"I understand you so far, " said Dors, sounding a little surprised.

"Well then, we know that some comparatively simple things are easy to simulate and as things grow more and more complex they become harder to simulate until finally they become impossible to simulate. But at what level of complexity does simulation cease to be possible? Well, what I have shown, making use of a mathematical technique first invented in this past century and barely usable even if one employs a large and very fast computer, our Galactic society falls short of that mark. It can be represented by a simulation simpler than itself. And I went on to show that this would result in the ability to predict future events in a statistical fashion-that is, by stating the probability for alternate sets of events, rather than flatly predicting that one set will take place."

"In that case, " said Hummin, "since you can profitably simulate Galactic society, it's only a matter of doing so. Why is it impractical?"

"All I have proved is that it will not take an infinite time to understand Galactic society, but if it takes a billion years it will still be impractical. That will be essentially the same as infinite time to us."

"Is that how long it would take? A billion years?"

"I haven't been able to work out how long it would take, but I strongly suspect that it will take at least a billion years, which is why I suggested that number."

"But you don't really know."

"I've been trying to work it out."

"Without success?"

"Without success."

"The University library does not help?" Hummin cast a look at Dors as he asked the question.

Seldon shook his head slowly. "Not at all."

"Dors can't help?"

Dors sighed. "I know nothing about the subject, Chetter. I can only suggest ways of looking. If Hari looks and doesn't find, I am helpless."

Hummin rose to his feet. "In that case, there is no great use in staying here at the University and I must chink of somewhere else to place you."

Seldon reached out and touched his sleeve. "Still, I have an idea."

Hummin stared at him with a faint narrowing of eyes that might have belied surprise--or suspicion. "When did you get the idea? Just now?"

"No. It's been buzzing in my head for a few days before I went Upperside. That little experience eclipsed it for a while, but asking about the library reminded me of it."

Hummin seated himself again. "Tell me your idea-if it's not something that's totally marinated in mathematics."

"No mathematics at all. It's just that reading history in the library reminded me that Galactic society was less complicated in the past. Twelve thousand years ago, when the Empire was on the way to being established, the Galaxy contained only about ten million inhabited worlds. Twenty thousand years ago, the pre-Imperial kingdoms included only about ten thousand worlds altogether. Still deeper in the past, who knows how society shrinks down? Perhaps even to a single world as in the legends you yourself once mentioned, Hummin."

Hummin said, "And you think you might be able to work out psychohistory if you dealt with a much simpler Galactic society?"

"Yes, it seems to me that I might be able to do so."

"Then too, " said Dors with sudden enthusiasm, "suppose you work out psychohistory for a smaller society of the past and suppose you can make predictions from a study of the pre-Imperial situation as to what might happen a thousand years after the formation of the Empire-you could then check the actual situation at that time and see how near the mark you were."

Hummin said coldly, "Considering that you would know in advance the situation of the year 1, 000 of the Galactic Era, it would scarcely be a fair test. You would be unconsciously swayed by your prior knowledge and you would be bound to choose values for your equation in such a way as to give you what you would know to be the solution."

"I don't think so, " said Dors. "We don't know the situation in 1, 000 G.E. very well and we would have to dig. After all, that was eleven millennia ago."

Seldon's face turned into a picture of dismay. "What do you mean we don't know the situation in 1, 000 G.E. very well? There were computers then, weren't there, Dors?"

"Of course."

"And memory storage units and recordings of ear and eye? We should have all the records of 1, 000 G.E. as we have of the present year of 12, 020 G.E."

"In theory, yes, but in actual practice- Well, you know, Hari, it's what you keep saying. It's possible to have full records of 1, 000 G.E., but it's not practical to expect to have it."

"Yes, but what I keep saying, Dors, refers to mathematical demonstrations. I don't see the applications to historical records."

