimate Reality. But although in this sense "only mystics can speak and write about mysticism," it is possible to accompany them in their exploration and to study the mystical phenomenon with a view to understanding not only their nature and manifestations, but their relation to the world of the spirit in general . The present work of Evelyn Underhill, one of the classic treatises on mysticism, seeks to restore, in the words of its author, the old meaning of mysticism as "science or art of spiritual life." Its value lies both in the richness of its documentation and in the clarity and security of its fundamental intuitions and theses, which make it an essential introduction. The first part of the book delimits the "mystical fact" in its relation with metaphysics, psychology, theology, magic and symbolism. The second part, more extensive, exposes the development of spiritual consciousness in the mystical way, entering into its main themes - the awakening of the self, its purification, voices and visions, introversion, ecstasy and outburst, night Dark soul and unitive life - through the reading of Eckhart, Teresa of Jesus or John of the Cross, among others. The appendix provides a historical sketch of European mysticism, from the early Christian era to William Blake.