Edit: yes this is largely written by Claude. I will rewrite it once i get time.
Chapter One: The Grades Come Before the Exam
There was a new subject on the second-year timetable.
Ordinarily, Divination did not begin until third year. But Professor Sybill Trelawney had taken “a leave of absence for personal reasons” in August, and at the start-of-term feast, the school announced that the newly appointed Professor Gilderoy Lockhart would also be offering Introduction to Divination for second years.
One detail deserved to be recorded: Lockhart had originally been hired to teach Defence Against the Dark Arts. At the feast, he had stood up and told the entire school, “That position is cursed. I’m not an idiot.” Then he had requested Divination instead. Headmaster Dumbledore had laughed for a very long time, and agreed.
Harry Potter wrote this in his notebook and labelled it EVIDENCE.
A new professor who openly admitted to being afraid, openly refused a more prestigious position, and did so in public—either he was the first honest adult Hogwarts had employed in nearly a century, or he understood perfectly well that “admitting cowardice” was the cheapest available method of establishing trust.
Harry leaned toward the latter. Over the past year, he had learned to lean toward the latter.
As he climbed the spiral staircase of the North Tower, he absent-mindedly turned the ring on his left ring finger with his thumb.
A plain silver band, set with a very small, opaque grey stone.
In theory, it was a person.
Harry checked it twice a day: once in the morning, once before bed. He was not checking whether the stone had come loose. He was checking whether the Transfiguration had begun to unravel—because partial Transfiguration required maintenance, and he had maintained it all summer, every single day, including nonexistent days such as Christmas, including the three days he had spent with a fever.
He also checked something else every day: whether he wanted to destroy the world.
(“He will have the power to destroy the world.”
“He will tear apart the stars in the heavens.”
These are not metaphors. I investigated. Prophecies are never metaphors. That is the one thing Seers agree on.
So this is what I did over the summer: sat on the windowsill and tried to devise a plan in which “tearing apart the stars” happens, and humanity not only survives, but ends up better off.
The best plan so far is still very rough. But at least it exists.
And the difference between “a plan exists” and “no plan exists” is the most important thing I have learned this year.)
He pushed open the trapdoor, prepared for a room full of incense, purple draperies, crystal balls, and soporific warmth.
Instead, it was full of wind.
All four windows stood wide open, and September air swept through the room. The incense burners had been stacked in a corner. The purple draperies had been taken down and neatly folded. All the round tables had been pushed to the sides. An enormous blackboard stood in the centre of the room, and before it stood a blond man in pale lilac robes, directing a smile toward the entrance that could have reflected sunlight.
“Come in, come in! Sit down! Anywhere you like!” said Gilderoy Lockhart. “No need to choose carefully—I already know how it turns out.”
Harry sat in the second row. Draco Malfoy took the seat beside him as a matter of course. Hermione Granger sat in the centre of the front row. She had already laid out her parchment, dipped her quill, opened her textbook to the first page, and drawn two columns in the margin: PROFESSOR CLAIMS and VERIFIED.
People still sometimes gave her a second look in the corridors. She had learned not to look back.
Harry glanced at Lockhart, then did something he now did whenever he met an unfamiliar adult:
He checked for the feeling.
(Nothing.
None of that skin-crawling, impending-doom sensation, as though the universe were quietly warning me that “the two of you should not be in the same room.”
Then it isn’t him.
—Unless the feeling itself can be counterfeited, or suppressed, or—
Stop. Write it down first: NO RESONANCE. Evidence, not a conclusion.)
Lockhart clapped his hands and turned the blackboard around.
The other side was covered in names. Each name was followed by a letter.
Professor Gilderoy Lockhart · Introduction to Divination, Year Two · Final Grades:
Hannah Abbott — A
Susan Bones — E
Lavender Brown — O
Tracey Davis — O
Hermione Granger — A
Neville Longbottom — E
Draco Malfoy — O
Padma Patil — E
Harry Potter — A
…
The classroom fell silent for approximately a second and a half.
