_Yoga_--Sanskrit, _union_--is a training of the different bodies of man by the will; its object is to make of those bodies complete and perfect instruments, capable of responding to the vibrations of the outer universe as well as to those of the individual soul. When this process is accomplished, man can receive, consciously and at will, in any one of his bodies, vibrations received by the soul primarily in one of the others; for instance, he may feel in the physical brain the direct action of his astral or higher bodies; he may also leave the physical, and feel directly in his astral body the action of the mental body, and so on. _Yoga_ can be practised only under the guidance of a Master, _i.e._, a highly developed being, capable of guiding the student safely through the dangers incidental to this training.] [Footnote 4: When the astral body is externalised, the subject cannot speak; he must await its return; when only partially externalised or not at all, and consciousness is centred in it, the subject can speak and relate what he sees afar off, for astral vision is possible at enormous distances. Such cases as these are frequently met with.] [Footnote 5: In 1876, in a Leipzic hospital, there was a patient possessed of neither sensibility nor muscular sense. He had only sight in the right eye and hearing in the left ear. If this eye and ear were closed, the patient immediately fell asleep. Neither by being touched nor shaken could he be awakened; to effect this, it was necessary to open his eye and unstop his ear. (_Archiv. fuer die ges. Physiologie_, vol. 15, p. 573).] [Footnote 6: These pictures are often visible in the astral world; they explain the prophetic faculty of ordinary seers.] [Footnote 7: In such cases, by association of ideas or any other influence, the soul dramatises the physical impression which calls forth the dream, and creates the long phantasmagoria of this dream in so short a time as to be scarcely appreciable. Between the sleeping physical body and the externalised astral body there is so close a degree of sympathy that the latter is conscious of everything that takes place in the former. This explains why the astral body returns so rapidly to the physical when a noise, light, or any other sensation impresses this latter.] [Footnote 8: We say "language of the physical plane" because the soul, in the astral body, sees in four dimensions, _i.e._, all the parts of an object at once, as though these parts were spread out on a two-dimensional plane. Consequently, the higher vision needs interpretation in order to be expressed on the physical plane.]