Dors said defensively, "Records don't last forever, Hari. Memory banks can be destroyed or defaced as a result of conflict or can simply deteriorate with time. Any memory bit, any record that is not referred to for a long time, eventually drowns in accumulated noise. They say that fully one third of the records in the Imperial Library are simply gibberish, but, of course, custom will not allow those records to be removed. Other libraries are less tradition bound. In the Streeling University library, we discard worthless items every ten years.

"Naturally, records frequently referred to and frequently duplicated on various worlds and in various libraries-governmental and private remain clear enough for thousands of years, so that many of the essential points of Galactic history remain known even if they took place in pre-Imperial times. However, the farther back you go, the less there is preserved."

"I can't believe that, " said Seldon. "I should think that new copies would be made of any record in danger of withering. How could you let knowledge disappear?"

"Undesired knowledge is useless knowledge, " said Dors. "Can you imagine all the time, effort, and energy expended in a continual refurbishing of unused data? And that wastage would grow steadily more extreme with time."

"Surely, you would have to allow for the fact that someone at some time might need the data being so carelessly disposed of."

"A particular item might be wanted once in a thousand years. To save it all just in case of such a need isn't cost-effective. Even in science. You spoke of the primitive equations of gravitation and say it is primitive because its discovery is lost in the mists of antiquity. Why should that be? Didn't you mathematicians and scientists save all data, all information, back and back to the misty primeval time when those equations were discovered?"

Seldon groaned and made no attempt to answer. He said, "Well, Hummin, so much for my idea. As we look back into the past and as society grows smaller, a useful psychohistory becomes more likely. But knowledge dwindles even more rapidly than size, so psychohistory becomes less likely--and the less outweighs the more. "

"To be sure, there is the Mycogen Sector, " said Dors, musing.

Hummin looked up quickly. "So there is and that would be the perfect place to put Seldon. I should have thought of it myself."

"Mycogen Sector, " repeated Hari, looking from one to the other. "What and where is Mycogen Sector?"

"Hari, please, I'll tell you later. Right now, I have preparations to make. You'll leave tonight."

33.

Dors had urged Seldon to sleep a bit. They would be leaving halfway between lights out and lights on, under cover of "night, " while the rest of the University slept. She insisted he could still use a little rest.

"And have you sleep on the floor again?" Seldon asked.

She shrugged. "The bed will only hold one and if we both try to crowd into it, neither of us will get much sleep."

He looked at her hungrily for a moment and said, "Then I'll sleep on the floor this time."

"No, you won't. I wasn't the one who lay in a coma in the sleet."

As it happened, neither slept. Though they darkened the room and though the perpetual hum of Trantor was only a drowsy sound in the relatively quiet confines of the University, Seldon found that he had to talk.

He said, "I've been so much trouble to you, Dors, here at the University. I've even been keeping you from your work. Still, I'm sorry I'll have to leave you."

Dors said, "You won't leave me. I'm coming with you. Hummin is arranging a leave of absence for me."

Seldon said, dismayed, "I can't ask you to do that."

"You're not. Hummin s asking it. I must guard you. After all, I faded in connection with Upperside and should make up for it."

"I told you. Please don't feel guilty about that. -Still, I must admit I would feel more comfortable with you at my side. If I could only be sure I wasn't interfering with your life . . ."

Dors said softly, "You're not, Hari. Please go to sleep."

Seldon lay silent for a while, then whispered, "Are you sure Hummin can really arrange everything, Dors?"

Dors said, "He's a remarkable man. He's got influence here at the University and everywhere else, I think. If he says he can arrange for an indefinite leave for me, I'm sure he can. He is a moat persuasive man."

I know, " said Seldon. "Sometimes I wonder what he really wants of me."

"fit he says, " said Dors. "He's a man of strong and idealistic ideas and dreams."

"You sound as though you know him well, Dors."

"Oh yes, I know him well."

"Intimately?"

Dors made an odd noise. "I'm not sure what you're implying, Hari, but, assuming the most insolent interpretation-No, I don't know him intimately. What business would that be of yours any-way?"

"I'm sorry.. said Seldon. "I just didn't want, inadvertently, to be invading someone else's--"

"Property? That's even more insulting. I think you had better go to sleep."