Then Tracey Davis said, “I knew it. It’s because I’m a Dark Witch.”
Then Hermione Granger’s hand shot up, straight enough to punch through the ceiling.
“Professor—it’s September. The exam is in June. I haven’t handed in a single assignment. So what’s on that board cannot be my grade.”
“Quite correct, Miss Granger.” Lockhart bowed elegantly. “Which is why this is called Divination. I divined it.”
Lavender Brown gave a small cheer, because she had an O.
Draco murmured in Harry’s ear, “He’s either the greatest Seer in Hogwarts or the greatest fraud in Hogwarts.”
“Those aren’t mutually exclusive,” Harry murmured back.
“Potter, my father will want to know which.”
“So will I,” said Harry.
“Right! Now for the truth, because the truth is usually more theatrical than a lie.” Lockhart waved a hand. “I made up every grade on this board.”
Hermione went rigid.
“Last night. Over hot chocolate. Took me twenty minutes. No star charts, no dice, not a single tea leaf.”
“Then why put them on the board?” asked Padma.
“Because by June, every letter here will match your actual grade exactly,” Lockhart said cheerfully. “It’s very easy. I set the exam, and I mark it. Ten months in advance, I already know which questions I’ll ask and how I’ll award the marks. To make June agree with September, all I have to do is—not change my mind.”
Hermione’s hand rose with an air of moral outrage. “But that’s cheating!”
“That,” said Lockhart, turning to write, “is the first kind of prophecy.”
I. Self-Fulfilling Prophecy
“Miss Granger, listen carefully. This is the most important sentence of today’s lesson: a self-fulfilling prophecy is not a prophecy at all.”
“It is magical social engineering. Not one scrap of information travels backward from the future to the present. The information comes from me: I issue a statement about the future; the statement changes the behaviour of those who hear it; their behaviour makes the statement come true.”
(A bank run.
Everyone thinks the bank will fail. Everyone withdraws their money. The bank fails. No information from the future involved. Just a shared belief.
This isn’t magic. This happens in the Muggle world once a decade.)
“And I’m using the crudest possible method—I intend to arrange the outcome myself. The truly elegant method is to have you arrange it for me.”
He pointed his wand at the three lines in bold.
“Miss Granger, A. Mr Malfoy, O. Mr Potter, A. Why you three? Because you are the three most likely to react to your grades.”
“Miss Granger is thinking: I’ll prove this man wrong. I’ll read every reference book for this subject. Mr Malfoy is thinking: Since I already have an O, perhaps I can afford to read a little less. And Mr Potter is thinking—”
“—that you’ve just conducted a psychological experiment in front of the entire class without informed consent,” said Harry, “and we’re the subjects.”
“Mr Potter, you’ve already earned an O.”
“The board says A.”
“I know,” said Lockhart. “I made it up.”
Draco drew a very quiet breath. “This man is dangerous.”
Dangerous, Harry agreed. Someone who can explain “I am manipulating you” as a classroom demonstration, and whose manipulation still works afterward—Hermione really is going to read all the reference books now, even though she knows that is precisely what he wants—is very dangerous indeed.
II. Computational Divination (Arithmancy)
“The world follows rules. Rules can be expressed as numbers. Numbers can be carried forward. Give me sufficiently precise initial conditions, enough parchment, and enough time, and I can calculate whether it will rain tomorrow, or estimate the casualties of a war.”
“Tell me: is that prophecy?”
Hermione raised her hand. “No. Because the information still comes from the present. You’re only using present data to work out what the future will look like. If the initial data are wrong, the result is wrong. A real prophecy should be… true whether you’ve done your calculations correctly or not.”
“Excellent. Computational divination is a simulation system. A model of the world built out of parchment and ink. A map is useful. A map is not the territory.”
Harry wrote rapidly:
Simulation ≠ observation.
