A Harry Potter fanfiction. Based on the world of "Harry Potter and the Methods of Rationality" by Eliezer Yudkowsky, diverging from canon.

This story was written collaboratively with Claude: Starting from the premise "HPMOR but with Chesterton's Fence", I brainstormed ideas with it and decided what to include and what to discard. Claude wrote down the result once I was satisfied with the plan, and I made final edits.

Harry had been having, by any objective measure, an excellent week.

On Monday he had demonstrated, to his own satisfaction and Professor Flitwick's visible alarm, that the Hover Charm could be generalized to any object regardless of mass if you conceptualized it as a momentum transfer rather than a force application. On Wednesday he had worked out why Neville's potions kept failing — the textbook instructions assumed clockwise stirring, but the underlying reaction was chirally sensitive, and Neville was left-handed. A trivial fix. Neville had cried.

On Friday evening, buoyed by the week's successes and looking for a specific reference on crystalline wand cores that he was certain would unlock a further generalization of his momentum framework, Harry was in the Restricted Section.

He had access. Professor McGonagall had granted it after the Hover Charm incident, in a tone that suggested she was choosing between supervised access and finding him there anyway at 2 AM. A reasonable calculation on her part.

The book he wanted wasn't where the index said it should be. In its place was something else — a slim volume, untitled, bound in leather that had gone dark and soft with age. No author. No date. No library markings at all, which was itself unusual; Madam Pince catalogued everything.

He opened it because he was Harry Potter and there was an uncatalogued book in front of him and not opening it was not a thing that was going to happen.

The first entry was dated in a system he didn't immediately recognize — then did. The Roman calendar. Before the Julian reform. Which put it somewhere around...

He did the arithmetic twice. The book was over two thousand years old.

The handwriting — once he adjusted to the Latin, which was oddly easy to read, closer to spell notation than classical prose — was precise, methodical, and deeply familiar. Not the content. The voice.

I have spent the summer months cataloguing what the elders call the "ancestral arts" and I find their taxonomy incoherent. They group spells by tradition and lineage rather than by underlying principle. When I asked Marcellus why the fire-calling and the forge-warming are taught as separate disciplines when they clearly operate on the same substrate, he told me that they come from different families and are therefore different magics. This is not a reason. This is genealogy dressed as ontology.

Harry's breath caught. Not at the content — at the recognition. He had written almost exactly this, in his own notes, three months ago. About Transfiguration and Charms.

He kept reading.

I have begun my own classification. If the elders will not systematize the arts, I will do it myself. The patterns are obvious once you abandon the traditional categories. There are at most seven fundamental interactions underlying all known magic, and the spells are simply different access points to the same underlying mechanisms. The ancestors must have known this. Why has it been forgotten? Why has no one else seen it?

The entries spanned what appeared to be several years. Harry read them in order, sitting cross-legged on the cold floor of the Restricted Section, the book in his lap, a Lumos hovering above him that he had long since stopped consciously maintaining.

The author — he never gave his name in the early entries, a habit of Roman-era wizards who considered written names a vulnerability — progressed rapidly. His early observations were sharp. His experiments were well-designed. Harry found himself nodding along, mentally annotating, sometimes wanting to reach through two millennia and suggest a control group.

By the middle entries, the author had begun to find things that disturbed him.

The incantations are not Latin. I have been operating under the assumption that our magical vocabulary derives from our common tongue, as all technical language does. I was wrong. I tested this with Cassia, who is gifted with languages. She confirms what I suspected: the derivation goes the wrong way. "Lumos" is not a Latin word adapted for magical use. The Latin words for light — lux, lumen, lucere — are corruptions of the incantation. The spell came first. The language came after.

I do not know what to make of this. It implies that the magical infrastructure predates Latin. Predates Rome. Predates, perhaps, all of our civilizations. If the spells are the original and the language is the echo, then who wrote the original?

Harry lowered the book for a moment. His hands were not shaking, because he was Harry Potter and his hands did not shake, but he noticed that his Lumos had brightened considerably, which was the sort of involuntary response that meant his emotional state was affecting his magic, which meant his emotional state was more affected than he was admitting to himself.

The etymology goes the wrong way.

He'd never thought about it. He'd never thought about it. He'd been casting spells in what he assumed was Latin for months and he'd never once asked why a language from an Italian peninsula was the universal interface for a fundamental force of nature.

He kept reading.

The author's investigation led him, inevitably, to the founders. Not of Hogwarts — of Rome.

I have secured an audience with the Elder of the Third House, who claims direct knowledge passed down from the time of Romulus. I was skeptical. I am no longer skeptical. He told me things about the founding that are not in any record, and which I have independently verified through architectural analysis of the oldest magical structures.

