Sennor thrust out a lower lip. "I'm sure you're wrong. You may have been misled by reading your own knowledge into various ambiguities you may have come across. No, without actual experience of Time-travel, the philosophic intricacies of Reality would be quite beyond the human mind. For instance, why does Reality possess inertia? We all know that it does. Any alteration in its flow must reach a certain magnitude before a Change, a true Change, is effected. Even then, Reality has a tendency to flow back to its original position.

"For instance, suppose a Change here in the 575th. Reality will change with increasing effects to perhaps the 600th. It will change, but with continually lesser effects to perhaps the 650th. Thereafter, Reality will be unchanged. We all know this is so, but do any of us know why it is so? Intuitive reasoning would suggest that any Reality Change would increase its effects without limit as the Centuries pass, yet that is not so.

"Take another point. Technician Harlan, I'm told, is excellent at selecting the exact Minimum Change Required for any situation. I'll wager he cannot explain how he arrives at his own choice.

"Consider how helpless the Primitives must be. They worry about a man killing his own grandfather because they do not understand the truth about Reality. Take a more likely and a more easily analyzed case and let's consider the man who in his travels through time meets himself--"

Harlan said sharply, "What about a man who meets himself?"

The fact that Harlan interrupted a Computer was a breach of manners in itself. His tone of voice worsened the breach to a scandalous extent, and all eyes turned reproachfully on the Technician.

Sennor harumphed, but spoke in the strained tone of one determined to be polite despite nearly insuperable difficulties. He said, continuing his broken sentence and thus avoiding the appearance of answering directly the unmannerly question addressed to him, "And the four subdivisions into which such an act can fall. Call the man earlier in physiotime, A, and the one later, B. Subdivision one, A and B may not see one another, or do anything that will significantly affect one another. In that case, they have not really met and we may dismiss this case as trivial.

"Or B, the later individual, may see A while A does not see B. Here, too, no serious consequences need be expected. B, seeing A, sees him in a position and engaged in activity of which he already has knowledge. Nothing new is involved.

"The third and fourth possibilities are that A sees B, while B does not see A, and that A and B see one another. In each possibility, the serious point is that A has seen B; the man at an earlier stage in his physiological existence sees himself at a later stage. Observe that he has learned he will be alive at the apparent age of B. He knows he will live long enough to perform the action he has witnessed. Now a man in knowing his own future in even the slightest detail can act on that knowledge and therefore changes his future. It follows that Reality must be changed to the extent of not allowing A and B to meet or, at the very least, of preventing A from seeing B. Then, since nothing in a Reality made un-Real can be detected, A never has met B. Similarly, in every apparent paradox of Time-travel, Reality always changes to avoid the paradox and we come to the conclusion that there are no paradoxes in Time-travel and that there can be none."

Sennor looked well pleased with himself and his exposition, but Twis sell rose to his feet.

Twis sell said, "I believe, gentlemen, that time presses."

Far more suddenly than Harlan would have thought the lunch was over. Five of the subcommittee members filed out, nodding at him, with the air of those whose curiosity, mild at best, had been assuaged. Only Sennor held out a hand and added a gruff "Good day, young man" to the nod.

With mixed feelings Harlan watched them go. What had been the purpose of the luncheon? Most of all, why the reference to men meeting themselves? They had made no mention of Noys. Were they there, then, only to study him? Survey him from top to bottom and leave him to Twissell's judging?

Twissell returned to the table, empty now of food and cutlery. He was alone with Harlan now, and almost as though to symbolize that he wielded a new cigarette between his fingers.

He said, "And now to work, Harlan. We have a great deal to do."

But Harlan would not, could not, wait longer. He said flatly, "Before we do anything, I have something to say."

Twissell looked surprised. The skin of his face puckered up about his faded eyes, and he tamped at the ash end of his cigarette thoughtfully.

He said, "By all means, speak if you wish, but first, sit down, sit down, boy."

Technician Andrew Harlan did not sit down. He strode up and back the length of the table, biting off his sentences hard to keep them from boiling and bubbling into incoherence. Senior Computer Laban Twissell's age-yellowed pippin of a head turned back and forth as he followed the other's nervous stride.

Harlan said, "For weeks now I've been going through films on the history of mathematics. Books from several Realities of the 575th. The Realities don't matter much. Mathematics doesn't change. The order of its development doesn't change either. No matter how else the Realities shifted, mathematical history stayed about the same. The mathematicians changed; different ones switched discoveries, but the end results-- Anyway, I pounded a lot of it into my head. How does that strike you?"

Twissell frowned and said, "A queer occupation for a Technician?"

"But I'm not just a Technician," said Harlan. "You know that."

"Go on," said Twissell and he looked at the timepiece he wore. The fingers that held his cigarette played with it with unwonted nervousness.

Harlan said, "There was a man named Vikkor Mallansohn who lived in the 24th Century. That was part of the Primitive era, you know. The thing he is known best for is the fact that he first successfully built a Temporal Field. That means, of course, that he invented Eternity, since Eternity is only one tremendous Temporal Field shortcircuiting ordinary Time and free of the limitations of ordinary Time."

"You were taught this as a Cub, boy."

"But I was not taught that Vikkor Mallansohn could not possibly have invented the Temporal Field in the 24th Century. Nor could anyone have. The mathematical basis for it didn't exist. The fundamental Lefebvre equations did not exist; nor could they exist until the researches of Jan Verdeer in the 27th Century."

If there was one sign by which Senior Computer Twissell could indicate complete astonishment, it was that of dropping his cigarette. He dropped it now. Even his smile was gone.

He said, "Were you taught the Lefebvre equations, boy?"

"No. And I don't say I understand them. But they're necessary for the Temporal Field. I've learned that. And they weren't discovered till the 27th. I know that, too."

Twissell bent to pick up his cigarette and regarded it dubiously. "What if Mallansohn had stumbled on the Temporal Field without being aware of the mathematical justification? What if it were simply an empirical discovery? There have been many such."

"I've thought of that. But after the Field was invented, it took three centuries to work out its implications and at the end of that time there was no one way in which Mallansohn's Field could be improved on. That could not be coincidence. In a hundred ways, Mallansohn's design showed that he must have used the Lefebvre equations. If he knew them or bad developed them without Verdeer's work, which is impossible, why didn't he say so?"

Twissell said, "You insist on talking like a mathematician. Who told you all this?"

"I've been viewing films."

"No more?"

"And thinking."