In reading THE SCIENCE OF STAR TREK, by Mark Brake, I'm naturally reminded of Vulcan telepathy (not discussed much if at all in this book, though). I don't recall the scope and nature of Spock's telepathic power being strictly defined in the original series. For complete access to the consciousness of another, Vulcans must perform a mind meld. From the episode with the alien Horta, we know language poses no barrier. Spock comprehends the thoughts of aliens through mind melds even if the other species aren't humanoid. However, he seems to exercise some limited form of telepathy without melding; in one later episode, we witness him silently "making a suggestion" to a humanoid antagonist who's not mentally on guard. The "Empath" episode introduces a young woman whose species, if she's typical, is mute. Rather than truly telepathic, they're empathic, sensing emotions but not thoughts. It seems unlikely that this species could have a technologically advanced culture, with no ability to communicate precise concepts, especially abstract ones.
Some theories of telepathy assume the participants must share a language for mutual understanding. Others postulate a universal mental "language" so that access to someone's thoughts automatically allows total comprehension. The title character of "The Mindworm," C. M. Kornbluth's classic psychic vampire tale, can hear the surface thoughts of everybody near him but can understand them only if the subject is mentally verbalizing in a language he knows (a limitation that proves his undoing when he clashes with Eastern European immigrants who recognize him from their native folklore).
Does a telepath "hear" only what the subject is thinking of at the moment or delve at will into all the contents of the person's mind? If the former, can you mask your secrets by deliberately thinking of something else? The telepath in Spider Robinson's VERY BAD DEATHS, so sensitive to the clamor of other people's minds that he lives as a hermit, picks up only surface thoughts. In Robert Heinlein's TIME FOR THE STARS, the telepathic twins "just talk," communicating silently in much the same way they do aloud. Trying to open themselves totally to each other's minds produces chaotic confusion, like being inside someone else's dream, so they don't bother.
On the other hand, some fictional telepaths can rummage through people's minds and quickly learn everything about the subject's past and present. Trying to conceal anything from a psychic with this power by simply thinking of pink elephants would be futile.
Here's a big question that I've never seen addressed, except implicitly in the STAR TREK "Empath" episode: Would a completely telepathic species have a language at all? It seems to me that they wouldn't have a reason to evolve it naturally. On the other hand, for any kind of advanced civilization to develop, surely they would have to invent language sooner or later. They would need a system of writing in order to keep records. They would need a way to communicate at long distance. Even if they got along without speech, surely written language would be a prerequisite for complex societies and any but the most rudimentary technology. It wouldn't evolve naturally, however. Geniuses among them would have to create it, as cultures on Earth invented mathematical notation. A first-contact premise of interstellar explorers from Earth meeting extraterrestrials whose only form of language is written, to whom audible speech is an alien concept, would make an exciting, challenging story.
Margaret L. CarterCarter's Crypt