The founders did not discover magic. They arrived with it. They came from somewhere else, carrying fragments of knowledge far beyond what we possess today, and they built the minimum necessary to sustain a civilization. What we call "Roman magic" is not a tradition developed over centuries. It is the residue of something much larger, distributed by people who understood only a fraction of it themselves.

I asked the Elder what the founders were fragments of. Where they came from. He became very still and told me I should stop this line of inquiry.

I will not be stopping this line of inquiry.

Harry heard himself laugh — a short, involuntary sound in the silent library. Of course the author wouldn't stop. Harry wouldn't have stopped either. That was the whole point of being the kind of person who —

He stopped laughing.

He kept reading.

The Elder has agreed to tell me more, though he is unhappy about it. I believe he has decided that refusing to answer will only drive my investigations in more dangerous directions, which is probably true.

He told me about Atlantis.

Not the myth. Not the garbled account that surfaces sometimes in Greek philosophy. The actual place. An actual civilization, so advanced that our magic is to theirs as a child's drawing is to the thing it depicts. They did not merely use the fundamental forces. They rewrote them. The magical substrate that we interact with — the spells, the wand movements, the magical creatures, the entire ecosystem that we treat as natural law — is not natural. It is infrastructure. Built by Atlantean artificers so long ago that their work has been mistaken for nature itself.

We are living inside their creation and we have forgotten that it was created.

I asked the Elder what happened to them.

He said: "What always happens."

I asked him to be more specific.

He was.

The next three entries were short and shaken. The author's handwriting, previously meticulous, had become uneven. He did not reproduce what the Elder told him. He referred to it only obliquely.

I have not slept. I keep thinking about the numbers. The Elder was not specific about the population of Atlantis at its height, but from the scale of what they built — and everything around us is what they built — it must have been vast. And it is all gone. Not conquered. Not declined. Erased so completely that the only evidence it existed is the infrastructure itself, still running, still shaping reality, maintained by no one, understood by no one.

A civilization capable of rewriting the laws of physics left nothing behind except the rewrite.

The entries resumed some weeks later. The author had regained his composure and — Harry felt a chill as he recognized this too — had begun to rationalize.

I have been thinking about the Elder's warning and I believe it is overstated. The Atlanteans destroyed themselves through what appears to have been unrestricted access to the deep substrate — the layer beneath the magical interface that we interact with. But we are not Atlanteans. We are working with the interface, not the source. The risk profile is entirely different.

Furthermore, the Elder's position is essentially conservative: because something went wrong once, we should never investigate again. This is not a principle. This is fear. By the same logic, we should never have built Rome because previous civilizations fell.

I do not intend to access the deep substrate. I intend merely to understand the interface more fully. There is a distinction between studying a tool and dismantling it.

Harry was nodding. The argument was sound. The distinction between studying and dismantling was real and important. You could investigate a system without —

He turned the page.

I have made a breakthrough. The warding structures on the oldest Roman buildings are not merely protective. They are computational. They are performing continuous calculations that maintain certain properties of local magical space. If I am right, then removing or modifying them would alter the behavior of all magic within their range.

I have identified a ward that appears to be suppressing something. I do not yet know what. But its structure suggests it was placed by the founders themselves, and it is consuming an enormous amount of magical energy to maintain. Whatever it is suppressing must be correspondingly powerful.

The obvious question: what would happen if it were removed?

I am not going to remove it. I am merely going to study it. There is a difference.

I have brought my findings to the Elder. He was not pleased. He used the word "fool," which I found unnecessarily personal.

He asked me: "Why not use this knowledge to protect Rome against Carthage?" I took this as a rhetorical point about the practical applications of my research and began to outline several defensive possibilities.

He cut me off. "Been there," he said. "Done that."

I asked him to explain.

He would not.

The entry ended there. The next one was dated six days later.

I have been researching Carthage independently. The military histories are straightforward. The magical histories are not. There are gaps. References that lead nowhere. Records that appear to have been deliberately destroyed.

I found one surviving account, hidden inside a genealogical registry where no one would think to look. It describes Carthage before the wars. A thriving magical civilization. Advanced. Innovative. In some ways more sophisticated than Rome.

The account was written by a Carthaginian wizard who was visiting Rome when his home ceased to exist. His description of what he returned to is...

The Romans salted the earth. I always assumed this was metaphorical, or at most a symbolic act of dominance. It was not. Nothing grows there because the magical substrate in that region was damaged so severely that it cannot support life properly. The salt was a cover story. Something happened to Carthage that had nothing to do with legions and warships.

"Been there. Done that."

I think the Elder was not speaking rhetorically.

The tone of the entries shifted after Carthage. The author became more cautious. More reflective. He wrote about his family — a wife, two children. He wrote about his garden. There were gaps of weeks between entries, then months.

Harry thought the journal was winding toward a conclusion. A decision to stop. A graceful retreat into domestic life, wisdom earned, lesson learned.