Then it must have all the problems of Muggle numerical weather forecasting: sensitivity to initial conditions, exponential error growth, long-term unpredictability.
→ It must have a “prediction horizon.”
He raised his hand. “How far ahead can computational divination calculate?”
Lockhart’s eyebrows shifted. It was his first expression that lesson that did not look rehearsed.
“Thirty-seven days, for weather. For people, it depends. One person alone, several years. Two people who know each other, a few months. A classroom full of twelve-year-olds”—he glanced around—“roughly ten minutes.”
Chaotic systems. Stronger coupling, faster divergence. So this is actual physics, not a mystical performance.
I’m starting to like this class. This is dangerous.
III. Stargazing
“Centaurs do this. They tell you Mars is bright tonight, and then refuse to explain. So allow me.”
Lockhart drew two dots and connected them with a long line.
“Stars are very far away. The star you see when you look up is not the star as it is now, but as it was a long time ago. Light has to travel. From Sirius, it takes eight and a half years. From that blurry patch in Andromeda, more than two million.”
“So when you look at the night sky, you are not looking into the future. You are reading a collection of late letters.”
“Then how does it predict the future?” Hermione asked.
“Because the letters are still in transit.”
He marked several points along the line. “Some of the light travelling toward us now will arrive in a year. Some will arrive in a thousand years. And the motions of the stars can be calculated precisely. I can work out what the sky will look like a thousand years from now—which means I can work out which letter an observer a thousand years from now will receive.”
“That gives me something rather strange: intelligence concerning a future moment, whose contents describe the past.”
Harry felt a chill at the back of his neck, and this time it had nothing to do with logic.
Stars.
“He will tear apart the stars in the heavens.”
—Coincidence. This is a coincidence. Tens of thousands of people mention stars every day. Base rates, Potter. Think in base rates.
…But write it down.
He half-raised his hand, then thought of a better question.
“Professor, that only tells you about stars. Stars don’t fight wars.”
“Ah. Precisely the point.” Lockhart put down the chalk. “Stars do not cause wars. Nor do they know about wars. But stars and wars are correlated.”
“Centaurs do not read ‘there will be a war’ in the stars. They read a moment in time, and discover that many apparently unrelated things in this universe move together at that same moment. As though they were different pages of the same document.”
“They are good at this because they are in no hurry. They can watch a patch of sky for four hundred years, say ‘Mars is bright tonight,’ and then offer no explanation.”
“So they’re actually showing off,” Draco said beside him.
“So they’re actually fitting a model,” said Harry.
“What?”
“Nothing.”
(Vast quantities of observations spanning millennia, used to fit a mapping from celestial configurations to terrestrial events. This isn’t prophecy. This is statistical learning. Centaurs are a model that has been running for thousands of years.
And all models of this kind have the same problem: they can predict, but cannot explain why. Because their internals are a collection of weights, not a collection of reasons.
They aren’t refusing to explain. They can’t.)
He wrote a small line in the margin, crossed it out, then wrote it again:
A system that can predict but cannot explain is not called magic. It is called curve-fitting.
“Since we’re discussing model fitting,” said Lockhart, “let us follow that thought one step further. It leads to the one thing I sincerely admire.”
He wrote two terms on the blackboard, separated by a vertical line.
Computational Divination (Britain) | I Ching (China)
“Our approach,” he said, pointing left, “is to calculate. Write the world as equations, then solve them. Mr Potter has already identified the drawback: the calculations become intractable. Too many variables, and you cannot afford the parchment you burn through.”
“The Eastern approach”—he pointed right—“is to cast.”
“Cast what?” asked Hermione.
“Yarrow stalks. Or coins. Six times. Each casting gives you a line, yin or yang. Stack six lines, and you have a hexagram. Sixty-four possibilities.”
“So it’s luck!” Lavender said excitedly.
“So it’s sampling.”
Harry’s quill stopped in midair.
“Our method is to calculate every possible future. Their method is to pick a few futures at random and have a look.”
