Harry Potter and the Methods of Rationality

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.

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