That is not what happened.

I have been away from this journal for four months. In that time I have tried to put my research aside. I have focused on teaching, on my family, on the ordinary satisfactions of a life well-lived.

I cannot do it.

The knowledge is there. The interface is not merely an interface — it is a doorway, and I have seen through it, and I cannot unsee what is on the other side. The Elder is right that the Atlanteans destroyed themselves. He is right that Carthage was destroyed by someone misusing recovered knowledge. He may even be right that I should stop.

But I am not going to access the deep substrate. I am merely going to remove one ward. One single suppression ward that is consuming enormous energy to hide something that may be entirely benign. I am not going to use what I find. I only want to know.

I will take every precaution.

The entries after that were technical. Dense. Excited. The author had found collaborators — "careful men, scholars, not reckless" — and they were mapping the ward structure in detail. The work was methodical. The safeguards were extensive. Every entry described another layer of caution, another fallback, another reason this was different from what had come before.

Harry read faster. Then slower.

The last entry was not dramatic. It was not a cry for help or a confession or a warning. It was a plan for the following week's work. A list of measurements to take. A note to bring lunch because last time they had worked through the meal and concentration suffered. A reminder to pick up something from the market for his daughter's birthday.

Then blank pages.

Harry turned them. One after another. Blank. Blank. Blank.

He turned them all.

The author's name was not in the journal. But there were enough identifying details — the Third House, the Elder, the specific ward locations — that it took Harry less than twenty minutes in the historical records to find him.

Marcus Valerius Corvus. Wizard of the Third Augural House. Born in the 154th year of Rome's founding. Noted scholar. Family man. Described in one secondary source as "the most gifted theoretical magician of his generation."

The secondary sources were sparse after a certain date. There was a gap in the records of the Third House. A fire, attributed to accident. Several members of the House dead or missing. A brief, clinical notation in a Senate record about "disturbances in the southern district" that required intervention. The word used for the intervention was one Harry had to look up.

It meant, roughly, "cauterization."

A later genealogical record listed the surviving members of the Corvus family. His wife. His daughter. His son. They had relocated to a rural settlement far from Rome. There was a single annotation next to his wife's name that Harry read three times before he understood it. It was a legal status marker.

It meant that her husband was not dead but had been declared non-person. Stripped of name, of citizenship, of family ties. Not executed. Not exiled. Something the Romans reserved for people who had committed offenses so severe that the punishment was un-being. Removal from all records, all lineages, all memory.

The man who had written the journal with such clarity and care and cautious optimism had his name scraped from the walls of his own house.

And the southern district of magical Rome — Harry checked — had been rebuilt. But the secondary sources noted, in the careful phrasing of historians who did not want to speculate, that the character of the magic there was different afterward. Weaker in some ways. Stranger in others. As if the substrate itself had been bruised.

Harry closed the genealogical record. He sat for a while in the silent library. His Lumos had dimmed to almost nothing and he had not noticed.

He thought about Marcus Valerius Corvus, who was the most gifted theoretical magician of his generation, who took every precaution, who only wanted to know, who was not going to use what he found, who was merely going to remove one ward —

He thought about Carthage. Five hundred thousand people. A salted plain.

He thought about the etymology going the wrong way, and what that meant, and what had built the system that everyone was living inside, and where they had gone.

He thought about the Weasleys' kitchen. The self-stirring pot. The clock on the wall that tracked the family. The pile of shoes by the door. The way Mrs. Weasley's cooking expanded to accommodate however many people showed up, not through efficiency but through abundance, and how the house itself seemed to grow rooms when rooms were needed, and how none of this struck any wizard as remarkable because it wasn't remarkable, it was just life when you had magic, and how he had looked at all of this and thought they could be so much more without ever asking more what? And why?

He thought about a civilization so advanced it could rewrite the laws of physics, and how they were gone so completely that the only evidence was everything.

His Lumos went out. He sat in the dark for a long time.

Then he picked up the journal and went to see the Headmaster.

It was late. Harry had expected to have to argue his way past the gargoyle, but it stepped aside before he spoke. The staircase was already moving. The door at the top was open, and the office was lit, and there was a teapot on the desk that was still steaming.

Dumbledore was in his chair. He looked at the book in Harry's hands and his expression did something complicated that ended in a kind of tired gentleness.

"Sit down, Harry."

Harry sat. Dumbledore poured tea. The cup was warm in Harry's hands and he held it without drinking.

"You knew I'd find it," Harry said.

"I knew you would find it, or something like it. You are not the first student of your particular... temperament."

"The book. Marcus Valerius Corvus. The southern district. All of it. You just — left it there? In the library?"

"Where would you suggest I put it?" Dumbledore said, gently. "It has been in that library for a very long time. It has been found before. It will be found again. The question has never been whether bright students will find it. The question is what they do after."

