Fraa Orolo had told me that since the Reconstitution there had been forty-eight instances in which radical change had occurred in a decade, and that two of these had culminated in Sacks—so perhaps the sudden ones were the most important. And yet ten years was a long enough span of time that people who lived extramuros, immersed in day-to-day goings-on, might be oblivious to change. So a Tenner reading an eleven-hundred-year-old questionnaire to an artisan could perform a service to the society extramuros (assuming anyone out there was paying attention). Which might help to explain why we were not only tolerated but protected (except when we weren’t) by the Sæcular Power. “The man who looks at a mole on his brow every day when he shaves may not see that it is changing; the physician who sees it once a year may easily recognize it as cancer.”

“Beautiful,” I’d said. “But you’ve never cared about the Sæcular Power before, so what’s your real reason?”

He had pretended to be bewildered by the question. But, seeing I wasn’t going to back off, he had shrugged and said “Just a routine check for CDS.”

“CDS?”

“Causal Domain Shear.”

This had as much as proved that Orolo was only having me on. But sometimes he had a point when he was doing that.

Correction: he always had a point. Sometimes I was able to see it. So I had rested my face on my hands and muttered, “Okay. Open the floodgates.”

“Well. A causal domain is just a collection of things linked by mutual cause-and-effect relationships.”

“But isn’t everything in the universe so linked?”

“Depends on how their light cones are arranged. We can’t affect things in our past. Some things are too far away to affect us in any way that matters.”

“But still, you can’t really draw hard and fast boundaries between causal domains.”

“In general, no. But you are much more strongly webbed together with me by cause and effect than you are with an alien in a faraway galaxy. So, depending on what level of approximation you’re willing to put up with, you could say that you and I belong together in one causal domain, and the alien belongs in another.”

“Okay,” I had said, “what level of approximation are you willing to put up with, Pa Orolo?”

“Well, the whole point of living in a cloistered math is to reduce our causal linkages with the extramuros world to the minimum, isn’t it?”

“Socially, yes. Culturally, yes. Ecologically, even. But we use the same atmosphere, we hear their mobes driving by—on a pure theoric level, there is no causal separation at all!”

He hadn’t seemed to have heard me. “If there were another universe, altogether separate from ours—no causal linkages whatsoever between Universes A and B—would time flow at the same rate between them?”

“It’s a meaningless question,” I’d said, after having thought about it for a moment.

“That’s funny, it seemed meaningful to me,” he’d retorted, a little cross.

“Well, it depends on how you measure time.”

He’d waited.

“It depends on what time is!” I’d said. I had spent a few minutes going up various avenues of explanation, only to find each of them a dead end.

“Well,” I’d said finally, “I guess I have to invoke the Steelyard. In the absence of a good argument to the contrary, I have to choose the simplest answer. And the simplest answer is that time runs independently in Universe A and Universe B.”

“Because they are separate causal domains.”

“Yes.”

Orolo said, “What if these two universes—each as big and as old and as complicated as ours—were entirely separate, except for a single photon that managed to travel somehow between them. Would that be enough to wrench A’s time and B’s time into perfect lockstep for all eternity?”

I had sighed, as I always did when one of Orolo’s traps closed over me.

“Or,” he’d said, “is it possible to have a little bit of time slippage—shear—between causal domains that are connected only loosely?”

“So—back to your interview with Artisan Flec—you want me to believe that you were just checking to see whether a thousand years might have gone by on the other side of that wall while only ten have gone by on this side!?”

“I saw no harm in making inquiries,” he’d said. Then he’d gotten a look as if something else were on the tip of his tongue. Something mischievous. I had headed him off before he could say it:

“Oh. Is this anything to do with your crazy stories about the wandering ten-thousand-year math?”

When we’d been new fids, Orolo had once claimed that he had found an instance in the Chronicles where a gate somewhere had ground open and some avout had walked out of it claiming to be Ten-thousanders celebrating Apert. Which was ridiculous because avout in their current form had only existed for (at that time) 3682 years. So we’d reckoned that the whole purpose of the story had been to see if we had been paying any attention whatsoever to our history lessons. But perhaps the story had been meant to convey a deeper point.

“You can get a lot done in ten millennia if you put your mind to it,” Orolo had said. “What if you found a way to sever all causal links to the world extramuros?”

“That is utterly ridiculous. You are giving Incanter-like powers to these people.”

“But if one could do it, then one’s math would become a separate universe and its time would no longer be synchronized with the rest of the world’s. Causal Domain Shear would become possible—”

“Nice thought experiment,” I’d said. “Point taken. Thank you for the calca. But please tell me you don’t really expect to see evidence of CDS when the gates open!”

“It is what you don’t expect,” he’d said, “that most needs looking for.”

“Do you have, in your wigwams or tents or skyscrapers or wherever you live—”

“Trailers without wheels mostly,” said Artisan Quin.

“Very well. In those, is it common to have things that can think, but that are not human?”

“We did for a while, but they all stopped working and we threw them away.”

“Can you read? And by that I don’t just mean interpreting Logotype…”

“No one uses that any more,” said Quin. “You’re talking about the symbols on your underwear that tell you not to use bleach. That sort of thing.”

“We don’t have underwear, or bleach—just the bolt, the chord, and the sphere,” said Fraa Orolo, patting the length of cloth thrown over his head, the rope knotted around his waist, and the sphere under his bottom. This was a weak joke at our expense to set Quin at ease.

Quin stood up and tossed his long body in a way that made his jacket fly off. He was not a thick-built man but he had muscles from working. He whirled the jacket round to his front and used his thumbs to thrust out a sheaf of tags sewn into the back of the collar. I could see the logo of a company, which I recognized from ten years ago, though they had made it simpler. Below it was a grid of tiny pictures that moved. “Kinagrams. They obsoleted Logotype.”

I felt old: a new feeling for me.

Orolo had been curious until he’d seen the Kinagrams; now he looked disappointed. “Oh,” he said, in a mild and polite tone of voice, “you are talking bulshytt.”

I got embarrassed. Quin was amazed. Then his face turned red. It looked as if he were talking himself into being angry.

“Fraa Orolo didn’t say what you think!” I told Quin, and tried to punctuate it with a chuckle, which came out as a gasp. “It is an ancient Orth word.”

“It sounded a lot like—”

“I know! But Fraa Orolo has forgotten all about the word you are thinking of. It’s not what he meant.”

“What did he mean, then?”

Fraa Orolo was fascinated that Quin and I were talking about him as if he weren’t there.

“He means that there’s no real distinction between Kinagrams and Logotype.”