Eliezer Yudkowsky - Harry Potter and the Methods of Rationality

Harry shot Professor Quirrell a look of helpless indignation. You weren't supposed to do that with books, enchanted or not.

Harry opened the book with ingrained, instinctive care. The pages seemed too thick, with a texture unlike either Muggle paper or wizarding parchment. And the contents were...

...blank?

"Am I supposed to be seeing -"

"Look nearer the beginning," said Professor Quirrell, and Harry (again with that helpless, ingrained care) turned a block of pages back.

The lettering was obviously handwritten, and very hard to read, but Harry thought the words might be Latin.

"What is this?" said Harry.

"That," said Professor Quirrell, "is a record of the magical researches of a Muggleborn who never came to Hogwarts. He refused his letter, and conducted his own small investigations, which never did get very far without a wand. From the description on the placard, I expect that his name bears rather more significance to you than to me. That, Harry Potter, is the diary of Roger Bacon."

Harry almost fainted.

Nestled up against the wall, where Professor Quirrell had stumbled, glistened the crushed remains of a beautiful blue beetle.

It wasn't every day you got to see Harry Potter beg.

" Pleeaaase, " whined Harry Potter.

Fred and George shook their heads again, smiling.

There was an agonized look on Harry Potter's face. "But I told you how I did the one with Kevin Entwhistle's cat, and Hermione and the vanishing soda, and I can't tell you about the Sorting Hat or the Remembrall or Professor Snape..."

Fred and George shrugged and turned to leave.

"If you ever do figure it out," said the Weasley twins, "be sure to let us know."

" You're evil! You're both evil! "

Fred and George firmly closed the door to the empty classroom behind them, and made sure to keep the grin on their faces for a while, just in case Harry Potter could see through doors.

Then they turned a corner and their faces sagged.

"I don't suppose Harry's guesses -"

"- gave you any ideas?" they said to each other at the same time, and then their shoulders slumped further.

Their last relevant memory was of Flume refusing to help them, though they couldn't remember what they'd asked him to do...

...but they must have looked elsewhere and found someone to help them do something illegal, or they wouldn't have agreed to be Obliviated afterward.

How had they possibly been able to get all that done on just forty Galleons?

At first they'd worried that they'd forged evidence so good that Harry actually would end up married to Ginny... but they'd thought of that too, it seemed. The Wizengamot proceedings had been tampered with again to put them back the way they'd been originally, the fake betrothal contract had vanished from its dragon-guarded vault in Gringotts, and so on. It was pretty scary, actually. Most people now thought the Daily Prophet had just made the whole thing up for unguessable reasons, and the Quibbler had helpfully twisted the knife deeper with the next day's headline, HARRY POTTER SECRETLY BETROTHED TO LUNA LOVEGOOD.

Whoever they'd hired would tell them after the statute of limitations expired, they desperately hoped. But meanwhile it was awful, they'd pulled their greatest prank ever, maybe the greatest prank in the history of pranking, and they couldn't remember how. It was crazy, they'd been able to think of a way the first time, so why couldn't they see it now after knowing everything they'd done?

Their only consolation was that Harry didn't know they didn't know.

Not even Mum had questioned them about it, despite the obvious Weasley connection. Whatever had been done, it was far out of the reach of any Hogwarts student... except possibly one , who, if certain rumors were true, might have done it by snapping his fingers. Harry had been questioned under Veritaserum, he'd told them... with Dumbledore present and giving the Aurors scary looks. The Aurors had asked just enough to determine that Harry hadn't pulled the prank himself or disappeared anyone, and then gotten the heck out of Hogwarts.

Fred and George had wondered whether to feel insulted about Harry Potter being questioned by the Aurors for their prank, but the look on Harry's face, probably for exactly the same reason, made everything worth it.

Unsurprisingly, Rita Skeeter and the editor of the Daily Prophet had both vanished and were probably in another country by now. They would've liked to be able to tell their family about that part. Dad would have congratulated them, they thought, after Mum had finished killing them and Ginny had burned the remains.

But everything was still all right, they'd tell Dad someday, and meanwhile...

...meanwhile Dumbledore had happened to sneeze while passing them in the hallway, and a small package had accidentally dropped out of his pockets, and inside had been two matched wardbreaker's monocles of incredible quality. The Weasley twins had tested their new monocles on the "forbidden" third-floor corridor, making a quick trip to the magic mirror and back, and they hadn't been able to see all the detection webs clearly, but the monocles had shown a lot more than they'd seen the first time.

Of course they would have to be very careful never to get caught with the monocles in their possession, or they would end up in the Headmaster's office getting a stern lecture and maybe even threats of expulsion.

It was good to know that not everyone who got Sorted into Gryffindor grew up to be Professor McGonagall.

Harry was in a white room, windowless, featureless, sitting before a desk, facing an expressionless man in formal robes of solid black.

The room was screened against detection, and the man had performed exactly twenty-seven spells before saying so much as "Hello, Mr. Potter."

It was oddly appropriate that the man in black was about to try reading Harry's mind.

"Prepare yourself," the man said tonelessly.

A human mind, Harry's Occlumency book had said, was only exposed to a Legilimens along certain surfaces . If you failed to defend your surfaces, the Legilimens would go through and be able to access any part of you which their own mind was able to comprehend...

...which tended not to be much. Human minds, it seemed, were hard for humans to understand on any level but the shallowest. Harry had wondered if knowing lots of cognitive science could make him an incredibly powerful Legilimens, but repeated experience had finally driven into him the lesson that he needed to get a little less excited in his anticipations about this sort of thing. It wasn't as if any cognitive scientist understood humans well enough to make one.

To learn the counter, Occlumency, the first step was to imagine yourself to be a different person, pretending it as thoroughly as you could, immersing yourself entirely in that alternate persona. You wouldn't always have to do that, but in the beginning, it was how you learned where your surfaces were. The Legilimens would try to read you, and you would feel it happening if you paid close enough attention, you would sense them trying to enter. And your job was to make sure that they always touched your imaginary persona and not the real one.

When you were good enough at that, you could imagine being a very simple sort of person, pretend to be a rock, and make a habit of leaving the pretense in place where all your surfaces were. That was a standard Occlumency barrier. Pretending to be a rock was hard to learn, but easy to do afterward, and the exposed surface of a mind was much shallower than its interior, so with enough practice you could keep it up as a background habit.