In more than 15 years of observing and researching the phenomena termed altered states of consciousness, I have been repeatedly impressed with the incredible range of phenomena encompassed by that term and with the high degree of unrelatedness of most of these phenomena. Hundreds of people have given me reports of radical alterations in the functioning of their consciousness, not only about such relatively familiar things as the changes caused by sleep and dreaming or by strong emotional states, but about changes associated with more exotic techniques, like various meditations, hypnosis, marijuana intoxication, intoxication with major psychedelic drugs, mediumistic trance states, out-of-the-body experiences, a variety of idiosyncratic states that seem to be unique to given persons, states that seem to be socially shared by groups of practitioners of particular spiritual disciplines, and experiences of that category that we vaguely label mystical experiences.