"Maybe you can believe it, then," said Brink. "Did you ever hear of a man named Hieronymus?" "No," said Fitzgerald in a numbed voice. "Who's he?" "He got a patent once," said Brink, matter-of-factly, "on a machine he believed detected something he called eloptic radiation. He thought it was a kind of radiation nobody had noticed before. He was wrong. It worked by something called psi." Sergeant Fitzgerald shook his head. It still needed clearing. "Psi still isn't fully understood," explained Brink, "but it will do a lot of things. For instance, it can change probability as magnetism can change temperature. You can establish a psi field in a suitable material, just as you can establish a magnetic field in steel or alnico. Now, if you spin a copper disk in a magnetic field, you get eddy currents. Keep it up, and the disk gets hot. If you're obstinate about it, you can melt the copper. It isn't the magnet, as such, that does the melting. It's the energy of the spinning disk that is changed into heat. The magnetic field simply sets up the conditions for the change of motion into heat. In the same way ... am I boring you?" "Confusing me," said Fitzgerald, "maybe. But keep on. Maybe I'll catch a glimmer presently." "In the same way," said Brink, "you can try to perform violent actions in a strong psi field--a field made especially to act on violence. When you first try it you get something like eddy currents. Warnings. It can be arranged that such psi eddy currents make your eyelids twitch. Keep it up, and probability changes to shift the most-likely consequences of the violence. This is like a spinning copper disk getting hot. Then, if you're obstinate about it, you get the equivalent of the copper disk melting. Probability gets so drastically changed that the violent thing you're trying to do becomes something that can't happen. Hm-m-m. ... You can't spin a copper disk in a magnetic field when it melts. You can't commit a murder in a certain kind of psi field when probability goes hog-wild. Any other thing can happen to anybody else--to you, for example--but no violence can happen to the thing or person you're trying to do something violent to. The psi field has melted down ordinary probabilities. The violence you intend has become the most improbable of all conceivable things. You see?" "I'm beginnin'," said Detective Sergeant Fitzgerald dizzily, "I'm beginnin' to get a toehold on what you mean. I'd hate to have to testify about it in court, but I'm receptive."