Harry Potter and the Methods of Rationality

can't do a police investigation in one day -"

The Potions Master raised his voice. "This is not Muggle Britain, Mr. Potter!" Severus's face was as expressionless as ever, but the bite in his voice was sharp. "The Aurors have an accusation under Veritaserum and a confession under Veritaserum. So far as they are concerned, the investigation is done."

"Not quite," said Dumbledore, just as Harry seemed ready to explode. "I have insisted to Amelia that this matter be given the utmost scrutiny. Unfortunately, as the ill-fated duel was at midnight -"

"Supposed duel," Harry said sharply.

"As the supposed duel was at midnight - yes, you're quite right, Harry - it is beyond the range of any Time-Turner -"

"Also supposedly," the Boy-Who-Lived said coldly. "And rather suspiciously, since the alleged murder suspect doesn't know about Time-Turners. I hope that an invisible Auror was immediately sent back in time as far as possible to observe -"

Dumbledore inclined his head. "I went myself, Harry, the moment I heard. But by the time I reached the trophy room, Mr. Malfoy was already unconscious and Miss Granger had gone -"

"No," said Harry Potter. "You reached the trophy room and saw Draco unconscious. That is all you observed, Headmaster. You did not observe Hermione there, or watch her leave. Let us distinguish observation from inference." The boy's head turned to look at her. "Imperius, Obliviation, False Memory Charm, Legilimency. Professor McGonagall, am I leaving out any mind-affecting spell that could have made Hermione do this or make her believe she'd done it?"

"The Confundus Charm," she said. And the Dark Arts had never been her study, but she knew - "And certain Dark rituals. But none of those could be performed in Hogwarts without alarm."

The boy nodded, his eyes still directly addressing her. "Which of those spells can be detected? Which would the Aurors try to detect?"

"The Confundus Charm would wear off in a few hours," she said, after a moment to gather her thoughts. "Miss Granger would remember the Imperius. Obliviation cannot be detected by any known means, but only a Professor could have cast that spell upon a student without alarm from the Hogwarts wards. Legilimency - can only be detected by another Legilimens, I think -"

"I requested that Miss Granger be examined by the court Legilimens," said Dumbledore. "The examination showed -"

"Do we trust him?" said Harry.

"Her," said Dumbledore. "Sophie McJorgenson, whom I remember as an honest student of Ravenclaw, and she is bound by the Unbreakable Vow to tell the truth of what she sees -"

"Could someone else be Polyjuiced as her?" Harry Potter interrupted again. "What did you observe, Headmaster?"

Albus said heavily, "A person who looked like Madam McJorgenson told us that a single Legilimens had lightly touched Miss Granger's mind some months ago. That is from January, Harry, when I communicated with Miss Granger about the matter of a certain Dementor. That was expected; but what I did not expect was the rest of what Sophie found." The old wizard turned to gaze into the Floo fire, letting the orange flames reflect on his face. "As you say, Harry, a False Memory Charm is one possibility; they are, when cast perfectly, indistinguishable from true memory -"

"That doesn't surprise me," Harry interrupted. "Studies show that human memories are more or less rewritten every time we remember them -"

"Harry," Minerva said softly, and the boy's mouth clamped shut.

The old wizard continued. "- but a False Memory Charm of such quality requires as much time to create as a true memory. Creating a detailed memory of ten minutes would be ten minutes' work. And according to the court Legilimens," Albus's face now seemed more tired and lined than before, "Miss Granger has been obsessing over Mr. Malfoy since the day that Severus... yelled at her. She has been thinking of how Mr. Malfoy might be in league with Professor Snape, how he might be planning to harm her and harm Harry - imagining it for hours every day - it would be impossible to create false memories for so much time."

"The appearance of insanity..." Severus murmured softly, as though he were speaking to himself. "Could it be natural? No, it is too disastrous to be pure accident; too convenient for someone, I have no doubt. A Muggle drug, perhaps? But that would not be enough - Miss Granger's madness would have to be guided -"

"Ah!" Harry said suddenly. "I get it now. The first False Memory Charm was cast on Hermione after Professor Snape yelled at her, and showed, say, Draco and Professor Snape plotting to kill her. Then last night that False Memory was removed by Obliviation, leaving behind the memories of her obsessing about Draco for no apparent reason, at the same time she and Draco were given false memories of the duel."

Minerva blinked in startlement. It would have been a thousand years before she thought of that possibility.

The Potions Master was frowning thoughtfully, eyes intent. "The reaction to a False Memory Charm is hard to predict in advance, Mr. Potter, without Legilimency. The subjects do not always act as expected, when they first remember the false memories. It would have been a risky ploy. But I suppose that is one way Professor Quirrell could have done it."

"Professor Quirrell?" said Harry. "What motive does he have to -"

The Potions Master said dryly, "The Defense Professor is always a suspect, Mr. Potter. You will notice a trend, given time."

Albus raised up a hand, a silencing gesture, and their heads all turned to look at him. "But in this case there is another suspect," Albus said quietly. "Voldemort."

That deadliest of unspeakable words seemed to echo around the room, canceling all the heat from the orange flames of the fireplace.