"I'm certain you couldn't. No one with emotions could have. Yet someone did. Someone killed it."

Panner said, "Do you intend to explain the paradox?"

"Easily explained. No one with emotions, A robot, however, does not have emotions. Suppose that somewhere on Jupiter Nine there is a robot, a mechanical man, in the perfect form of a human being?"

"You mean a humanoid?" exploded Commander Donahue. "Impossible. Such things exist only in fairy tales."

Lucky said, "I think, Commander, you are not aware of how skillful the Sirians are in the manufacture of robots. I think they might be able to use some man on Jupiter Nine, some thoroughly loyal man, as model; build a robot in his shape and substitute it for him. Such a humanoid robot could have special senses that would enable it to be the perfect spy. It might, for instance, be able to see in the dark or sense things through thicknesses of matter. It would certainly be able to transmit information through the subether by some built-in device."

The commander shook his head. "Ridiculous. A man could easily have killed the V-frog. A desperate man frightened to an extreme might have overcome this- this mental influence the animal exerted. Have you thought of that?"

"Yes, I have," said Lucky. "But why should a man be so desperate, why so wild to kill a harmless V-frog? The most obvious reason is that the V-frog represented a desperate danger, that it was not harmless at all. The only danger a V-frog might have to the killer would involve the animal's capacity to detect and transmit the killer's emotions. Suppose those emotions would be an immediate giveaway to the fact that the killer was a spy?"

"How could it be?" Panner asked.

Lucky turned to look at him. "What if our killer had no emotions at all? Wouldn't a man without emotions be revealed at once as a robot?… Or take it another way altogether. Why kill only the V-frog? Having gotten into our rooms, having risked so much, having found one of us in the shower and one at the intercom and both unsuspecting and unready, why did not the killer kill us instead of the V-frog? For that matter, why not kill us and the V-frog?"

"No time, probably," said the commander.

"There's another and more plausible reason," said Lucky. "Do you know the Three Laws of Robotics, the rules of behavior that all robots are built to follow?"

"I know them generally," the commander said. "I can't quote them."

"I can," said Lucky, "and with your permission I will, so that I may make a point. The First Law is this: A robot may not injure a human being or, through inaction, allow a human being to come to harm. The Second Law is: A robot must obey the orders given it by human beings except where such orders would conflict with the First Law. The Third Law is: A robot must protect its own existence as long as such protection does not conflict with the First or Second Law."

Panner nodded. "All right, Councilman, what does that prove?"

"A robot can be ordered to kill the V-frog, which is an animal. It will risk its existence, since self-preservation is only Third Law, to obey orders, which is Second Law. But it cannot be ordered to kill Bigman or myself, since we are humans, and First Law takes precedence over all. A human spy would have killed us and the V-frog; a robot spy would have killed only the V-frog. It all points to the same thing, Commander."

The commander considered that for long minutes, sitting motionless, the lines on his tired face grooving deeper. Then he said, "What do you propose to do? X-ray every man on the project?"

"No," said Lucky at once. "It's not that simple. Successful espionage is going on elsewhere than here. If there is a humanoid robot here, there are probably others elsewhere. It would be well to catch as many of the humanoids as possible; all of them if we can. If we act too eagerly and openly to catch the one under our hands, the others may be snatched away for use at another time."

"Then what do you propose doing?"

"To work slowly. Once you suspect a robot, there are ways of making it give itself away without its being aware of it. And I don't start completely from scratch. For instance, Commander, I know you are not a robot, since I detected emotion in you yesterday. In fact, I deliberately induced anger in you to test my V-frog, and for that I ask your pardon."

Donahue's face had gone mauve. "I, a robot?"

"As I said, I used you only to test my V-frog."

Panner said dryly, "You have no reason to feel sure about me, Councilman. I never faced your V-frog."

"That is right," said Lucky. "You are not cleared yet. Remove your shirt."

"What!" cried Panner indignantly. "Why?"

Lucky said mildly, "You have just been cleared. A robot would have had to obey that order."

The commander's fist banged down on his desk. "Stop it! This ends right here. I will not have you testing or annoying my men in any way. I have a job to do on this satellite, Councilman Starr; I have an Agrav ship to get into space, and I'm getting it into space. My men have been investigated and they're clear. Your story about a robot is flimsy, and I'm not going along with it.

"I told you yesterday, Starr, that I didn't want you on this satellite disturbing my men and wrecking their morale. You saw fit yesterday to address me in insulting fashion. You say now it was just to test your animal, which makes it no less insulting. For that reason, I feel no need to co-operate with you and I am not doing so. Let me tell you exactly what I have done.

"I've cut off all communication with Earth. I've put Jupiter Nine under emergency orders. I have the powers of a military dictator now. Do you understand?"

Lucky's eyes narrowed a trifle. "As councilman of the Council of Science, I outrank you."

"How do you intend to enforce your rank? My men will obey me and they have their orders. You will be restrained forcibly if you try in any way, by word or deed, to interfere with my orders."

"And what are your orders?"

"Tomorrow," said Commander Donahue, "at 6 p.m., Solar Standard Time, the first functioning Agrav ship in existence will make its first flight from Jupiter Nine to Jupiter One, the satellite lo. After we're back- after we're back, Councilman Starr, and not one hour sooner-you may conduct your investigation. And if you then want to get in touch with Earth and arrange court-martial proceedings, I will be ready for you."

Commander Donahue stared firmly at Lucky Starr.

Lucky said to Panner, "Is the ship ready?"

PFanner said, "I think so."

Donahue said scornfully. "We leave tomorrow. Well, Councilman Starr, do you go along with me or will I have to have you arrested?"

The silence that followed was a tense one. Bigman virtually held his breath. The commander's hands were clenching and unclenching, and his nose was white and pinched. Panner slowly fumbled a stick of gum out of his shirt pocket, stripped it of its plastofoil coating with one hand, and crumpled it into his mouth.

And then Lucky clasped his hands loosely, sat back in his chair, and said, "I'll be glad to co-operate with you, Commander."