Eliezer Yudkowsky - Harry Potter and the Methods of Rationality

It didn't take long after that.

It was a wide portrait, but the three people in it were looking rather crowded. There was a middle-aged man from the twelfth century, dressed in black swathes of cloth; who spoke to a sad-looking young woman from the fourteenth century, with hair that seemed to constantly frizz about her head as if she'd been charged up by a static spell; and she spoke to a dignified, wizened old man from the seventeenth century with a solid gold bowtie; and him they could understand.

They had asked about Dementors.

They had asked about phoenixes.

They had asked about dragons and trolls and house elves.

Harry had frowned, pointed out that creatures which needed the most magic could just be dying out entirely, and had asked for the most powerful magical creatures known.

There wasn't anything unfamiliar on the list, except for a species of Dark creature called mind flayers which the translator noted had finally been exterminated by Harold Shea, and those didn't sound half as scary as Dementors.

Magical creatures were as powerful now as they'd ever been, apparently.

The sickness in Draco's stomach was easing, and now he just felt confused.

"Harry," Draco said in the middle of the old man translating a list of all eleven powers of a beholder's eyes, "what does this mean?"

Harry held up a finger and the old man finished the list.

Then Harry thanked all the portraits for helping - Draco, pretty much on automatic, did so as well and more graciously - and they headed back to the classroom.

And Harry brought out the original parchment with the hypotheses, and began scribbling.

Observation:

Wizardry isn't as powerful now as it was when Hogwarts was founded.

Hypotheses:

1. Magic itself is fading.

2. Wizards are interbreeding with Muggles and Squibs.

3. Knowledge to cast powerful spells is being lost.

4. Wizards are eating the wrong foods as children, or something else besides blood is making them grow up weaker.

5. Muggle technology is interfering with magic. (Since 800 years ago?)

6. Stronger wizards are having fewer children. (Draco = only child? Check if 3 powerful wizards, Quirrell / Dumbledore / Dark Lord, had any children.)

Tests:

A. Are there spells we know but can't cast (1 or 2) or are the lost spells no longer known (3)? Result: Inconclusive due to Interdict of Merlin. No known uncastable spell, but could simply have not been passed on.

B. Did ancient first-year students cast the same sort of spells, with the same power, as now? (Weak evidence for 1 over 2, but blood could also be losing powerful wizardry only.) Result: Same level of first-year spells then as now.

C. Additional test that distinguishes 1 and 2 using scientific knowledge of blood, will explain later. Result: There's only one place in the recipe that makes you a wizard, and either you have two papers saying 'magic' or you don't.

D. Are magical creatures losing their powers? Distinguishes 1 from (2 or 3). Result: Magical creatures seem to be as strong as they ever were.

"A failed," said Harry Potter. "B is weak evidence for 1 over 2. C falsifies 2. D falsifies 1. 4 was unlikely and B argues against 4 as well. 5 was unlikely and D argues against it. 6 is falsified along with 2. That leaves 3. Interdict of Merlin or not, I didn't actually find any known spell that couldn't be cast. So when you add it all up, it looks like knowledge is being lost."

And the trap snapped shut.

As soon as the panic went away, as soon as Draco understood that magic wasn't fading out, it took all of five seconds to realize.

Draco shoved himself away from the desk and stood up so hard that his chair skittered with a scraping noise across the floor and fell over.

"So it was all just a stupid trick, then."

Harry Potter stared at him for a moment, still sitting. When he spoke, his voice was quiet. "It was a fair test, Draco. If it had come out a different way, I would have accepted it. That's not something I would ever cheat on. Ever. I didn't look at your data before I made my predictions. I told you up front when the Interdict of Merlin invalidated the first experiment -"

"Oh," Draco said, the anger starting to come out into his voice, "you didn't know how the whole thing was going to come out?"

"I didn't know anything you didn't know," Harry said, still quietly. "I admit that I suspected. Hermione Granger was too powerful, she should have been barely magical and she wasn't, how can a Muggleborn be the best spellcaster in Hogwarts? And she's getting the best grades on her essays too, it's too much coincidence for one girl to be the strongest magically and academically unless there's a single cause. Hermione Granger's existence pointed to there being only one thing that makes you a wizard, something you either have or you don't, and the power differences coming from how much we know and how much we practice. And there weren't different classes for purebloods and Muggleborns, and so on. There were too many ways the world didn't look the way it would look if you were right. But Draco, I didn't see anything you couldn't see too. I didn't perform any tests I didn't tell you about. I didn't cheat, Draco. I wanted us to work out the answer together. And I never thought that magic might be fading out of the world until you said it. It was a scary idea for me, too."

"Whatever," Draco said. He was working very hard to control his voice and not just start screaming at Harry. "You claim you're not going to run off and tell anyone else about this."

"Not without consulting you first," Harry said. He opened his hands in a pleading gesture. "Draco, I'm being as nice as I can but the world turned out to just not be that way ."

"Fine. Then you and I are through. I'm going to just walk away and forget any of this ever happened."

Draco spun around, feeling the burning sensation in his throat, the sense of betrayal, and that was when he realized he really had liked Harry Potter, and that thought didn't slow him down for a moment as he strode toward the classroom door.

And Harry Potter's voice came, now louder, and worried:

"Draco... you can't forget. Don't you understand? That was your sacrifice."

Draco stopped in midstride and turned around. " What are you talking about?"

But there was already a freezing coldness in Draco's spine.

He knew even before Harry Potter said it.

"To become a scientist. You questioned one of your beliefs, not just a small belief but something that had great significance to you. You did experiments, gathered data, and the outcome proved the belief was wrong. You saw the results and understood what they meant." Harry Potter's voice was faltering. "Remember, Draco, you can't sacrifice a true belief that way, because the experiments will confirm it instead of falsifying it. Your sacrifice to become a scientist was your false belief that wizard blood was mixing and getting weaker."

" That's not true! " said Draco. "I didn't sacrifice the belief. I still believe that!" His voice was getting louder, and the chill was getting worse.

Harry Potter shook his head. His voice came in a whisper. "Draco... I'm sorry, Draco, you don't believe it, not anymore." Harry's voice rose again. "I'll prove it to you. Imagine that someone tells you they're keeping a dragon in their house. You tell them you want to see it. They say it's an invisible dragon. You say fine, you'll listen to it move. They say it's an inaudible dragon. You say you'll throw some cooking flour into the air and see the outline of the dragon. They say the dragon is permeable to flour. And the telling thing is that they know, in advance, exactly which experimental results they'll have to explain away. They know everything will come out the way it does if there's no dragon, they know in advance just which excuses they'll have to make. So maybe they say there's a dragon. Maybe they believe they believe there's a dragon, it's called belief-in-belief. But they don't actually believe it. You can be mistaken about what you believe, most people never realize there's a difference between believing something and thinking it's good to believe it." Harry Potter had risen from the desk now, and taken a few steps toward Draco. "And Draco, you don't believe any more in blood purism, I'll show you that you don't. If blood purism is true, then Hermione Granger doesn't make sense, so what could explain her? Maybe she's a wizarding orphan raised by Muggles, just like I was? I could go to Granger and ask to see pictures of her parents, to see if she looks like them. Would you expect her to look different? Should we go perform that test?"