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Charles Tart, Altered States of Consciousness (New York: Harper, 1990); William James, Varieties of Religious Experience (N.p.: CreateSpace, 2009); Mihaly Csikszentmihalyi, Flow (New York: Harper Perennial, 2008), pp. 43–93; Bruce Greyson, «The Near-Death Experience Scale», Journal of Nervous and Mental Disease 171, no. 6 (1983): 369–74; Erich Studerus, Alex Gamma, and Franz Vollenweider, «Psychometric Evaluation of the Altered States of Consciousness», PLoS One, August 31, 2010; Ronald Pekala, Quantifying Consciousness (Boston: Springer, 1991). Also see Rick Strassman, «Hallucinogenic Drugs in Psychiatric Research and Treatment», Journal of Nervous and Mental Disease 183, no. 3 (1995): 127–38, Robert Thurman, The Tibetan Book of the Dead: The Great Book of Natural Liberation Through Understanding in the Between (New York: Bantam Books, 1993).