“Would you explain that, sir?” said Miss Dalessandro.

“At a later meeting,” Dajani replied. He went on, “Naturally, we must not confine time travel exclusively to tourists. Historians must have access to all significant events of the past, since it is necessary to revise all existing views of history in the light of the revelation of the real story. We set aside out of the profits of our tourist business a certain number of scholarships for qualified historians, enabling them to visit periods of their research without cost. These tours, too, are conducted by Courier. However, you will not be concerned with this aspect of our work. We anticipate assigning all of you who qualify as Couriers to the tourist division.

“The other division of the Time Service is the Time Patrol, whose task it is to prevent abuses of Benchley Effect devices and to guard against the emergence of paradoxes. At our next lesson we will consider in detail the nature of these paradoxes and how they may be avoided. Dismissed.”

We had a small social session after Dajani left the room. Miss Dalessandro, moving in a determined whirl of hairy armpits, closed in on blonde, delicate Miss Chambers, who promptly fled toward Mr. Chudnik, a brawny, towering gentleman with the vaguely noble look of a Roman bronze. Mr. Chudnik, however, was in the process of trying to reach an accommodation with Mr. Burlingame, a dapper young man who could not possibly have been as homosexual as he looked and acted. And so, seeking some other shelter from the predatory Miss Dalessandro, Miss Chambers turned to me and invited me to take her home. I accepted. It developed that Miss Chambers was a student of later Roman imperial history, which meant that her field of interest dovetailed with mine. We sexed in a perfunctory and mechanical way, since she was not really very interested in sex but was just doing it out of politeness, and then we talked about the conversion of Constantine to Christianity until the early hours of the morning. I think she fell in love with me. I gave her no encouragement, though, and it didn’t last. I admired her scholarship but her pale little body was quite a bore.

At our next lesson we considered in detail the nature of the time-travel paradoxes and how they may be avoided.

“Our greatest challenge,” Dajani began, “lies in maintaining the sanctity of now-time. The development of Benchley Effect devices has opened a Pandora’s Box of potential paradoxes. No longer is the past a fixed quantity, since we are free now to travel up the line to any given point and alter the so-called ‘real’ events. The results of such alteration would of course be catastrophic, creating a widening vector of disruption that, by the time it had reached our own era, might transform every aspect of society.” Dajani yawned politely. “Consider, if you will, the consequences of permitting a time-traveler to journey to the year 600 and assassinate the youthful Mohammed. The whole dynamic movement of Islam will thus be arrested at its starting point; there will be no Arab conquest of the Near East and southern Europe; the Crusades will not have taken place; millions who died as a result of the Islamic invasions will now not have died, and numerous lines of progeny that would not otherwise have existed at all will come into being, with incalculable effects. All this stems simply from the slaying of a certain young merchant of Mecca. And therefore—”

“Perhaps,” suggested Miss Dalessandro, “there’s a Law of Conservation of History which would provide that if Mohammed didn’t happen, some other charismatic Arab would arise and play precisely the same role?”

Dajani glowered at her.

“We do not care to risk it,” he said. “We prefer to see to it that all ‘past’ events, as recorded in the annals of history as compiled prior to the era of time-travel, go untouched. For the past fifty years of now-time the entire previous span of history, thought to be fixed, has been potentially fluid; yet we struggle to keep it fixed. Thus we employ the Time Patrol to make certain that everything will happen in the past exactly as it did happen, no matter how unfortunate an event it might be. Disasters, assassinations, tragedies of all kinds must occur on schedule, for otherwise the future — our now-time — may be irreparably changed.”

Miss Chambers said, “But isn’t the very fact of our presence in the past a changing of the past?”

“I was about to reach that point,” said Dajani, displeased. “If we assume that the past and present form a single continuum, then obviously visitors from the twenty-first century were present at all the great events of the past, unobtrusively enough so that no mention of them found its way into the annals of the fixed-time era. So we take great care to camouflage everyone who goes up the line in the costume of the time being visited. One must watch the past without meddling, as a silent bystander, as inconspicuous as possible. This is a rule that the Time Patrol enforces with absolute inflexibility. I will discuss the nature of that enforcement shortly.

“I spoke the other day of cumulative audience paradox. This is a severe philosophical problem which has not yet been resolved, and which I will present to you now purely as a theoretical exercise, to give you some insight into the complexities of our undertaking. Consider this: the first time-traveler to go up the line to view the Crucifixion of Jesus was the experimentalist Barney Navarre, in 2012. Over the succeeding two decades, another fifteen or twenty experimentalists made the same journey. Since the commencement of commercial excursions to Golgotha in 2041, approximately one tourist group a month — or 100 tourists a year — has viewed the scene. Thus about 1800 individuals of the twenty-first century, so far, have observed the Crucifixion. Now, then: each of these groups is leaving from a different month, but every one of them is converging on the same day! If tourists continue to go up the line at a rate of 100 a year to see the Crucifixion, the crowd at Golgotha will consist of at least 10,000 time-travelers by the middle of the twenty-second century, and — assuming no increase in the permissible tourist trade — by the early thirtieth century, some 100,000 time-travelers will have made the trip, all of them necessarily congregating simultaneously at the site of the Passion. Yet obviously no such crowds are present there now, only a few thousand Palestinians — when I say ‘now,’ I mean of course the time of the Crucifixion relative to now-time 2059 — and just as obviously those crowds will continue to grow in the centuries of now-time. Taken to its ultimate, the cumulative audience paradox yields us the picture of an audience of billions of time-travelers piled up in the past to witness the Crucifixion, filling all the Holy Land and spreading out into Turkey, into Arabia, even to India and Iran. Similarly for every other significant event in human history: as commercial time-travel progresses, it must inevitably smother every event in a horde of spectators, yet at the original occurrence of those events, no such hordes were present! How is this paradox to be resolved?”

Miss Dalessandro had no suggestions. For once, she was stumped. So were the rest of us. So was Dajani. So are the finest minds of our era.

Meanwhile, the past fills up with time-traveling sightseers.

Dajani tossed one final twister at us before he let us go. “I may add,” he said, “that I myself, as a Courier, have done the Crucifixion run twenty-two times, with twenty-two different groups. If you were to attend the Crucifixion yourselves tomorrow, you would find twenty-two Najeeb Dajanis at the hill of Golgotha simultaneously, each of me occupying a different position at the event explaining the happening to my clients. Is this multiplication of Dajanis not a fascinating thing to consider? Why are there not twenty-two Dajanis at loose in now-time? It stretches the intellect to revolve such thoughts. Dismissed, dear ladies and gentlemen, dismissed.”