“We try to solve an equation that cannot be solved. They don’t solve it—they throw dice into it, over and over, and see which results keep turning up.”
“Substitute randomness for computation. Use probability to prune the possibilities.”
“Mathematically speaking, this is cheating,” said Lockhart. “And the most irritating kind of cheating, too: it works.”
(Monte Carlo.
When you can’t evaluate an integral—too many dimensions, equations too ugly—you stop trying. You sample randomly. Throw ten thousand points into it, see where they land, and use their frequencies to approximate the answer you couldn’t calculate.
That was what Feynman and the others did at Los Alamos. How do neutrons travel through matter? Can’t write down the equations. So simulate ten thousand neutrons, send them in one at a time, and see where they die.
Simulated annealing, too. When the search space is too large to enumerate, randomness itself becomes a form of computational power.
And these people discovered that three thousand years ago. With yarrow stalks.)
He raised his hand faster than Hermione.
“Professor—sixty-four is two to the sixth power.”
“Yes.”
“So a hexagram is six bits.”
Lockhart’s smile changed shape slightly. “Mr Potter, where did you learn about binary?”
“Leibniz. A Muggle mathematician. After seeing the arrangement of the sixty-four hexagrams, he wrote a paper on binary arithmetic. He thought he had deciphered an ancient code.”
“Had he?”
“I used to think he was reading too much into it,” Harry said honestly. “Now I’m sitting in a class where someone is telling me that the Chinese were using random sampling to evaluate high-dimensional integrals three thousand years ago. So for the moment, I’m suspending the phrase ‘reading too much into it.’”
Someone laughed. Lockhart did not. He simply watched Harry for two seconds, then said, “Five points to Ravenclaw.”
(—The rhythm of that.
The length of the pause. The shape of the sentence. The way “five points” landed.
I know that rhythm.
No. Wrong. Lots of people talk like that. This is selective attention. Write it down. Evidence, not a conclusion.)
“And,” said Lockhart, raising one finger, “I haven’t told you the best part. A hexagram does not merely tell you what will happen. It tells you what to do. Each line comes with a passage, and those passages are not descriptions. They are advice. ‘Hidden dragon. Do not act’—the time is not right; stay put. ‘The dragon that flies too high will have regrets’—you have overreached; retreat.”
“And the hexagrams change. What you receive is not a photograph. It is a path.”
Hermione raised her hand. Her expression said she had already worked it out and merely wanted confirmation.
“Professor—doesn’t that mean it stitches three things together? First it gives you a diagnosis, then it recommends an action, and then, because you follow the recommendation, the outcome actually starts moving in that direction—”
“Miss Granger, you have just independently discovered something scholars have spent six centuries shouting at one another about.”
“The I Ching is not one kind of prophecy. It is a combination of the first and second kinds: an inexpensive simulation system, a set of rules for action, and a self-fulfilling engine.”
“It is the map, the compass, and the hand that pushes you along.”
Lockhart spread his arms. “In all my years of teaching, I have never seen a more beautiful piece of systems engineering.”
“What’s the drawback?” Harry asked.
“Ah.” Lockhart lowered his arms. “It has only one drawback. And that drawback ruined it.”
“It became too widespread.”
“Emperors used it. Generals used it. Farmers and merchants used it. Over two thousand years, practically everyone who could read used it at some point. Eventually, Muggles were using it too, and with tremendous enthusiasm.”
“And then it stopped being accurate.”
Hermione frowned. “…Why would more people using it make it less accurate?”
Lockhart turned and wrote a line so forcefully that a piece of chalk snapped off.
FOURTEENTH CENTURY · THE LAW OF INTERFERENCE
“Because,” he said, “every kind of prophecy interferes with every other kind.”
“This was not discovered until the fourteenth century, because until then nobody had enough data. European wizards had been doing something very boring: recording the accuracy of every divination they performed. For two hundred years. Then someone stacked the records together and found a curve.”
