When a man sinks into sleep the connection between his principles changes, as described earlier in this work. The part of the sleeping man which lies upon his couch comprises the physical and etheric bodies, but not the astral body and not the ego. It is because the etheric body remains bound to the physical body in sleep that the life-activities continue. For the moment the physical body is left to itself, it must of necessity fall into decay. The things that are extinguished in sleep are ideas, pain, pleasure, joy, grief, the ability to express conscious will, and similar facts of existence. But the astral body is the vehicle of all these things. That the astral body, with all its joy and sorrow, its realm of thought and will, is annihilated in sleep is an opinion that cannot be entertained by an unbiased judgment; it exists still, but in another condition. In order that the human ego and the astral body may not only be endowed with pleasure and pain and all the other things we have named, but also have a conscious perception of them, it is necessary that the astral body should be united with the physical and etheric bodies. This is the case during waking life, but not in sleep. The astral body has withdrawn itself from the other bodies. It has adopted another kind of existence than that which it possesses while united with the physical and etheric bodies. Now it is the task of occult science to study this other kind of existence in the astral body. During sleep, the astral body withdraws from the possibility of external observation and occult science must trace it in its hidden life, until it again takes possession of its physical and etheric bodies on waking. As in all cases when knowledge of the hidden things and events of life have to be dealt with, clairvoyant observation is necessary for the discovery of the real facts of the sleep state in its true nature, but if that which may be discovered by this means has once been made clear, it is comprehensible to really unprejudiced thought without further demonstration. For events in the unseen world show themselves by their effects in the manifested world. If what is revealed by clairvoyant vision is an explanation of visible events, such a confirmation by life itself is the proof which may rightly be demanded. Even one who will not use the means to be given later for the attainment of clairvoyant vision, may have the following experience: he may, in the first place, take the statements of the clairvoyant for granted, and then apply them to the material events within his experience. He will then find that life thereby becomes clear and comprehensible; and the more exact and minute his observations of ordinary life, the more readily will he come to this conclusion.