Hallucinations. A state produced sometimes by physiological disorders, sometimes by mediumship, and at others by drunkenness. But the cause that produces the visions has to be sought deeper than physiology. All such, particularly when produced through mediumship, are preceded by a relaxation of the nervous system, generating invariably an abnormal magnetic condition which attracts to the sufferer waves of astral light. It is these latter that furnish the various hallucinations, which, however, are not always, as physicians would explain them, mere empty and unreal dreams. No one can see that which does not exist — i. e., which is not impressed — in or on the astral waves. But a seer may perceive objects and scenes (whether past, present or future) which have no relation whatever to himself; and perceive, moreover, several things entirely disconnected with each other at one and the same time, so as to produce the most grotesque and absurd combinations. But drunkard and seer, medium and adept see their respective visions in the astral light; only while the drunkard, the madman, and the untrained medium, or one in a brain fever, see, because they cannot help it, and evoke jumbled visions unconsciously to themselves without being able to control them, the adept and the trained Seer have the choice and the control of such visions. They know where to fix their gaze, how to steady the scenes they wish to observe, and how to see beyond the upper outward layers of the astral light. With the former such glimpses into the waves are hallucinations; with the latter they become the faithful reproduction of what actually has been, is, or will be taking place. The glimpses at random, caught by the medium, and his flickering visions in the deceptive light, are transformed under the guiding will of the adept and seer into steady pictures, the truthful representation of that which he wills to come within the focus of his perception.
Source: H.P.Blavatsky - The Key to Theosophy, The theosophical glossary