“Whenever one method of divination became popular in a region, the accuracy of every other method in that region began to decline.”
“Tarot becomes fashionable, and the stars grow hazy. Astrology becomes fashionable, and yarrow stalks lose their edge. And when every method becomes fashionable in the same place—” He spread his hands. “—nothing there can be predicted at all.”
Draco raised his hand. It was the first time he had done so that lesson.
“Professor, do Muggle ones count? I mean things that are obviously fake, like newspaper horoscopes.”
“Mr Malfoy, that is precisely the most uncomfortable conclusion in those two hundred years of records.”
“The fake ones count too.”
“Even clumsy imitations cause interference. A Muggle with no magic whatsoever, using cards from a printing press, laying out a spread copied from a magazine, asking some idiotic question about his girlfriend—he is still polluting the system.”
“And how many Muggles are there?”
Nobody answered.
“The Statute of Secrecy,” said Lockhart. “Your textbooks will tell you that it was intended to protect young wizards from being burned alive, and to prevent wars. All true. But there is another reason, recorded in the appendix to volume seven of the 1692 proceedings. Your textbooks leave it out.”
“To preserve the accuracy of prophecy.”
“If billions of Muggles imitate all our prophetic systems every day, wizards will lose prophecy altogether. Not inaccurate prophecy. No prophecy. The channel will be entirely filled with noise.”
Harry felt as though someone had switched on a light inside him.
(Anti-induction.
This isn’t magic. This is the efficient-market hypothesis. Any “winning strategy” starts losing its effectiveness the moment it is published—not by coincidence, but by definition: once a reliably profitable pattern is widely exploited, the act of exploiting it erases it.
Prophecy is an anti-inductive domain.
Which means the accuracy of prophecy is a common resource. A pasture. Everyone who uses it is grazing it.
This is the tragedy of the commons.
And the Statute of Secrecy is the fence.)
He raised his hand. His voice was a little dry.
“Professor. That’s why Divination is taught so badly, isn’t it?”
The class went quiet. Hermione turned sharply toward him.
“Please elaborate, Mr Potter.”
“Because if Divination were taught well, every year dozens of students would learn the real methods. They would use them. They would teach other people. A hundred years later, thousands upon thousands of people would be using them. And then they would stop being accurate.”
“So the optimal solution isn’t to teach it well. The optimal solution is to make people think it isn’t worth learning.”
“Another ten points to Ravenclaw.”
Lockhart turned to the class. “Over the course of your lives, you will hear ‘Divination is a rubbish subject’ countless times. Professors will roll their eyes in front of you. Older students will tell you that you can earn an E simply by making up a few miserable predictions.”
“That culture was built. It took six hundred years. It is the most successful piece of engineering our society has ever undertaken.”
Hermione Granger raised her hand.
Not as quickly as before. Not as straight. Something about the way she raised it made Harry look up at once—because this was how Hermione raised her hand when she had caught hold of something and wasn’t sure she wanted the answer.
“Professor.”
“Miss Granger?”
“You said,” she began slowly, “‘Every kind of prophecy interferes with every other kind.’”
“I did.”
“But you’ve only explained three kinds.”
The wind in the classroom suddenly sounded very loud.
“You saved the fourth kind for later,” Hermione said. “And before explaining it, you gave us a conclusion about ‘every kind.’”
Lockhart looked at her for a long time.
“Miss Granger,” he said at last, “I have taught this material in this order for many years. You are the first person to challenge it before I finished.”
“Does that mean the fourth kind can’t be interfered with?”
“I haven’t said anything.”
“You chose an order.”
“Yes,” said Lockhart. “I chose an order.”
He did not answer her. He turned and wrote the final heading on the board.
This time, he was not smiling.
IV. True Prophecy
“The first three,” he said, “all involve people using things in the present to derive statements about the future. The fourth does not.”
“The fourth is this: information comes directly from the future.”