Harry looked down at his tea.

"Harry, do you know how many people lived in Carthage?"

"At its height? Estimates vary. Somewhere around five hundred thousand."

Dumbledore said nothing. He let the number sit in the room.

A long silence.

"You are not the worst case I have dealt with, if that offers any comfort," Dumbledore said, in the tone of a man offering what comfort he could. "In 1971 I had to physically restrain a student who had found a reference to something called — and I wish I were not saying these words — the Torment Nexus, and was attempting to access it because, and I quote, 'it probably isn't really that bad, the name is most likely metaphorical.'"

A beat.

"It was not metaphorical."

Harry, numbly: "Was that Voldemort?"

"It was not Voldemort. There are many bright students, Harry."

Another silence. The fire crackled. Somewhere in the castle, a clock chimed a late hour.

"Grindelwald read that journal in his fifth year," Dumbledore said, quietly. "He drew ambitious conclusions. I read it the year after. I had the advantage of watching what those conclusions did to my closest friend."

He set down his teacup.

"I was as clever as you, once. Cleverer, perhaps. I looked at the wizarding world and I saw everything you see — the inefficiency, the waste, the tradition without reason, the power unused. Gellert and I were going to remake everything. For the greater good." The words came out with the particular care of a man handling something that still cut. "It was Gellert who wanted to move fast. I was the one who wanted to be systematic. I was going to be careful. I was only going to remove the unnecessary constraints. I had safeguards planned. Precautions. I was not going to be reckless."

He looked at Harry.

"Do you know what the difference is between Gellert Grindelwald and Marcus Valerius Corvus?"

Harry shook his head.

"Scale. Only scale. The reasoning is always the same. 'I am not going to use it, I only want to know. I will take every precaution. This is different from what came before.' I have heard it from every brilliant student who has sat where you are sitting. The words barely vary."

Harry stared at the journal in his lap. The leather was warm where his hands had been holding it.

"What do I do?" he asked. His voice was smaller than he wanted it to be.

"I have found," Dumbledore said, "that the question is less about what to do than about what to want. The wanting is where it all goes wrong. Not the knowing. Not the doing. The wanting."

He picked up the teapot and refilled Harry's cup, though Harry had not drunk any.

"Molly Weasley tells me she is making two pies for the Christmas holiday. Apparently your friend Ron found the first insufficient last year and has formally requested a second. I understand it will be treacle."

Harry looked up. Dumbledore's eyes were bright and kind and ancient and sad all at once.

"Go to the Weasleys for Christmas, Harry. Eat pie. Let Molly fuss over you. Watch Arthur get excited about batteries. These are not small things. In a world that has already been optimized, they are the only things that matter."

Harry walked back to the Gryffindor common room slowly. The castle was quiet. His footsteps echoed in the empty corridors and he listened to them the way you listen to something when your mind is too full for thought.

He thought about Marcus Valerius Corvus, who took every precaution and only wanted to know.

He thought about Grindelwald, who drew ambitious conclusions.

He thought about a brilliant student in 1971 who tried to open something called the Torment Nexus because the name was probably metaphorical.

He thought about the Weasley kitchen, and the self-repairing house, and the clock that tracked the family, and the way Ron talked about his mum's cooking with the unselfconscious happiness of a person who had never once doubted that there would be enough.

He thought about a civilization that could rewrite physics. Gone. Infrastructure still running. No one left to read the manual.

He thought about Dumbledore, seventeen years old, clever as anyone who ever lived, choosing between more and enough, and choosing wrong, and spending the rest of his life gently steering other clever children away from the same door.

He thought about the journal, which he was still carrying, and which he was going to return to the Restricted Section in the morning. Not because it should be hidden. Because it should be findable. Because someday another student with his particular temperament would need to read it at exactly the right moment, and the library needed to be ready.

He climbed through the portrait hole. The common room was empty except for a low fire. Ron had fallen asleep on the couch with a Chudley Cannons scarf over his face. He was snoring.

Harry stood there for a while.

The optimization engine in his head — the one that never stopped, the one that saw every system as a problem and every problem as solvable and every solution as a step toward the next solution — was still running. It would probably always be running. He didn't think you could turn it off. But for the first time since he'd come to Hogwarts, it was reaching a conclusion he hadn't expected.

The system was already optimized. Not by him. Not for him. By someone so far beyond him that the comparison wasn't even meaningful, and then by centuries of people who'd learned, through suffering, which parts to leave alone. The Weasleys' kitchen was the output. The pies were the output. Ron, asleep on the couch, content in a way Harry had never been — Ron was the output.

The fire crackled. Ron shifted in his sleep and murmured something about Quidditch.

Harry put the journal on the table. He sat down in the chair across from his friend. He didn't pick up a book. He didn't start planning. He didn't optimize anything.

He just sat there, in the warmth, and let it be enough.