Eliezer Yudkowsky - Harry Potter and the Methods of Rationality

For an instant Harry considered sacrificing Bellatrix to save himself -

And in that moment, some of the dim orange gas-light seemed to flee the corridor, a touch of cold crept over Harry's fingertips. And he knew, then, that to think of leaving Bellatrix to the shadows of Death, would make him vulnerable once more. Even in the moment of making the decision, he might become unable to cast the Patronus Charm, for he would have given up the thought that had saved him before.

It occurred to Harry that he could still take the Cloak from Bellatrix afterward, even if he couldn't cast the Patronus Charm; and then he had to wrench his thoughts away from that option, focus firmly on his decision not to do it, or he might have just fallen over where he stood. For the whirlpool of emptiness swirling around him was now deadly strong; there were screams coming from above , and the screams below had stopped.

This is ridiculous, said his logical side. Rational agents shouldn't have to put up with this sort of censored reasoning process, all the theorems assume that how you think doesn't affect reality apart from your actual actions, which is why you're free to choose an optimal algorithm without worrying about how your thoughts interact with Dementors -

...

That is a really dumb idea, said Gryffindor. Even I think it's a dumb idea and I'm your Gryffindor side. You're not seriously going to just stand there and -

"We have a fix!" shouted Ora, holding up her magic mirror as though in triumph. "The Dementor outside the inner wall pointed to level seven, C spiral, that's where they are!"

Her Aurors were looking at her expectantly.

"No," Amelia said in a level voice. "That's where one of them is. The Dementors still can't find Bellatrix Black. We are not running down there and letting her through in the confusion, and we are not dividing our forces to be ambushed. So long as we move with caution, we can't lose. Tell Scrimgeour and Shacklebolt to keep going down level by level, same as before -"

The old wizard was already striding forward. Amelia didn't even bother cursing him, this time, as once again their carefully constructed shields parted like water and rippled gently in his wake.

Harry waited at the beginning of the corridor, just next to the stairs leading upward. Bellatrix and the snake were behind him, concealed by the Deathly Hallow that Harry had mastered; he knew, though he could not see, that the emaciated sorceress was sitting upon the stairs, slumped back, since Harry had withdrawn his Hover Charm to free up his mind and magic.

Harry's eyes were fixed on the far end of the corridor, next to the stairs that led downward. Not in his mind now, but in true reality, the light in the corridor had dimmed, the temperature had fallen. The fear thundered over him and around him like a sea whipped by hurricane winds, and the sucking emptiness had become a howling draw toward some approaching black hole.

Up the stairs at the far end, floating smoothly through the dying air, came the voids, the absences, the wounds in the world.

And Harry expected them to stop.

With all the will and focus he could muster, Harry expected them to stop.

Anticipated their stopping.

Believed they would stop.

...that was the idea, anyway...

Harry shut down the dangerous stray thought, and expected the Dementors to halt. They had no intelligence of their own, they were just wounds in the world, their form and structure was borrowed from others' expectations. People had been able to negotiate with them, offer them victims in exchange for cooperation, only because they believed Dementors would bargain. So if Harry believed hard enough that the voids would turn and go, they would turn and go.

But the wounds in the world kept coming, the swirling fear seemed like a solid thing now, the emptiness tearing at matter as well as mind, substance as well as spirit, you could see the metal beginning to tarnish as the holes in the world passed.

A small sound came from behind him, from Bellatrix, but she said no word, for she had been instructed to remain silent.

Don't think of them as creatures, think of them as psychosensitive objects, they can be controlled if I can control myself -

The problem was that he couldn't control himself so easily, couldn't make himself believe blue was green by an act of will. Couldn't suppress all those thoughts about how irrational it was to make yourself believe something. How impossible it was to trick yourself into believing something if you knew that was what you were doing. All the training Harry had given himself against self-deception was refusing to switch off no matter how harmful it was in this unique special case -

The shadows of Death crossed the halfway point of the corridor, and Harry held up his hand, fingers spread, and said in a voice of firm and confident command, "Stop."

The shadows of Death stopped.

Behind Harry, Bellatrix gave a strangled gasp, like it was being torn out of her.

Harry gestured to her, the signal he had set up in advance which meant, repeat what you heard the Dementors say.

"They say," Bellatrix said, her voice was shaking, "they said, 'Bellatrix Black was promised us. Tell us where she hides, and you will be spared.'"

"Bellatrix?" Harry said, making his voice sound amused. "She escaped a while ago."

A moment later, Harry realized that he should have said that Bellatrix was among the Aurors in the top level, that would have caused more confusion -

No, it was wrong to think of the Dementors as trickable, they were merely things, they were controlled only by expectations -

"They say," Bellatrix said in a cracked voice, "they say they know you're lying."

The voids began to move forward again.

Her anticipations are more solidly believed than mine; she is controlling them, unwittingly -

"Don't resist," Harry said, pointing his wand behind him.

"I, I love you, farewell, my Lord -"

" Somnium. "

It had helped, strangely enough, hearing those particular awful words, understanding Bellatrix's mistake; it reminded Harry why he was fighting.

"Stop," Harry said again. Bellatrix was asleep; now only his own will, his own expectations rather, should control those spheres of annihilation -

But they kept on gliding forward, and Harry couldn't stop himself from worrying that the previous experience had damaged his confidence, which meant that he wouldn't be able to stop them, and as he noticed himself thinking that, he doubted even more - he needed more time to prepare, really ought to practice controlling just one Dementor in a cage first -

There was only a quarter of corridor now between Harry and the shadows of death, the empty winds were so strong that Harry could feel the erosion beginning in the cracks of himself.

And the thought came to Harry that maybe he was wrong, maybe Dementors did have their own desires and planning capability. Or maybe they were controlled by how everyone thought they worked, not just whoever was closest to them. And in either case -

Harry drew up his wand into the starting position for the Patronus Charm, and spoke.

"One of your number went to Hogwarts and did not return. It no longer exists; that Death is dead."

The Dementors halted, a dozen wounds in the world stood motionless, while the emptiness screamed around them like a deadly wind to nowhere.

"Turn and go and do not speak of this to anyone, little shadows, or I will destroy you as well."

Harry's fingers slid into the starting position for the Patronus Charm, and readied himself to cast it; in his mind, the Earth shone among the stars, the day side bright and blue with reflected sunlight, the night side glimmering with the light of human cities. Harry wasn't bluffing, wasn't trying to do anything tricky with his thoughts. The shadows of Death would move forward and be annihilated, or they would depart, he was equally ready for either...