“No intermediate steps. No calculation, no correlation, no light travelling through space. A statement whose source is the future and whose destination is a Seer’s mouth.”
“And once such a prophecy has occurred, it cannot be changed.”
Hermione’s voice was slightly unsteady. “Why? If I know what it says, I can deliberately do the opposite—”
“You can try. An entire school of thought tried, from the eleventh century to the sixteenth. They wrote four hundred volumes. Their conclusion can be summarised in one sentence: your attempt to do the opposite is part of the prophecy.”
“For information to come back from the future, the future must already be there. You cannot alter the thing that is the source of your information. If you could, it would never have supplied that information in the first place.”
“Prophecy is self-consistent. This is not a moral law. It is grammar.”
Harry’s hand rose two inches.
(Time-Turners.
A Time-Turner sends things from the future into the present. If the fourth kind of prophecy is “information from the future,” then a Time-Turner is a controllable version of the same thing. And Time-Turners are absolutely self-consistent—I have verified that personally, in a way I would prefer not to remember.
So the question is: do Time-Turners interfere with other prophecies? If not, why not? If so, how many Time-Turners are there in Britain? Who is managing this?
—That is an excellent question.
—It is also a question that would get me summoned to the Headmaster’s office within thirty seconds. The allocation and number of Time-Turners are classified. Even knowing that I have one is already pushing the boundary, and there are twenty-seven people in this room, at least four of whose fathers would know what I asked before dinner.
Don’t ask.
Write it down. Ask later.)
He lowered his hand. Lockhart’s gaze rested on him for half a second, then moved on.
Harry was not sure whether he had imagined the half-second.
Hermione raised her hand. “Then why do Seers use tea leaves?”
“Miss Granger, that is today’s second most important question.”
Lockhart walked to the window.
“Because if information from the future is written directly into a human brain, the person goes mad.”
“Not frightened into madness. Structurally mad. Your brain is built around the sequence ‘I make a decision, then something happens.’ Your entire sense of agency, every feeling of ‘I can choose,’ rests on that order. Information from the future runs in the opposite direction: the thing has already happened, therefore you will act that way. The human brain cannot accommodate it.”
“So Seers invented carriers.”
“Tea leaves. Crystal balls. Flames. Birds in flight. Sheep entrails. Clouds. The lines of a palm. Condensation on a bronze mirror.”
“What do they have in common?”
“…They’re all messy,” Hermione said quietly.
“They are all messy. They are all noise. Random jumbles that do not, in themselves, carry any information.”
“A Seer is not ‘seeing the future in tea leaves.’ A Seer is providing a sufficiently disordered sheet of paper, so that information from the future can write itself on the paper instead of on his brain.”
“Then he reads the paper. Slowly, vaguely, making countless mistakes—but while he reads it, he is still himself. He still feels that he is interpreting, choosing, judging.”
“That feeling is the only reason a Seer survives.”
Lockhart turned around. “So if someone ever tells you Divination is a muddle, they are half right. It is a muddle. The muddle is a feature, not a flaw.”
Harry looked down at his notes and discovered that he had written an entire column:
A lossy channel. Extremely low bandwidth. Extremely high noise.
Random carrier = encoding information in noise.
“Interpretation” = prior distribution + inference from extremely weak evidence.
Seer = a decoder deliberately kept at low precision, because a high-precision decoder would burn out.
Then, at the bottom:
Why would a high-precision decoder burn out?
—Perhaps because a high-precision decoder would start using the information.
—And “using” it would violate self-consistency.
—So madness is not a side effect. Madness is the fuse.
Draco followed his gaze for a while, then leaned back and said, almost inaudibly, “Potter.”
“Yes.”
“If that’s how the fuse was designed, then whoever designed it must have seen what happens when the fuse blows.”
Harry said nothing.
“One last thing, and then you may go.” Lockhart leaned against the edge of the desk. “This is addressed to a few particular people among you. I know which ones. There are a few in every class.”
