From Mysticism, by Evelyn Underhill:
1. True mysticism is active and practical, not passive and theoretical. It is an organic life-process, a somthing which the whole self does; not something as to which its intellect holds an opinion.
2. Its aims are wholly transcendental and spiritual. It is in no way concerned with adding to, exploring, re-arranging, or improving anything in the visible universe. The mystic brushes aside that universe, even in its supernormal manifestations. Though he does not, as his enemies declare, neglect his duty to the many, his heart is always set upon the changeless One.
3. This One is for the mystic, not merely the Reality of all that is, but also a living and personal Object of Love; never an object of exploration. It draws his whole being homeward, but always under the guidance of the heart.
4. Living union with this One–which is the term of his adventure–is a definite state or form of enhanced life. It is obtained neither from an intellectual realization of its delights, nor from the most acute emotional longings. Though these must be present, they are not enough. It is arrived at by an arduous psychological and spiritual process–the so-called Mystic Way–entailing the complete remaking of character and the liberation of a new, or rather latent, form of consciousness; which imposes on the self the condition which is sometimes inaccurately called “ecstasy,” but is better named the Unitive State.
Mysticism, then, is not an opinion: it is not a philosophy. It has nothing in common with the pursuit of occult knowledge. On the one hand it is not merely the power of contemplating Eternity: on the other, it is not to be identified with any kind of religious queerness. It is the name of that organic process which involves the perfect consummation of the Love of God; the achievement here and now of the immortal heritage of man. Or, if you like it better–for this means exactly the same thing–it is the art of establishing a conscious relation with the Absolute.
Underhill, Evelyn. Mysticism. 1911. Image Edition. New York: Doubleday, 1990, p. 81.