It was Hamo's sums that gave him the idea, in the end, though the monk never knew it.
Thomas had spent the two days since their conversation by the brazier turning over the plain fact of it: that a column of water didn't care what a man believed, that the numbers held true whether or not Thomas himself was there to read them. He had built an entire valley on knowledge that lived in exactly one place — a cracked slab of glass in his own tunic, and the single skull behind his own eyes. If the glass died, or if he did, the valley would not lose a leader. It would lose a library, all at once, with no copy anywhere.
He raised it with Victoria first, over the ledger-work she still did most evenings out of habit rather than necessity.
"I want to write it down," he said. "Not the ledgers — those are yours and they're safe. Everything else. The metallurgy, the sanitation, the medicine, the mathematics. All of it, in plain hand, on vellum that will outlast both of us."
"That's years of work."
"Then I'd better not do it alone."
Victoria set down her quill, and he watched her turn the idea over with the same care she gave a doubtful column of figures — testing it from several directions before she'd trust it to stand. "You mean to choose people."
"I mean to choose people who won't just copy what I already know. People who understand it well enough to correct me when I'm wrong, and to keep building after I'm not here to ask." He hesitated, aware of how large a thing he was about to say. "I want to name them properly. Not apprentices. Something that means they're trusted to carry it forward, not just told what to do."
"You want Disciples," Victoria said, and there was no mockery in the word at all, only a kind of recognition. "Say what you actually mean, Thomas. You're building a priesthood for a religion with no god in it but arithmetic."
"I hadn't thought of it that way."
"You should. It changes how carefully you choose them." She was already reaching for a fresh sheet, already thinking in structure, in names and columns. "Who did you have in mind?"
He named Wat first, which surprised neither of them. Then Elias, whose maps and now whose towers had outgrown anything Thomas could have taught him. Then, after a pause, Diccon — the boy who had once demonstrated buoyancy with a tub of water for the Archbishop's scholars, and who had spent every year since growing quietly, steadily, into the best hand at the press and the sharpest mind in the schoolhouse besides Thomas's own.
"That's three," Victoria said. "You said five."
"I said Wat, Elias, and Diccon. I want your recommendation for the fourth."
That, more than anything he'd said yet, seemed to catch her off guard. She considered it for a long moment. "Aldith," she said finally. "The midwife's apprentice. You've barely spoken two words to her, so you won't have noticed, but she's rebuilt half of what you taught the nurses about the fever wards on her own initiative, and she's begun keeping her own notes on which remedies actually work and which the old wives simply believe work. She doesn't ask you for answers, Thomas. She tests them. I'd trust her with the medicine before I'd trust either of us."
"Then Aldith. And the fifth is you."
Victoria's expression didn't change, but something in the set of her shoulders did. "I already carry half of what this valley knows. You don't need to name me anything."
"I'm not asking because I need to. I'm asking because if something happens to me and the valley has to choose who speaks for what I built, I want it written, in ink, that it was always meant to be you as much as anyone. Not your husband's shadow. A keeper in your own name."
She didn't answer that immediately, and he let the silence hold, the way Hamo had taught him without meaning to.
"Then I accept," she said at last, "on the condition that the Codex is written honestly. Not a monument to you, Thomas. A working document, full of the places where you were wrong the first time, so the next person doesn't have to make the same mistakes twice."
"That's the only kind worth writing."
They gathered the five of them two evenings later in the undercroft, around the same brazier where Thomas and Hamo had talked, because it was the only room large enough to hold both the people and the weight of what they were about to say. Wat came straight from the forge, still smelling of hot iron. Elias arrived with a fresh sheet of vellum already tucked under his arm, unable to resist the occasion for a new map even when the territory in question was entirely abstract. Diccon stood a little too straight, visibly aware that he was being asked to stand among adults as something other than the youngest person in the room. Aldith came last, wiping her hands on her apron, wary in the particular way of someone who has been summoned somewhere without being told why.
"I'm not going to make a speech," Thomas said, which drew a short, disbelieving grunt from Wat. "I built this valley on things I remembered from a world none of you will ever see, and it occurred to me recently that I could disappear tomorrow and take most of it with me. I don't want that. I want it to survive whether I'm here or not, which means it can't live only in my head anymore."
He looked at each of them in turn. "I'm asking the five of you to become the keepers of it. Not my apprentices. Not people who repeat what I say. People who understand it well enough to argue with me, and to keep it alive long after I've stopped being useful to it. We'll call it the Codex — everything we know, written plainly enough that a stranger could rebuild the wire and the wheel from nothing but the pages, if it ever came to that."
"Everything," Elias repeated, testing the word. "Including the parts you've kept hidden even from us."
"Including those."
Wat crossed his arms, unimpressed by ceremony as always, though Thomas had learned by now to read the particular quality of his silence. "You're asking me to write things down. I forge iron, Thomas. I don't have a scholar's hand."
"I don't need your hand, Wat. I need what's in it. Elias can do the writing if yours embarrasses you. I need to know that after I'm gone, someone in this valley still knows exactly how hot the crucible needs to run and exactly why, instead of guessing and hoping the metal forgives them."
That seemed to settle something in the blacksmith, who gave a short nod and said nothing further, which was, from Wat, as good as an oath.
Aldith spoke last, and quietly. "What happens if we disagree with you? In the Codex, I mean. Written down, where it can't be unsaid."
"Then we disagree in the Codex, in ink, where the next person can read both arguments and decide for themselves which of us was right." Thomas met her eyes. "That's the whole point, Aldith. I don't want a book of my own certainty. I've been wrong about enough things in this valley to fill a chapter by itself."
She studied him a moment longer, as if weighing that answer against whatever she'd expected to hear, and then nodded, satisfied in a way that told him Victoria's judgment of her had been exactly right.
Victoria produced the first blank quire of vellum herself, already bound, already waiting, and set it on the bench between them like a foundation stone.
"Then let's begin properly," she said, "before any of us loses our nerve."
They worked past midnight, the five of them, arguing gently over the first page — what belonged first, what could wait, whose hand would carry which section — and Thomas found, watching them, that it was the first time in three years the valley's whole future had not depended on the charge remaining in his pocket.