An Outline of Occult Science (4th Edition). By Rudolf Steiner, Ph.D. Originally Published 1922.

---Much complementary and additional matter has been inserted in this edition in the part dealing with "Perception of the Higher Worlds." The endeavour has been made to represent in a graphic way the kind of inner soul-processes by which the power of cognition liberates itself from the limits which confine it in the world of sense and thereby becomes qualified for experiencing the supersensible world. The attempt has been made to show that these experiences, even though gained by entirely inner ways and methods, still do not have a merely subjective significance for the particular individual who gains them. The description attempts to show that within the soul stripped of its individuality and personal peculiarities, an experience takes place which every human being may have in the same way, if he will only work at his development from out his subjective experiences. It is only when "knowledge of supersensible worlds" is thought of as bearing this character that it may be differentiated from old experiences of merely subjective mysticism. Of this mysticism it may be said that it is after all more or less a subjective concern of the mystic. The scientific spiritual training of the soul, however, as it is described here, strives for objective experiences, the truth of which, although recognized in an entirely inner way, may yet, for that very reason, be found to be universally valid.---Rudolf Steiner