Eliezer Yudkowsky - Harry Potter and the Methods of Rationality

He'd sustained that Transfiguration for seventeen days, and would now need to start over.

Could've been worse. He could've done this fourteen days later, after Professor McGonagall had approved him to Transfigure his father's rock. That was one very good lesson to learn the easy way.

Note to self: Always remove ring from finger before completely exhausting magic.

Harry pushed himself up, making rather hard going of it. Using up your magic didn't exhaust your muscles, but dodging around trees certainly did.

He staggered over to the iridescent hemisphere that contained Draco Malfoy, who was holding his wand aloft to sustain the shield, and smiling coldly at Harry.

"Where's the fifth soldier?" said Harry.

"Um..." said a boy whose name Harry couldn't remember at the moment. "I fired a Sleep Hex at the shield and it bounced off and hit Lavender, I mean the angle shouldn't have been right but it did..."

Draco was smirking inside the shield.

"So let me guess," Harry said, looking Draco directly in the eyes, "those neat little trios are the formation used by professional magical militaries? Made up of trained soldiers who can easily hit moving targets if their own hands are steady, and who can combine their defensive powers so long as they stay together? Unlike your soldiers?"

The smirk had vanished from Draco's face, which was now hard and grim.

"You know," Harry said lightly, knowing that none of the others would understand the real message passing between them, "it just goes to show that you should always question everything you see your role models doing, and ask why it's being done, and whether it makes sense in context for you to do it too. Don't forget to apply that advice to real life, by the way. And thanks for the slow-moving clustered targets."

Because Draco had already gotten that lecture, and, Harry suspected, discounted it out of suspicion that Harry was trying to shift his loyalties further away from pureblood tradition. Which of course Harry was. But this example would make an excellent excuse, next Saturday, to claim that questioning authority was a merely practical technique for real life. And Harry would also mention the experiments he'd run, first with individuals and then with groups, to check that his ideas about the importance of speed had actually been correct, by way of hammering home the point of Draco needing to keep an eye out at all times for chances to apply the methods in everyday practice.

"You haven't won yet, General Potter!" snarled Draco. "Maybe we'll run out of time, and Professor Quirrell will call it a draw."

A fair and worrisome point. The war only ended when Professor Quirrell, in his personal judgment, decided one army had won by practical real-world standards. There was no formal victory condition, Professor Quirrell had explained, because then Harry would figure out how to game the rules. Harry had to admit this was a fair cop.

And Harry couldn't blame Professor Quirrell for not calling an end, because it was plausible that the last soldier of Dragon Army could take out all five survivors of the Chaos Legion.

"All right," Harry said. "Does anyone know anything about General Malfoy's shield spell?"

It developed that Draco's shield was a version of the standard Protego which had several disadvantages, the most important of which was that the shield couldn't move with the caster.

The upside - or from Harry's perspective, downside - was that it was easier to learn, easier to cast, and much easier to sustain for long times.

They would need to hammer the shield with attack spells in order to bring it down.

And Draco could apparently exert some control over the angle of reflection at which the spells would bounce off.

The thought occurred to Harry that they could use Wingardium Leviosa to pile up heavy rocks on the shield until Draco couldn't sustain it against the pressure... but then the rocks might fall in afterward and hit Draco, and injuring the enemy general for real was not among today's goals.

"So," said Harry. "Are there such things as specialized shield-piercing spells?"

There were.

Harry asked if any of his soldiers knew them.

No one did.

Draco was smirking again, inside his shield.

Harry asked if there was any sort of attack spell that wouldn't bounce.

Lightning bolts, it seemed, were usually absorbed by shields instead of bouncing off them.

...No one knew how to cast any sort of lightning-related spell.

Draco sniggered.

Harry sighed.

He quite deliberately laid his wand on the ground.

And Harry announced, with some weariness in his voice, that he would just go ahead and take down the shield himself, using some method that would remain mysterious; and everyone else was to fire on Draco as soon as his shield went down.

The Chaos Legionnaires looked nervous.

Draco looked calm, which was to say, controlled.

A thin, folded blanket came out of Harry's pouch.

Harry sat down next to the shimmering shield, and pulled the blanket over his head so no one could see what he did - except Draco, of course.

From Harry's pouch came a car battery and a set of jumper cables.

...it wasn't like he'd been about to leave the Muggle world to start a new era of magical research, and not take along any way of generating electricity.

Shortly after, the Chaos Legionnaires heard the sound of fingers snapping, followed by a crackling noise from beneath the blanket. The shield started glowing more brightly, and Harry's voice said, "Don't be distracted please, eyes on General Malfoy."

The strain was showing on Draco's face, along with the fury and annoyance and frustration.

Harry smiled up at him, and mouthed, Tell you later.

And that was when a spiral of green energy shot out of the forest and smashed into Draco's shield, which shrieked like pieces of sharp glass being rubbed together, and Draco staggered.

In sudden, frantic panic, Harry took the jumper cables off the battery and fed them into the pouch, then he fed the battery itself into the pouch, and then he tore off the blanket and grabbed his wand and stood up.

All of his soldiers were still there and glancing around frantically.

" Contego, " Harry said, and his soldiers followed suit, but Harry didn't even know which direction the shield ought to be pointing in. "Did anyone see where that came from?" Shaken heads. "And General Malfoy, would you mind telling me if you got General Granger?"

"Why yes," Draco said acidly, "I mind."

Oh, hell.

Harry's mind began calculating, Draco inside the shield, Draco worn out now to some degree, Harry worn out too, Hermione in the woods who-knew-where, Harry and four other Chaotics left...

"You know, General Granger," Harry said out loud, "you really should've waited to attack until after I'd fought General Malfoy. You might've been able to get all the survivors."

From somewhere came a girl's high-pitched laughter.

Harry froze.

That wasn't Hermione.

And that was when the dreadful, eerie, cheerful chant began to rise, coming from all around them.

" Don't be frightened, don't be sad,

We'll only hurt you if you're bad... "

" Granger cheated! " burst out Draco inside the shield. "She woke up her soldiers! Why doesn't Professor Quirrell -"

"Let me guess," Harry said, the sickness already churning in his stomach. He really hated losing. "It was a very easy battle, right? They dropped like flies?"

"Yes," Draco said. "We got them all on the first shot -"

The look of horrified realization spread from Draco to the Chaos Legionnaires.

"No," Harry said, "we didn't."