The theorization of modern spiritualism was mainly due to L. H. D. Rivail (1803-1869) a doctor of medicine who became celebrated under the pseudonym Allan Kardec.

Kardec's theories were simple enough: After death the soul becomes a spirit and seeks reincarnation, which, as Pythagoras taught, is the destiny of all human souls; spirits know the past, present, and future; sometimes they can materialize and act on matter. We should let ourselves be guided by good spirits, Kardec maintained, and refuse to listen to bad spirits.

Kardec wrote many books which achieved enormous popularity in his own lifetime. His works also spread to Brazil, where he still has a huge following, and where postage stamps were recently issued in his honor. His intellectual energy certainly deserves admiration. However, he built his theory on the untenable hypothesis that mediums, embodying a so-called spirit, are never mistaken, unless their utterances are prompted by evil spirits. This notion does not of course, take into account the possi-bilities of suggestion, multiple personality, or unconscious influences which were quickly developed as alternative hypotheses to outright fraud by skeptical scientific investigators such as Michael Faraday.