The City and the Stars / The Sands of Mars by Arthur C. Clarke

long story, but I think you will be interested.”
    “I am interested in everything,” said Alvin, truthfully enough.
    “Very well. The men— if they were men, which I sometimes doubt— who designed Diaspar had to solve an incredibly complex problem. Diaspar is not merely a machine, you know— it is a living organism, and an immortal one. We are so accustomed to our society that we can’t appreciate how strange it would have seemed to our first ancestors. Here we have a tiny, closed world which never changes except in its minor details, and yet which is perfectly stable, age after age. It has probably lasted longer than the rest of human history— yet in that history there were, so it is believed, countless thousands of separate cultures and civilizations which endured for a little while and then perished. How did Diaspar achieve its extraordinary stability?”
    Alvin was surprised that anyone should ask so elementary a question, and his hopes of learning something new began to wane.
    “Through the Memory Banks, of course,” he replied. “Diaspar is always composed of the same people, though their actual groupings change as their bodies are created or destroyed.”
    Khedron shook his head.
    “That is only a very small part of the answer. With exactly the same people, you could build many different patterns of society. I can’t prove that, and I’ve no direct evidence of it, but I believe it’s true. The designers of the city did not merely fix its population; they fixed the laws governing its behavior. We’re scarcely aware that those laws exist, but we obey them. Diaspar is a frozen culture, which cannot change outside of narrow limits. The Memory Banks store many other things outside the patterns of our bodies and personalities. They store the image of the city itself, holding its every atom rigid against all the changes that time can bring. Look at this pavement— it was laid down millions of years ago, and countless feet have walked upon it. Can you see any sign of wear? Unprotected matter, however adamant, would have been ground to dust ages ago. But as long as there is power to operate the Memory Banks, and as long as the matrices they contain can still control the patterns of the city, the physical structure of Diaspar will never change.”
    “But there have been some changes,” protested Alvin. “Many buildings have been torn down since the city was built, and new ones erected.”
    “Of course— but only by discharging the information stored in the Memory Banks and then setting up new patterns. In any case, I was merely mentioning that as an example of the way the city preserves itself physically. The point I want to make is that in the same way there are machines in Diaspar that preserve our social structure. They watch for any changes, and correct them before they become too great. How do they do it? I don’t know— perhaps by selecting those who emerge from the Hall of Creation. Perhaps by tampering with our personality patterns; we may think we have free will, but can we be certain of that?
    “In any event, the problem was solved. Diaspar has survived and come safely down the ages, like a great ship carrying as its cargo all that is left of the human race. It is a tremendous achievement in social engineering, though whether it is worth doing is quite another matter.
    “Stability, however, is not enough. It leads too easily to stagnation, and thence to decadence. The designers of the city took elaborate steps to avoid this, though these deserted buildings suggest that they did not entirely succeed. I, Khedron the Jester, am part of that plan. A very small part, perhaps. I like to think otherwise, but I can never be sure.”
    “And just what is that part?” asked Alvin, still very much in the dark, and becoming a little exasperated.
    “Let us say that I introduce calculated amounts of disorder into the city. To explain my operations would be to destroy their effectiveness. Judge me by my