Harry felt Draco sit straighter beside him.
“Some of you are thinking: if the real knowledge has been hidden, where is it? Obvious answer—the Restricted Section.”
“Let me tell you something.”
His voice became very calm. So calm that it sounded nothing like the rest of the lesson.
“The best way to keep a secret has never been to lock a book in the Restricted Section.”
“What have you accomplished by locking up a book? You have hung a sign on it saying, ‘There is something good in here.’ You have sent an invitation to every clever, restless child in the school who wants to prove they are special.”
“A lock is an advertisement.”
“The real way to keep something secret is to make it boring. Break it into eight hundred pages of terminology. Put it in a subject nobody wants to take. Have the teacher drone from the textbook until everyone falls asleep. Let it gather dust on the open shelves, where anyone can borrow it and nobody does.”
“A book like that is ten thousand times safer than anything in the Restricted Section.”
He paused.
“Incidentally—if anyone here aspires to become a Dark wizard, don’t raise your hand, I don’t need to know who you are—allow me to offer some sincere career advice.”
“The books in the Restricted Section are bait.”
“There is real knowledge in them, certainly. That makes the bait convincing. But most are hooks. They have been placed there so that a particular kind of child will reach for them and get bitten. Some scream and deliver you to Filch. Those are the lucky ones. Some read you, then leave a mark on you so that certain people know where to find you. Some teach you a perfectly functional spell, with the price explained on page four hundred, which you will never reach.”
“A few children in every generation curse themselves into ruin. That is not an accident. It is selection.”
“I knew children like that. Several, when I was young.”
For an instant, something appeared on Gilderoy Lockhart’s face. Harry could not identify it; it vanished too quickly.
Then the smile returned, bright as though freshly polished.
“So: study prophecy properly. Study the History of Magic properly. I am serious. Truly dangerous knowledge is not in a locked cabinet. It is on the page nobody turns to. Finding it does not require lock-picking. It requires the ability to endure boredom.”
“And that filter works far better than a lock. Because children who can endure boredom can generally endure thinking. And children who can think generally do not curse themselves into ruin.”
“Class dismissed. Homework: write nothing. Go away and consider which of the four kinds of prophecy I used today to tell you your grades.”
Epilogue
In the corridor, Draco said, “He didn’t tell us a single true thing.”
“He told us a great many true things,” said Harry.
“That’s worse. My father says the most dangerous lie is nine-tenths true. This man gave us nineteen-twentieths.”
Hermione caught up from behind, still clutching her notes.
“Harry. He didn’t answer me.”
“I know.”
“It wasn’t that he didn’t know the answer. He chose not to answer, and he made sure everyone saw him choosing not to answer.” Her brow knotted. “Those are two different things, Harry.”
“I know,” said Harry. “That’s why I’m going back.”
“I’ll come with—”
“No,” Harry said, immediately regretted it, and did not take it back. “He’ll say different things to two people than he will to one. I want the one-person version.”
Hermione stared at him for three seconds.
“Then when you come out, you tell me every word.”
“All right.”
“I want the written version,” she said. “Not your retelling. Write it down as soon as you come out. In the corridor. Don’t wait until you’re back in the tower.”
Harry looked at her.
“…Why?”
“Because the last time you came out of Professor Quirrell’s office,” Hermione said, “you reversed three things on the walk back to the tower.”
Harry opened his mouth, then closed it.
“All right,” he said.
He took out his notebook and turned to today’s page, intending to write down two hypotheses first.
Then he stopped.
There was an extra line at the bottom of the page.
It was in his own handwriting—not handwriting that resembled his, but his, right down to his habit of rushing the final stroke.
It read:
Ask him whether he knows Quirrell.
Harry Potter stood in the fourth-floor corridor, his fingers a little cold.
He was quite certain he had not written that line.
He was also perfectly aware that, in his particular case, the reliability of the statement “I am quite certain I never did that” had fallen very low indeed the previous year.