Robert Sheckley

“ As one of the Guardian ships protecting Earth, the crew had a problem to solve. Just how do you protect a race from an enemy who can take over a man's mind without seeming effort or warning?
"That hand didn't move, did it?" Edwardson asked, standing at the port, looking at the stars.
"No," Morse said. He had been staring fixedly at the Attison Detector for over an hour. Now he blinked three times rapidly, and looked again. "Not a millimeter." ”

“ Edwardson said, his thin face twisted in scorn. "They're telepathic. They must have read every bit of stuff in Everset's mind."
"Everset didn't know we had a detection unit," Morse said, his eyes returning to the dial. "He was captured before we had it."
"Look," Edwardson said, "They ask him, 'Boy, what would you do if you knew a telepathic race was coming to take over Earth? How would you guard the planet?'"
"Idle speculation," Cassel said. "Maybe Everset didn't think of this."
"He thinks like a man, doesn't he? Everyone agreed on this defense. Everset would, too." ”

“ The answer, from a technological planet, was a technological one. Build a space fleet and equip it with some sort of a detection-fire network.
This was done in record time. The Attison Detector was developed, a cross between radar and the electroencephalograph. Any alteration from the typical human brain wave pattern of the occupants of a Detector-equipped ship would boost the indicator around the dial. Even a bad dream or a case of indigestion would jar it.
It seemed probable that any attempt to take over a human mind would disturb something. ”