“You can see from this how devastating it can be to meddle with the past.”

“Yes,” I said, “but—”

Lieutenant Sanderson gave me a frigid smile. “You are about to observe that we do not, in fact, live under a repressive Turkish tyranny. I agree. Our present pattern of existence was saved by the following procedure:

“The murder of the young Jesus was detected by a Time Courier who went up the line late in April, 2052, escorting a party of tourists to witness the Crucifixion. When the group arrived at the time and place of the Crucifixion, they found two thieves undergoing execution; no one, however, had heard of Jesus of Nazareth. The Courier instantly notified the Time Patrol, which began a paradox search. Jesus’ time line was followed from birth through boyhood and was seen to be unchanged; but no trace of him could be found after mid-adolescence, and inquiry in the neighborhood finally turned up the information that he had died suddenly and mysteriously in the year 11. It was a simple matter then to maintain surveillance until we observed the arrival of the illegal time-traveler.

“What do you think we did then?”

Hands went up. Lieutenant Sanderson recognized Mr. Chudnik, who said, “You arrested the criminal five minutes before he could give the poison to Jesus, thus preventing the changing of history, and took him back down the line for trial.”

Lieutenant Sanderson smiled genially. “Wrong,” he said. “We let him give the poison to Jesus.”

Uproar.

The Time Patrol man said benignly, “As you surely know, the maximum penalty for unauthorized interference in past events is death — the only capital offense now recognized in law. But before so severe a penalty can be invoked, absolute proof of the crime is necessary. Therefore, whenever a crime of this kind is detected, Time Patrolmen allow it to proceed and surreptitiously make a full record of it.”

“But how,” Miss Dalessandro demanded, “does the past get unchanged that way?”

“Aha!” cried Lieutenant Sanderson. “Once we have a proper record of the commission of the crime, we can obtain a quick conviction and secure permission to carry out sentence. This was done. The Time Patrol investigators returned with their evidence to the night of April 4, 2052. This was the date of the departure up the line of the would-be murderer of Jesus. They presented their proof of the crime to the Time Patrol commissioners, who ordered the execution of the criminal. Time Patrol executioners were dispatched to the home of the criminal, seized his timer, and painlessly put him to death an hour before his intended trip into the past. Thus he was erased from the time-stream and the main current of the past was preserved, for in fact he did not make his trip and Jesus lived on to preach his creed. In this way — through detection of unlawful changes and eradication of the changers in advance of their departure up the line — we preserve the sanctity of now-time.”

How beautiful, I thought.

I’m too easily satisfied. Miss Dalessandro, that arch-troublemaker, put up her fleshy hand, and when called on, said, “I’d like one clarification, though. Presumably when your Time Patrolmen returned to April, 2052, with the evidence of the crime, they were returning to a changed world run by Turkish dictators. Where would they find Time Patrol commissioners? Where would they even find the murderer? He might have ceased to exist as a consequence of his own crime, because by murdering Jesus he set in motion some train of events that eliminated his own ancestors. For that matter, maybe time-travel itself was never invented in that world where Jesus didn’t live, and so the moment Jesus was killed all Time Patrolmen and Time Couriers and tourists would become impossibilities, and cease to exist.”

Lieutenant Sanderson did not look pleased.

“You bring up,” he said slowly, “a number of interesting subsidiary paradoxes. I’m afraid that the time at my disposal isn’t sufficient to deal with them properly. Briefly, though: if the timecrime of 11A.D. had not been detected relatively quickly, the focus of change would indeed have widened over the centuries and eventually transformed the entire future, possibly preventing the emergence of the Benchley Effect and the Time Patrol itself, leading to what we call the Ultimate Paradox, in which time-travel becomes its own negation. In fact, though, the vast potential consequences of the poisoning of Jesus never occurred because of the detection of the crime by the Time Courier visiting the Crucifixion. Since that event took place inA.D. 33, only the years 11 to 33 were ever affected by the timecrime, and the changes created by the absence of Jesus from those years were insignificant, because Jesus’ influence on history emerged only long after the Crucifixion. Meanwhile the retroactive deletion of the timecrime canceled even the slight changes that had taken place in the 22-year period affected; those two decades were pinched off into another track of time, inaccessible to us and in effect nonexistent, and the basic and authentic track was restored in full continuity fromA.D. 11 to the present.”

Miss Dalessandro wasn’t satisfied. “There’s something circular here. Shouldn’t the Ultimate Paradox have occurred all the way down the line, the instant Jesus was poisoned? How did any of the Couriers and Patrolmen manage to continue to exist, let alone to remember how the past should have gone? It seems to me that there ought not to be any way of correcting a timecrime sweeping enough to bring on the Ultimate Paradox.”

“You forget, or perhaps you don’t yet know,” said Sanderson, “that time-travelers currently up the line at the moment of a timecrime are unaffected by any change in the past, since they’re detached from their time matrices. A time-traveler in transit is a drifting bubble of now-time ripped loose from the matrix of the continuum, immune to the transformations of paradox. This means that anyone currently up the line may observe and correct an alteration of the true past, and will continue to retain memories both of the temporary false condition and of his role in correcting it. Of course, any time-traveler leaving the sanctuary of the transit state is vulnerable once he comes back to his starting point down the line. That is, if you go up the line and kill your grandfather before his marriage, you won’t instantaneously wink out of existence, since you’re shielded from paradox by the Benchley Effect. But the moment you return to the present you will cease ever to have existed, since as a result of your alteration of your own past you no longer have a time-link to the present. Clear?”

No, I thought. But I kept quiet.

Miss Dalessandro pressed onward. “Those in transit are protected by—”

“The Paradox of Transit Displacement, we call it.”

“The Paradox of Transit Displacement. They’re encapsulated, and as they travel they’re free to compare what they see with what they remember true time to have been like, and if necessary they can make changes to restore the true order if it’s been changed.”

“Yes.”

“Why? Why should they be immune? I know I keep coming back to this point, but—”

Lieutenant Sanderson sighed. “Because,” he said, “if they were affected by a past-change while they were in the past themselves, this would be the Ultimate Paradox: a time-traveler changing the era that produced time-travel. This is even more paradoxical than the Paradox of Transit Displacement. By the Law of Lesser Paradoxes, the Paradox of Transit Displacement, being less improbable, holds precedence. Do you see?”

“No, but—”

“I’m afraid I can’t dwell on this in greater detail,” said the Patrolman. “However, no doubt Mr. Dajani will go into these matters at later instruction sessions.”

He gave Dajani a sickly smile and excused himself fast.

Dajani, you can bet on it, didn’t deal with Miss Dalessandro’s paradoxes properly, or at all. He found cunning ways to sidetrack her every time she brought up the issue. “You can be sure,” he said, “that the past is restored whenever it is changed. The hypothetical worlds created by unlawful change cease retroactively to exist the moment the changer is apprehended. Q.E.D.”