"Mysticism" for Underhill is a process by which the individual "soul" moves toward final and utterly ineffable union with the "absolute."
"Broadly speaking I understand [mysticism] to be the innate tendency of the human spirit towards the complete harmony with the transcendental order, whatever be the theological formula under which that order is understood. This tendency, in great mystics, gradually captures the whole field of consciousness, it dominates their life and, in the experience called ‘mystic union,’ attains its end…I believe this process to represent the true line of development of the highest form of human consciousness. (Mysticism, p. XV)
In her book, Mysticism, EU distills from the writings and experiences of Western mystics what she considers to be the characteristic stages or experiences that mark the progress of the mystic.
AWAKENING: the first step toward the mystic goal in which a person awakens to the existence of a divine order or reality.
"This experience, usually abrupt and well-marked, is accompanied by intense feelings of joy and exhaltation." (p. 169)
Psychologically, this awakening of transcendental consciousness is a form of conversion.
PURGATION: This first phase of awakening soon shades into the second – purgation.
The mystic increasingly realizes his or her imperfection and finiteness in contrast to the sacred and attempts by means of discipline and or mortification to eliminate all that stands in the way of coming into deeper contact with the transcendent.
"The first thing that the self observes when it turns back upon itself in that awful moment of lucidity [of awakening] is the horrible contrast between its clouded contours and the pure sharp radiance of the Real; between its muddled faulty life, its perverse self-centered drifting, and [the clarity of the transcendent]." (p.200)
ILLUMINATION: the pain and effort of purgation open before long upon the third phase of mystic life -- illumination.
Illumination results from having purged oneself of attachments to the things of the senses and having substituted for them an attachment to the ttranscendent.
It is "a state which includes in itself many of the stages of contemplation, visions and adventures of the soul described in " the writings of the great mystics.
But deep as its joys may be, illumination is not finally true union with the absolute, and Underhill warns of its dangers:
"In persons opf feeble or diffuse intelligence…and above all in victims of self-regarding spirituality, this deep absorption in the sense of Divine Reality may easily degenerate into a monoideism. Then the ‘shady side’ of Illumination, a selfish preoccupation with transcendental joys, the ‘spiritual gluttony’ condemned by St. John of the Cross, comes out. (p. 246)"
AWAKENING, PURGATION and ILLUMINATION form the "first mystic life" and many mystics never go beyond it. (Many people not usually considered "mystics" have shared to some extent at least the experience of the Illuminated state.)
THE DARK NIGHT OF THE SOUL: In the strongest mystics, Illumination gives way to what St. John of the Cross called "the dark night of the soul."
This fourth phase of mystic development is the final, deepest and most radical purgation the mystic experiences in his movement into the divine.
The heart and mind are arid; no longer does the mystic experience the joys (the mere pleasures) of Illumination.
The task now is to learn to "dissociate the personal satisfaction of mystical vision from the reality of mystical life." (p. 170)
In other words, the mystic must overcome all attachment to self and ego (to selfish ego) in order to merge with the great One in, under, behind and through all things.
To do this, the mystic must finally give up the greatest good he or she has ever known – the powerful attachment to the transcendent pleasures and visions of Illumination.
Giving them up, all things seem at first to have become ashes – the Divine seems to have fled.
UNION: But passing through the dark night, the mystic enters at last into Union, the fifth and last phase of mystic expansion into the transcendent.
Union is essentially ineffable.
In it, one no longer merely perceives and enjoys the transcendent (as
with Illumination) but becomes one with it.