inertia."

"You know, sir, I don't really understand this paratime stuff," the pilot confessed. "I know that all time is totally present, and that every moment has its own past-future line of event-sequence, and that all events in space-time occur according to maximum probability, but I just don't get this alternate probability stuff, at all. If something exists, it's because it's the maximum-probability effect of prior causes; why does anything else exist on any other time-line?"

Verkan Vall blew smoke at the air-renovator. A lecture on paratime theory would nicely fill in the three-hour interval until the landing at Dhergabar. At least, this kid was asking intelligent questions.

"Well, you know the principal of time-passage, I suppose?" he began.

"Yes, of course; Rhogom's Doctrine. The basis of most of our psychical science. We exist perpetually at all moments within our life-span; our extraphysical ego component passes from the ego existing at one moment to the ego existing at the next. During unconsciousness, the EPC is 'time-free'; it may detach, and connect at some other moment, with the ego existing at that time-point. That's how we precog. We take an autohypno and recover memories brought back from the future moment and buried in the subconscious mind."

"That's right," Verkan Vall told him. "And even without the autohypno, a lot of precognitive matter leaks out of the subconscious and into the conscious mind, usually in distorted forms, or else inspires 'instinctive' acts, the motivation for which is not brought to the level of consciousness. For instance, suppose, you're walking along North Promenade, in Dhergabar, and you come to the Martian Palace Café, and you go in for a drink, and meet some girl, and strike up an acquaintance with her. This chance acquaintance develops into a love affair, and a year later, out of jealousy, she rays you half a dozen times with a needler."

"Just about that happened to a friend of mine, not long ago," the pilot said. "Go on, sir."

"Well, in the microsecond or so before you die—or afterward, for that matter, because we know that the extraphysical component survives physical destruction—your EPC slips back a couple of years, and re-connects at some point pastward of your first meeting with this girl, and carries with it memories of everything up to the moment of detachment, all of which are indelibly recorded in your subconscious mind. So, when you re-experience the event of standing outside the Martian Palace with a thirst, you go on to the Starway, or Nhergal's, or some other bar. In both cases, on both time-lines, you follow the line of maximum probability; in the second case, your subconscious future memories are an added causal factor."

"And when I back-slip, after I've been needled, I generate a new time-line? Is that it?"

Verkan Vall made a small sound of impatience. "No such thing!" he exclaimed. "It's semantically inadmissible to talk about the total presence of time with one breath and about generating new time-lines with the next. All time-lines are totally present, in perpetual co-existence. The theory is that the EPC passes from one moment, on one time-line, to the next moment on the next line, so that the true passage of the EPC from moment to moment is a two-dimensional diagonal. So, in the case we're using, the event of your going into the Martian Palace exists on one time-line, and the event of your passing along to the Starway exists on another, but both are events in real existence.

"Now, what we do, in paratime transposition, is to build up a hypertemporal field to include the time-line we want to reach, and then shift over to it. Same point in the plenum; same point in primary time—plus primary time elapsed during mechanical and electronic lag in the relays—but a different line of secondary time."

"Then why don't we have past-future time travel on our own time-line?" the pilot wanted to know.

That was a question every paratimer has to answer, every time he talks paratime to the laity. Verkan Vall had been expecting it; he answered patiently.

"The Ghaldron-Hesthor field-generator is like every other mechanism; it can operate only in the area of primary time in which it exists. It can transpose to any other time-line, and carry with it anything inside its field, but it can't go outside its own temporal area of existence, any more than a bullet from that rifle can hit the target a week before it's fired," Verkan Vall pointed out. "Anything inside the field is supposed to be unaffected by anything outside. Supposed to be is the way to put it; it doesn't always work. Once in a while, something pretty nasty gets picked up in transit." He thought, briefly, of the man in the black tunic. "That's why we have armed guards at terminals."

"Suppose you pick up a blast from a nucleonic bomb," the pilot asked, "or something red-hot, or radioactive?"

"We have a monument, at Paratime Police Headquarters, in Dhergabar, bearing the names of our own personnel who didn't make it back. It's a large monument; over the past ten thousand years, it's been inscribed with quite a few names."

"You can have it; I'll stick to rockets!" the pilot replied. "Tell me another thing, though: What's all this about levels, and sectors, and belts? What's the difference?"

"Purely arbitrary terms. There are five main probability levels, derived from the five possible outcomes of the attempt to colonize this planet, seventy-five thousand years ago. We're on the First Level—complete success, and colony fully established. The Fifth Level is the probability of complete failure—no human population established on this planet, and indigenous quasi-human life evolved indigenously. On the Fourth Level, the colonists evidently met with some disaster and lost all memory of their extraterrestrial origin, as well as all extraterrestrial culture. As far as they know, they are an indigenous race; they have a long pre-history of stone-age savagery.

"Sectors are areas of paratime on any level in which the prevalent culture has a common origin and common characteristics. They are divided more or less arbitrarily into sub-sectors. Belts are areas within sub-sectors where conditions are the result of recent alternate probabilities. For instance, I've just come from the Europo-American Sector of the Fourth Level, an area of about ten thousand parayears in depth, in which the dominant civilization developed on the North-West Continent of the Major Land Mass, and spread from there to the Minor Land Mass. The line on which I was operating is also part of a sub-sector of about three thousand parayears' depth, and a belt developing from one of several probable outcomes of a war concluded about three elapsed years ago. On that time-line, the field at the Hagraban Synthetics Works, where we took off, is part of an abandoned farm; on the site of Hagraban City is a little farming village. Those things are there, right now, both in primary time and in the plenum. They are about two hundred and fifty thousand parayears perpendicular to each other, and each is of the same general order of reality."

The red light overhead flashed on. The pilot looked into his visor and put his hands to the manual controls, in case of failure of the robot controls. The rocket landed smoothly, however; there was a slight jar as it was grappled by the crane and hoisted upright, the seats turning in their gimbals. Pilot and passenger unstrapped themselves and hurried through the refrigerated outlet and away from the glowing-hot rocket.

An air-taxi, emblazoned with the device of the Paratime Police, was waiting. Verkan Vall said good-by to the rocket-pilot and took his seat beside the pilot of the aircab; the latter lifted his vehicle above the building level and then set it down on the landing-stage of the Paratime Police Building in a long, side-swooping glide. An