It was Powell who broke the desperate silence. "In the first place," he said, "Speedy isn't drunk - not in the human sense - because he's a robot, and robots don't get drunk. However, there's something wrong with him which is the robotic equivalent of drunkenness"
"To me, he's drunk," stated Donovan, emphatically, "and all I know is that he thinks we're playing games. And we're not. It's a matter of life and very gruesome death."
"All right. Don't hurry me. A robot's only a robot. Once we find out what's wrong with him, we can fix it and go on."
"Once," said Donovan, sourly.
Powell ignored him. "Speedy is perfectly adapted to normal Mercurian environment. But this region" - and his arm swept wide - "is definitely abnormal. There's our clue. Now where do these crystals come from? They might have formed from a slowly cooling liquid; but where would you get liquid so hot that it would cool in Mercury's sun?"
"Volcanic action," suggested Donovan, instantly, and Powell's body tensed.
"Out of the mouths of sucklings," he said in a small, strange voice and remained very still for five minutes.
Then, he said, "Listen, Mike, what did you say to Speedy when you sent him after the selenium?"
Donovan was taken aback. "Well damn it - I don't know. I just told him to get it."
"Yes, I know, but how? Try to remember the exact words."
"I said... uh... I said: 'Speedy, we need some selenium. You can get it such-and-such a place. Go get it - that's all. What more did you want me to say?"
"You didn't put any urgency into the order, did you?"
"What for? It was pure routine."
Powell sighed. "Well, it can't be helped now - but we're in a fine fix." He had dismounted from his robot, and was sitting, back against the cliff. Donovan joined him and they linked arms: In the distance the burning sunlight seemed to wait cat-and-mouse for them, and just next them, the two giant robots were invisible but for the dull red of their photoelectric eyes that stared down at them, unblinking, unwavering and unconcerned.
Unconcerned! As was all this poisonous Mercury, as large in jinx as it was small in size.
Powell's radio voice was tense in Donovan's ear: "Now, look, let's start with the three fundamental Rules of Robotics - the three rules that are built most deeply into a robot's positronic brain." In the darkness, his gloved fingers ticked off each point.
"We have: One, a robot may not injure a human being, or, through inaction, allow a human being to come to harm."
"Right!"
"Two," continued Powell, "a robot must obey the orders given it by human beings except where such orders would conflict with the First Law."
"Right"
"And three, a robot must protect its own existence as long as such protection does not conflict with the First or Second Laws."
"Right! Now where are we?"
"Exactly at the explanation. The conflict between the various rules is ironed out by the different positronic potentials in the brain. We'll say that a robot is walking into danger and knows it. The automatic potential that Rule 3 sets up turns him back. But suppose you order him to walk into that danger. In that case, Rule 2 sets up a counterpotential higher than the previous one and the robot follows orders at the risk of existence."
"Well, I know that. What about it?"
"Let's take Speedy's case. Speedy is one of the latest models, extremely specialized, and as expensive as a battleship. It's not a thing to be lightly destroyed"
"So?"
"So Rule 3 has been strengthened - that was specifically mentioned, by the way, in the advance notices on the SPD models - so that his allergy to danger is unusually high. At the same time, when you sent him out after the selenium, you gave him his order casually and without special emphasis, so that the Rule 2 potential set-up was rather weak. Now, hold on; I'm just stating facts."
"All right, go ahead. I think I get it."
"You see how it works, don't you? There's some sort of danger centering at the selenium pool. It increases as he approaches, and at a certain distance from it the Rule 3 potential, unusually high to start with, exactly balances the Rule 2 potential, unusually low to start with."
Donovan rose to his feet in excitement. " And it strikes an equilibrium. I see. Rule 3 drives him back and Rule 2 drives him forward-"
"So he follows a circle around the selenium pool, staying on the locus of all points of potential equilibrium. And unless we do something about it, he'll stay on that circle forever, giving us the good old runaround." Then, more thoughtfully: "And that, by the way, is what makes him drunk. At potential equilibrium, half the positronic paths of his brain are out of kilter. I'm not a robot specialist, but that seems obvious. Probably he's lost control of just those parts of his voluntary mechanism that a human drunk has. Ve-e-ery pretty."
"But what's the danger? If we knew what he was running from-"?
"You suggested it. Volcanic action. Somewhere right above the selenium pool is a seepage of gas from the bowels of Mercury. Sulphur dioxide, carbon dioxide - and carbon monoxide. Lots of it and at this temperature."
Donovan gulped audibly. "Carbon monoxide plus iron gives the volatile iron carbonyl."
"And a robot," added Powell, "is essentially iron." Then, grimly: "There's nothing like deduction. We've determined everything about our problem but the solution. We can't get the selenium ourselves. It's still too far. We can't send these robot horses, because they can't go themselves, and they can't carry us fast enough to keep us from crisping. And we can't catch Speedy, because the dope thinks we're playing games, and he can run sixty miles to our four."