Scientists have always struggled with their inability to explain psychological states in which information appears to pass from one mind to another without intervention. In the 1700's, Huygen's exposition on the wave theory of light used the analogy of onion skins that travel from an object to the eye, and then that are reflected from the eye and travel back to the object. Riemann's theory of higher-dimensional manifolds led to a renaissance in the analysis of mystical phenomenon. Public essays were solicited, with prizes awarded for the most compelling description of life in a four-dimensional space. The scientific proponents were humiliated in a public lawsuit.
Mentalists were energized by the theory of electromagnetism, which explained how a magnet could induce motion in iron that was otherwise inert. The recent discovery of ferromagnetic structures in birds is a concrete manifestation of their aspirations: they reasoned that magnetic structures in the human body, and in particular in the nervous system, could explain the long-range and acausal side-effects of intimacy. The term "animal magnetism" was coined in this era.
Mentalists performed rigorous studies of the surprising impact of hypnotic suggestion. An individual under hypnotic suggestion is able to manifest feats of physical strength and endurance that would be completely inaccessible when conscious. One experimenter speaks of stretching out between two chairs, supported by only feet and head, and then having another person actually sit on her stomach - all without fatigue or injury. Other studies document the use of gestures - not actual touch - near specific sites of the skull that would induce predictable physical responses in a subject.
Typical refutations of these studies resorted to claims of collusion between mentalist and subject. This was easy, as much effort was spent studying the extreme responses of highly susceptible individuals, as opposed to establishing universal norms. And of course, the counter-response - that there was no collusion - is a universal negative that cannot be proven.
Interest in mentalists remained strong among popular circles. The most recent popular figure may have been Uri Geller, famous for telepathy and bending spoons. As others of his line, it was discovered that his feats could not be reliably reproduced under controlled circumstances. Much laughter was occasioned over the magical explanation: "There is too much negative energy in the room."
I believe that the mentalists live on, in Las Vegas and children's birthday parties. They hide in plain site, and make a decent living, as magicians.
Then again, I have begun to encounter them as lawyers, art vendors, and auto mechanics.