^Stoke, Douglas M. (1997) The Nature of Mind. McFarland. p. 51. ISBN978-0786477548
^Neher, Andrew. (2011). Paranormal and Transcendental Experience: A Psychological Examination. Dover Publications. p. 310. ISBN0-486-26167-0
^Carroll, Robert Todd. (2003). The Skeptic's Dictionary: A Collection of Strange Beliefs, Amusing Deceptions, and Dangerous Delusions. John Wiley & Sons. p. 311. ISBN978-0-471-27242-7