Eliezer Yudkowsky - Harry Potter and the Methods of Rationality

Or if you were a perfect Occlumens , you could race ahead of any probes, answering queries as fast as they were asked, so that the Legilimens would enter through your surfaces and see a mind indistinguishable from whoever you were pretending to be.

Even the best Legilimens could be fooled that way. If a perfect Occlumens claimed they were dropping their Occlumency barriers, there was no way to know if they were lying. Worse, you might not know you were dealing with a perfect Occlumens. They were rare, but the fact that they existed meant you couldn't trust Legilimency on anyone.

It was a sad commentary on how little human beings understood each other, how little any wizard comprehended the depths lying beneath the mind's surface, that you could fool the best human telepaths by pretending to be someone else.

But then human beings only understood each other in the first place by pretending. You didn't make predictions about people by modeling the hundred trillion synapses in their brain as separate objects. Ask the best social manipulator on Earth to build you an Artificial Intelligence from scratch, and they'd just give you a dumb look. You predicted people by telling your brain to act like theirs. You put yourself in their place . If you wanted to know what an angry person would do, you activated your own brain's anger circuitry, and whatever that circuitry output, that was your prediction. What did the neural circuitry for anger actually look like inside? Who knew? The best social manipulator on Earth might not know what neurons were , and neither might the best Legilimens.

Anything a Legilimens could understand , an Occlumens could pretend to be. It was the same trick either way - probably implemented by the same neural circuitry in both cases, a single set of control circuits for reconfiguring your own brain to act as a model of someone else's.

And so the race between telepathic offense and telepathic defense had been a decisive win for defense. Otherwise the entire magical world, maybe even the whole Earth, would have been a very different place...

Harry took a deep breath, and concentrated. There was a slight smile on his face.

For once, just once, Harry hadn't gotten shortchanged in the mysterious powers department.

After almost a month of work, and more on a whim than any real hunch, Harry had decided to make himself coldly angry and then try the book's Occlumency exercises again. At that point he'd mostly given up hope on that sort of thing, but it had still seemed worth a quick try -

He'd run through all the book's hardest exercises in two hours, and the next day he'd gone and told Professor Quirrell he was ready.

His dark side, it had turned out, was very, very good at pretending to be other people.

Harry thought of his standard trigger, from the first time he'd gone over entirely to his dark side...

Severus paused, looking quite pleased with himself. "And that will be... five points? No, let us make it an even ten points from Ravenclaw for backchat."

Harry's smile grew chillier, and he regarded the black-robed man who thought he was going to read Harry's mind.

And then Harry turned into someone else entirely, someone who had seemed appropriate to the occasion.

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

Kimball Kinnison regarded the black-robed man who thought he was going to read the mind of a Second-Stage Lensman of the Galactic Patrol.

To say that Kimball Kinnison was confident of the outcome would be an understatement. He had been trained by Mentor of Arisia, the most powerful mind known to this or any other universe, and the mere wizard sitting across from him would see precisely what the Gray Lensman wanted him to see...

...the mind of the boy he was currently disguised as, an innocent child named Harry Potter.

"I'm ready," said Kimball Kinnison in nervous tones that were exactly appropriate for an eleven-year-old boy.

" Legilimens, " said the black-robed wizard.

There was a pause.

The black-robed wizard blinked, as if he'd seen something so shocking that it had been enough to make even his eyelids move. His voice wasn't quite toneless as he said, "The Boy-Who-Lived has a mysterious dark side? "

The heat slowly crept up into Harry's cheeks.

"Well," the man said. His face had now settled back into perfect calm. "Excuse me. Mr. Potter, it is good to know your advantages, but that is not the same as being wildly overconfident in them. You may indeed be able to learn Occlumency at eleven years of age. This astounds me. I had thought Mr. Dumbledore was pretending to be insane again. Your dissociative talent is so strong that I am surprised to find no other signs of childhood abuse, and you may become a perfect Occlumens in time. But there is a considerable difference between that and expecting to put up a successful Occlumency barrier on your first attempt. That is merely ridiculous. Did you feel anything as I read your mind?"

Harry shook his head, now blushing furiously.

"Then pay closer attention next time. The goal is not to create a perfect image on your first day of lessons. The goal is to learn where your surfaces are. Prepare yourself."

Harry tried to pretend to be Kimball Kinnison again, tried to pay more attention, but his thoughts were a little scattered and he was suddenly aware of all the things he shouldn't be thinking about...

Oh, this was going to suck.

Harry gritted his teeth. At least the instructor would be Obliviated afterward.

" Legilimens. "

There was a pause -

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

It was their fourth day, on a Sunday evening. When you paid this much, you got your sessions any darned time you wanted, never mind the concept of weekends.

"Hello, Mr. Potter," the telepath said tonelessly, having cast the full suite of privacy spells.

"Hello, Mr. Bester," Harry said wearily. "Let's just get the initial shock out of the way, shall we?"

"You managed to surprise me?" the man said, now sounding slightly interested. "Well then." He pointed his wand and stared into Harry's eyes. " Legilimens. "

There was a pause, and then the black-robed wizard jerked as if someone had touched him with a cattle prod.

"The Dark Lord is alive? " he choked. His eyes were suddenly wild. " Dumbledore turns himself invisible and sneaks into girls' dorm rooms? "

Harry sighed and looked down at his watch. In about another three seconds...

"So," the man said. He hadn't quite recovered his tonelessness. "You genuinely believe you're going to discover the secret rules of magic and become all-powerful."

"That's right," Harry said evenly, still looking at his watch. "I'm that overconfident."

"I wonder. It seems the Sorting Hat thinks you'll be the next Dark Lord."

"And you know I'm trying pretty hard not to be, and you saw that we already had a long discussion about whether you were willing to teach me Occlumency, and in the end you decided to do it, so can we just get this over with?"

"All right," said the man exactly six seconds later, same as last time. "Prepare yourself." He paused, and then said, his voice rather wistful, "Though I do wish I could remember that trick with the gold and silver."

Harry was finding himself very disturbed by how reproducible human thoughts were when you reset people back to the same initial conditions and exposed them to the same stimuli. It was dispelling illusions that a good reductionist wasn't supposed to have in the first place.