Telepathy was the communication of messages or thoughts directly from one mind to another without use of ordinary vocal and auditory mechanisms. Telepathy existed in various forms among various races.
A touch telepath could communicate directly with the mind of another being, but only if in physical contact. The Vulcan mind meld was a form of touch telepathy (although there was more to it than that). (TOS: "Dagger of the Mind")
Some telepaths could exchange thoughts only with others of their own species (for example, Cairns and Lumerians), some could send but not receive, and some could receive but not send. Betazoids, for one example, could send their thoughts only to other empaths and telepaths, but could perceive thoughts and feelings from almost any being's mind, telepathically gifted or not. (TNG: "Encounter at Farpoint").
In some races, telepathy was present in all healthy individuals (those not born with congenital problems and not the victims of accidents or disease). In other species, such as Humans, telepathy appeared only infrequently. And certain species, notably the Ferengi, were never telepathic. Often such species could not be read even by telepaths normally capable of reading alien minds.
Telepathy made possible some forms of interaction that would otherwise be impossible. The Medusans were an incorporeal race so ugly (or perhaps merely so alien) that the sight of them drove Humans insane. Interaction with them was very limited until 2268, when Miranda Jones established a telepathic link with Ambassador Kollos, proving that it could be done. (TOS: "Is There in Truth No Beauty?")
Extraordinarily powerful telepathic species exised. They were rare but extremely formidable, as they could immerse other minds in a reality of their own fabrication, much like a holodeck, but in which the telepath made all the rules. The xenophobic Melkot used their telepathic powers to punish aliens who encroached upon their space without permission (which, prior to 2268, they never extended). (TOS: "Spectre of the Gun")
Under rare circumstances, individuals could gain telepathic powers. In 2265, exposure to the galactic barrier altered Gary Mitchell, who developed telepathic powers. In 2266, Captain Kirk and the USS Enterprise took aboard a passenger, Charles Evans. Subsequent events revealed that Evans had been granted various skills by the enigmatic Thasians so that he might survive on their world. (TOS: "Where No Man Has Gone Before", "Charlie X")
Telepathic races such as the Betazoids and Vulcans usually developed a moral code (sometimes making it actual law, as did the Ullians) that precisely dictated under what circumstances such powers could be used. Vulcan telepathy, though, was more limited, usually practiced in the form of the mind meld. That, coupled with Vulcan logical and mental conditioning made unauthorized use all but unheard of. Telepathy, misused, could be the ultimate invasion of privacy, and for that reason, it was feared by some. (TNG: "Violations"; Star Trek Nemesis) The Devore considered trust to be a concept that was alien to telepathic species and therefore deeply mistrusted telepaths, searching all ships that passed through their space for them and sending any found to relocation centers. (VOY: "Counterpoint") Forcibly intruding into another's mind and/or manipulating ones actions was considered, among most telepathic species, to be a form of assault that bordered on rape. Prior to the Time of the Awakening, Vulcans used a device — the psionic resonator — to telepathically kill others. An extremist group had hoped to employ the same tactic in their efforts. (TNG: "Gambit, Part II")
A few civilizations developed mechanical telepathy. Landru, a gifted engineer and philosopher, constructed a computer able to telepathically control most of the population of the planet Beta III. (TOS: "The Return of the Archons") The engimatic Flint built a series of robots that could be controlled telepathically, and that could respond to mental emanations consistent with distress or danger. (TOS: "Requiem for Methuselah") Sargon, Thalassa, and Henoch, the last survivors of an extremely advanced civilization, knew how to build robots their minds could inhabit and control. (TOS: "Return to Tomorrow") Furthermore, the inhabitants of the Think Tank met by Voyager had built a mechanical means for communication, as their species differed so profoundly that they never would be able to communicate by verbal means. (VOY: "Think Tank")
In 2369, the Klingon crew of the Vor'cha-class attack cruiser IKS Toh'Kaht found several energy spheres on a planet in the Gamma Quadrant, which they described as a telepathic archive. (DS9: "Dramatis Personae")
- Extrasensory perception
- Neurogenic telepathy
- Psychoprojective telepathy
- Telepathic species