The City and the Stars Page 0,73

planned when the city was laid down?"

"That can be said of all men."

This reply made Alvin pause It was true enough; the human inhabitants of Diaspar had been designed as carefully as its machines. The fact that he was a Unique gave Alvin rarity, but there was no necessary virtue in that.

He knew that he could learn nothing further here regarding the mystery of his origin. It was useless to try to trick this vast intelligence, or to hope that it would disclose information it had been ordered to conceal. Alvin was not unduly disappointed; he felt that he had already begun to glimpse the truth, and in any case this was not the main purpose of his visit.

He looked at the robot he had brought from Lys, and wondered bow to make his next step. It might react violently if it knew what he was planning, so it was essential that it should not overhear what he intended to say to the Central Computer.

"Can you arrange a zone of silence?" he asked.

Instantly, be sensed the unmistakable "dead" feeling, the total blanketing of all sounds, which descended when one was inside such a zone. The voice of the Computer, now curiously flat and sinister, spoke to him: "No one can hear us now. Say what you wish."

Alvin glanced at the robot; it bad not moved from its position. Perhaps it suspected nothing, and he had been quite wrong in ever imagining that it had plans of its own. It might have followed him into Diaspar like a faithful, trusting servant, in which case what he was planning now seemed a particularly churlish trick.

"You have heard bow I met this robot," Alvin began. "It must possess priceless knowledge about the past, going back to the days before the city as we know it existed. It may even be able to tell us about the other worlds than Earth, since it followed the Master on his travels. Unfortunately, its speech circuits are blocked. I do not know how effective that block is, but I am asking you to clear it."

His voice sounded dead and hollow as the zone of silence absorbed every word before it could form an echo. He waited, within that invisible and unreverberant void, for his request to be obeyed or rejected.

"Your order involves two problems," replied the Computer. "One is moral, one technical. This robot was designed to obey the orders of a certain man. What right have I to override them, even if I can?"

It was a question which Alvin had anticipated and for which he had prepared several answers.

"We do not know what exact form the Master's prohibition took," be replied. "If you can talk to the robot, you may be able to persuade it that the circumstances in which the block was imposed have now changed."

It was, of course, the obvious approach. Alvin had attempted it himself, without success, but he hoped that the Central Computer, with its infinitely greater mental resources, might accomplish what he had failed to do.

"That depends entirely upon the nature of the block," came the reply. "It is possible to set up a block which, if tampered with, will cause the contents of the memory cells to be erased. However, I think it unlikely that the Master possessed sufficient skill to do that; it requires somewhat specialized techniques. I will ask your machine if an erasing circuit has been set up in its memory units."

"But suppose," said Alvin in sudden alarm, "it causes erasure of memory merely to ask if an erasing circuit exists?"

"There is a standard procedure for such cases, which I shall follow. I shall set up secondary instructions, telling the machine to ignore my question if such a situation exists. It is then simple to insure that it will become involved in a logical paradox, so that whether it answers me or whether it says nothing it will be forced to disobey its' instructions. In such . an event all robots act in the same manner, for their own protection. They clear their input circuits and act as if no question has been asked."

Alvin felt rather sorry that he had raised the point, and after a moment's mental struggle decided that he too would adopt the same tactics and pretend that he had never been asked the question. At least he was reassured on one pointthe Central Computer was fully prepared to deal with any booby traps that might exist in the robot's memory units. Alvin had