"I'm sorry again, Dors, but I can't sleep. Let me at least change the subject. You haven't explained what the Mycogen Sector is. Why will it be good for me to go there? What's it like?"

"It's a small sector with a population of only about two million if I remember correctly. The thing is that the Mycogenians cling rightly to a set of traditions about early history and are supposed to have very ancient records not available to anyone else. It's just possible they would be of more use to you in your attempted examination of pre-Imperial times than orthodox historians might be. All our talk about early history brought the sector to mind."

"Have you ever seen their records?"

"No. I don't know anyone who has."

"Can you be sure that the records really exist, then?"

"Actually, I can't say. The assumption among non-Mycogenians is that they're a bunch of madcaps, but that may be quite unfair. They certainly ray they have records, so perhaps they do. In any case, we would be out of sight there. The Mycogenians keep strictly to themselves. --and now please do go to sleep."

And somehow Seldon finally did.

34.

Hari Seldon and Dors Venabili left the University grounds at 0300. Seldon realized that Dors had to be the leader. She knew Trantor better than he did-two years better. She was obviously a close friend of Hummin (how close? the question kept nagging at him) and she understood his instructions.

Both she and Seldon were swathed in light swirling docks with tight-fitting hoods. The style had been a short-lived clothing fad at the University (and among young intellectuals, generally) some years back and though right now it might provoke laughter, it had the saving grace of covering them well and of making them unrecognizable-at least at a cursory glance.

Hummin had said, "There's a possibility that the event Upperside was completely innocent and that there are no agents after you, Seldon, but lets he prepared for the worst."

Seldon had asked anxiously, "Won't you come with us?"

"I would like to, " said Hummin, "but I must limit my absence from work if I am not to become a target myself. You understand?"

Seldon sighed. He understood.

They entered an Expressway car and found a seat as far as possible from the few who had already boarded. (Seldom wondered why anyone should be on the Expressways at three in the morning---and then thought that it was lucky some were or he and Dors would be entirely too conspicuous.) Seldon fell to watching the endless panorama that passed in review as the equally endless line of coaches moved along the endless monorail on an endless electromagnetic field.

The Expressway passed row upon row of dwelling units, few of them very tall, but some, for all he knew, very deep. Still, if tens of millions of square kilometers formed an urbanized total, even forty billion people would not require very tall structures or very closely packed ones. They did pass open areas, in most of which crops seemed to be growing--but some of which were clearly parklike. And there were numerous structures whose nature he couldn't guess. Factories? Office buildings? Who knew? One large featureless cylinder struck him as though it might be a water tank. After all, Trantor had to have a fresh water supply. Did they sluice rain from Upperside, filter and treat it, then store it? It seemed inevitable that they should.

Seldon did not have very long to study the view, however.

Dors muttered, "This is about where we should be getting off." She stood up and her strong fingers gripped his arm.

They were off the Expressway now, standing on solid flooring while Dors studied the directional signs.

The signs were unobtrusive and there were many of them. Seldon's heart sank. Most of them were in pictographs and initials, which were undoubtedly understandable to native Trantorians, but which were alien to him .

"This way, " said Dors.

"Which way? How do you know?"

"See that? Two wings and an arrow."

"Two wings? Oh." He had thought of it as an upside-down "w, " wide and shallow, but he could see where it might be the stylized wings of a bird.

"Why don't they use words?" he said sullenly.

"Because words vary from world to world. What an 'air-jet' is here could be a 'soar' on Cinna or a 'swoop' on other worlds. The two wings and an arrow are a Galactic symbol for an sir vessel and the symbol is understood everywhere. -Don't you use them on Helicon?"

"Not much. Helicon is a fairly homogeneous world, culturally speaking, and we tend to cling to our private ways firmly because we're overshadowed by our neighbours."

"See?" said Dors. "There's where your psychohistory might come in. You could show that even with different dialects the use of set symbols, Galaxy-wide, is a unifying force."