"That raises an interesting point," said Lucky. "Do you know anything about robotics?"

"I?" said Panner. "Never dealt with a robot in my life."

"Exactly," said Lucky. "Earthmen invented the positronic robot and developed most of the refinements, yet, except for a few specialists, the Earth technician knows nothing about robotics, simply because we don't use robots to any extent. It isn't taught in the schools and it doesn't come up in practice. I myself know the Three Laws and not too much more. Commander Donahue couldn't even quote the Three Laws. The Sirians, on the other hand, with a robot-saturated economy, must be past masters at all the subtleties of robotics.

"Now I spent a good deal of time yesterday and today with a book-film on advanced robotics, that I found in the project library. It was the only book on the subject, by the way."

"So?" said Panner.

"It became obvious to me that the Three Laws aren't as simple as one might think… Let us move on, by the way. We can give the engine levels a double check on the way back." He was moving across this lowest level as he spoke, looking with keen interest at his surroundings.

Lucky continued, "For instance, I might think it would only be necessary to give each man on the ship a ridiculous order and note whether it be obeyed. As a matter of fact, I did think so. But that isn't necessarily true. It is theoretically possible to adjust the positronic brain of a robot to obey only those orders that belong naturally to the line of its duties. Orders that are contrary to those duties or irrelevant to them may still be obeyed provided that they are preceded by certain words which act as a code or by the person who gives the orders identifying himself in a certain way. In this manner a robot can be handled in all ways by its proper overseers and yet be insensitive to strangers."

Panner, who had placed his hands on the holds that would guide the men up to the next higher level, released them. He turned to face Lucky.

He said, "You mean when you told me to take off my shirt and I didn't obey, that meant nothing?"

"I say it could have meant nothing, Dr. Panner, since taking off your shirt at that moment was no part of your regular duties, and my order might not have been stated in the proper form."

"Then you're accusing me of being a robot?"

"No. It isn't likely that you are. The Sirians, in choosing some member of the project to replace by a robot, would scarcely choose the chief engineer. For the robot to do that job properly, it would have to know so much about Agrav that the Sirians couldn't supply the knowledge. Or, if they could, they would have no need to spy."

"Thanks," said Panner, sourly, turning toward the hand holds again, but now Bigman's voice rang out.

"Hold it, Panner!" The small Martian had his ready needle-gun in his fist. He said, "Wait a minute, Lucky, how do we know he knows anything about Agrav? We're just assuming that. He never showed us any knowledge. When the Jovian Moon shifted to Agrav, where was he? Sitting on his squatter in his quarters with us, that's where he was."

Lucky said, "I thought of that, too, Bigman, and that's one reason I brought Panner down here. He's obviously acquainted with the engines. I've watched him inspect everything and he couldn't have done it with such assurance if he weren't an expert on the workings."

"Does that suit you, Martian?" Panner demanded with suppressed anger.

Bigman put his needle-gun away, and without a further word Panner scrambled up the ladder.

They stopped off at the next level, working through it a second time.

Panner said, "All right, that leaves ten men: two army officers, four engineers, four workmen. What dp you propose to do? X-ray each of them separately? Something like that?"

Lucky shook his head. "That's too risky. Apparently the Sirians have been known to use a cute little trick to protect themselves. They've been known to use robots to carry messages or to perform tasks which the individual giving the orders wanted to be kept secret. Now obviously a robot can't keep a secret if a human being asks him, in the proper fashion, to reveal it. What the Sirians do, then, is to install an explosive device in the robot which is triggered by any attempt to force the robot to give away the secret."

"You mean if you put an X-ray on the robot, it will explode?"

"There's a very good chance that it would. Its greatest secret is its identity, and it may be triggered for every attempt to discover that identity that the Sirians could think of." Lucky added regretfully, "They hadn't counted on a V-frog; there was no trigger against that. They had to order the robot to kill the V-frog directly. Or that might have been preferable anyway, since it managed to keep the robot alive undetected."

"Wouldn't the robot be harming humans nearby if it exploded? Wouldn't it be breaking First Law?" asked Panner with a trace of sarcasm.

"It wouldn't. It would have no control over the explosion. The triggering would be the result of the sound of a certain question or the sight of a certain action, not the result of anything the robot itself would do."

They crawled up to still another level.

"Then what do you expect to do, Councilman?" demanded Panner.

"I don't know," Lucky said frankly. "The robot must be made to give itself away somehow. The Three Laws, however modified and fancified, must apply. It's only a question of being sufficiently acquainted with robotics to know how to take advantage of those Laws. If I knew how to force the robot into some action that would show it to be non-human without activating any explosive device with which it might be equipped; if I could manipulate the Three Laws so as to force one to conflict with another sufficiently strongly to paralyze the creature completely; if I-"

Panner broke in impatiently, "Well, if you expect help from me, Councilman, it's no use. I've told you already I know nothing of robotics." He whirled suddenly. "What's that?"

Bigman looked about, too. "I didn't hear anything."

Wordlessly Panner squeezed past them, dwarfed by the bending metal tube on either side.

He had gone almost as far as he could, the other two following, when he muttered, "Someone might have squeezed in among the rectifiers. Let me pass again."

Lucky stared, frowning, into what was almost a forest of twisting cables that enclosed them in a complete dead end.

Lucky said, "It seems clear to me."

"We can test it for sure," Panner said tightly. He had opened a panel in the wall nearby and now he reached in cautiously, looking over his shoulder.

"Don't move," he said.

Bigman said testily, "Nothing's happened. There's nothing there."

Panner relaxed. "I know it. I asked you not to move because I didn't want to slice an arm off when I established the force field."

"What force field?"

"I've shorted a force field right across the corridor. You can't move out of there any more than you could if you were encased in solid steel three feet thick."

Bigman yelled, "Sands of Mars, Lucky, he is the robot!" His hand lunged.

Panner cried at once, "Don't try the needle-gun. Kill me and how do you ever get out?" He stared at them, dark eyes sparking, his broad shoulders hunched. "Remember, energy can get through a force field but matter can't, not even air molecules. You're airtight in there. Kill me and you'll suffocate long before anyone happens to come across you down here."

"I said he was the robot," said Bigman in raging despair.

Panner laughed shortly, "You're wrong. I'm not a robot. But if there is one, I know who it is."