How to stymie telepathic intruders (in the Historical Dictionary of Science Fiction)

There are many terms from classic and modern SF that remain unresearched, and the Historical Dictionary of Science Fiction will be continually updated, especially as additional resources are put online. Boing Boing is syndicating new entries from the HDSF on a regular basis. (Read the series introduction.)

Thought screen, Thought shield, Thought wave

We've previously discussed the history of paranormal mental powers in science fiction, when we added telepathize and mind-control. Today's new entries reflect a similar background. The first, thought wave, originated in mid-nineteenth-century poetry, representing an impulse of thought emanating from someone's brain; by later that century, it was used in more clearly parapsychological contexts.

Pretty much as soon as pulp science fiction began, the term was adapted into the gee-whiz technical style of that era; a character in 1926 described tuning into a person's "thought-wave length" the same way a radio works. The trope of telepathic emanations was used by a range of prominent authors—Edgar Rice Burroughs, A. E. van Vogt, Philip K. Dick, Stephen King—and the term remains in use today.

An ongoing concern with telepaths is maintaining one's privacy from them; it is a core theme of Alfred Bester's greatest work, The Demolished Man (in that case, it is a criminal who wants to prevent the police from proving his guilt). Several terms have been used for mechanisms to block telepathic examination, all arising in the 1930s: thought screen, thought shield, and mind shield. (There are also a few examples of mind screen in science fiction, and of these few, many are in the different sense 'a notional area where one visualizes one's thoughts', like a TV screen; we are not adding an entry for this yet.)

Both thought screen and thought shield are frequently associated with E. E. "Doc" Smith, who used them in his Lensman space-opera series, and thought screen in particular has become rare except in reference to Smith. Precisely how any of these operate is almost always hand-waved away: they can be gadgets that block intrusive thought waves; they can be some kind of mental effort on the part of the would-be subject; they can be entirely unexplained. Regardless of how they operate, though, one imagines that they will remain useful in any universe where telepaths exist.