"We've all faced it," Donegan said. "When I was an Operative I went through it, too. It's part of the job." "But--" "And I'm not going to tell you how to do the job," Donegan went on firmly. "Either you know that by now, or you don't belong here." He got up to leave, slowly. "It's a fine way to find out," he said mournfully. Donegan rose, too. "Good luck," he said. And meant it, too. That was the chief for you, he thought. Send you out into God knows what with no weapons, no instructions, lots of help planted for the man who wanted to kill you--and then wish you good luck at the end of it. Sometimes he wondered why he didn't go in for some nice, peaceful job of work--like rocket testing, for instance. * * * * * Fredericks, downstairs, was deciding to do things the subtle way. The man upstairs--Jones, Brady or whatever his name was--deserved what he was going to get. Psi powers were all very well, but there were defenses against them. Briefly he thought of the man who'd sold him the special equipment, and wondered why more criminals didn't know the equipment existed. It worked; he was sure of that. Fredericks knew enough of general psi theory to know when somebody was handing him a snow job. And the equipment was no snow job. A force shield, that was the basic thing. A shield with no points of entrance for anything larger than air molecules. Sight and sound could get through, because the shield was constructed to allow selected vibrations and frequencies. But no psi force could crack the shield. Fredericks has sat through a long explanation. Psi wasn't a physical force; it was more like the application of a mental "set," in the mathematical sense, to the existing order. But it could be detected by specially built instruments--and a shield could be set up behind which no detection was possible. It wasn't accurate to say that a psi force was blocked by the shield; no construct can block that which has no real physical existence. It was, more simply, that the shield created a framework inside of which the universe existed in the absence of psi. That wasn't very clear, either, Fredericks thought; but mathematics was the only adequate language for talking about psi, anyhow. It had been the theory of sets that had led to the first ideas of structure and rationality within the field, and the math had gotten progressively more complex ever since. Psi couldn't get through the shield, at any rate; that was quite certain. And very little else could get in, or out. There was only one point of exit. Unholstering his gun and aiming it automatically keyed the shield to allow passage of a bullet, and the point of exit was controlled by the gun's aiming. It was efficient and simple to handle.