Telepathy is the communication of messages or thoughts directly from one mind to another without use of ordinary vocal and auditory mechanisms. Telepathy exists across space; it has appeared in various forms among various races.
A touch telepath can communicate directly with the mind of another being, but only if in physical contact. The Vulcan mind meld is a form of touch telepathy (although there is more to it than that). (TOS: "Dagger of the Mind")
Some telepaths can exchange thoughts only with others of their own species (for example, Cairns and Lumerians), some can send but not receive, and some can receive but not send. Betazoids, for one example, can send their thoughts only to other empaths and telepaths, but can receive thoughts and feelings from almost any being's mind, psionically gifted or not. (TNG: "Encounter at Farpoint").
In some races, telepathy is 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 appears only infrequently. And certain species, notably the Ferengi, are never telepathic. Often such species cannot be read even by telepaths normally capable of reading alien minds.
Telepathy makes possible some forms of interaction that would otherwise be impossible. The Medusans are an incorporeal race so ugly (or perhaps merely so alien) that the sight of them drives 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 exist. They are rare, but are extremely formidable, as they can immerse other minds in a reality of their own fabrication, much like a holodeck, but in which the telepath makes 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 can 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 psionic 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 develop a moral code (sometimes making it actual law, as do the Ullians) that precisely dictates under what circumstances such powers may be used. Vulcan telepathy, though, is more limited, usually practiced in the form of the mind meld. That, coupled with Vulcan logical and mental conditioning makes unauthorized use all but unheard of. Telepathy, misused, can be the ultimate invasion of privacy, and for that reason, it is feared by some. (TNG: "Violations"; Star Trek Nemesis) Forcibly intruding into another's mind and/or manipulating ones actions is considered, among most telepathic species, to be a form of assault that borders 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 have 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