The theory of the existence of a sixth sense as a convenient explanation of paranormal phenomena was first put forward in the era of animal magnetism by Tardy de Monravel in his Essai sur la Théorie du Somnambolisme Magnétique (1785). Departing from his mesmerist contemporaries, he considered the sixth sense as the source and sum of all our partial senses. (His colleagues attempted to explain clairvoyance and prevision by positing the existence of a "magnetic fluid.")
More recently the sixth sense has been given prominence as Charles Richet 's comprehensive term for the phenomena of telepathy, clairvoyance, psychometry, premonition, prediction, crystal gazing, and phantasmal appearances. They were, in Richet's view, manifestations of a new unknown sense that perceives the vibrations of reality. The conception is largely an attempt to do away with the spirit hypothesis, making its invocation unnecessary. Richet admitted, however, that the working of this sense is incomprehensible when a choice has to be made between vibrations of reality, for instance in the case of a book test, when the sensitive is called upon to read a certain line on a certain page in a certain book that nobody has opened.
His main argument in favor of his theory was that the hypothesis of the sixth sense as a new physiological notion contradicted nothing that we learn from physiology, whereas the spirit hypothesis does. A hint of Richet's term survived in the concept of extrasensory perception as used by J. B. Rhine.
Richet, Charles Notre Sixième Sens. Paris: Editions Montaigne, 1928.
Sinel, Joseph. The Sixth Sense. London: T. W. Laurie, 1927.