Eliezer Yudkowsky - Harry Potter and the Methods of Rationality

Dumbledore took a soda can from Harry, and Harry tossed another to Professor Quirrell. The two older men each muttered identical charms over the can and frowned briefly at the result. Harry, for his part, simply popped the top and drank.

The Defense Professor and the Headmaster of Hogwarts politely followed suit.

Harry said, "I thought of my absolute rejection of death as the natural order."

It might not be the right kind of warm feeling you needed to cast a Patronus Charm, but it was going into Harry's Top 10 nonetheless.

The looks he got from the Defense Professor and the Headmaster briefly made Harry nervous, as the spilled Comed-Tea faded out of existence; but then the two of them each glanced at the other and both apparently decided that they couldn't get away with doing anything really awful to Harry in the other's presence.

"Mr. Potter," said Professor Quirrell, "even I know that is not how things are supposed to work."

"Indeed," said Dumbledore. "Explain."

Harry opened his mouth, and then, as realization hit him, rapidly snapped his mouth shut again. Godric hadn't told anyone, nor had Rowena if she'd known; there might have been any number of wizards who'd figured it out and kept their mouths shut. You couldn't forget if you knew that was what you were trying to do; once you realized how it worked, the animal form of the Patronus Charm would never work for you again - and most wizards didn't have the right upbringing to turn on Dementors and destroy them -

"Erm, sorry about this," said Harry. "But I've just this instant realized that explaining would be an incredibly bad idea until you work some things out on your own."

"Is that the truth, Harry?" Dumbledore said slowly. "Or are you just pretending to be wise -"

" Headmaster! " said Professor Quirrell, sounding genuinely shocked. "Mr. Potter has told you that this spell is not spoken of with those who cannot cast it! You do not press a wizard on such matters!"

"If I told you -" Harry began.

"No," Professor Quirrell said, sounding rather severe. "You don't tell us why, Mr. Potter, you simply tell us that we are not to know. If you wish to devise a hint, you do so carefully, at leisure, not in the midst of conversation."

Harry nodded.

"But," said the Headmaster. "But, but what am I to tell the Ministry? You can't just lose a Dementor!"

"Tell them I ate it," said Professor Quirrell, causing Harry to choke on the soda he had unthinkingly raised to his lips. "I don't mind. Shall we head on back, Mr. Potter?"

The two of them began to walk the dirt path back to Hogwarts, leaving behind Albus Dumbledore staring forlornly at the empty cage and the three sleeping Aurors awaiting their Memory Charms.

Aftermath, Harry Potter and Professor Quirrell:

They walked for a while before Professor Quirrell spoke, and all background noise dropped into silence when he did.

"You are exceptionally good at killing things, my student," said Professor Quirrell.

"Thank you," Harry said sincerely.

"I am not prying," said Professor Quirrell, "but on the off-chance that it was only the Headmaster who you did not trust with the secret...?"

Harry considered this. Professor Quirrell already couldn't cast the animal Patronus Charm.

But you couldn't untell a secret, and Harry was a fast enough learner to realize that he ought to at least think for a while before unleashing this one upon the world.

Harry shook his head, and Professor Quirrell nodded acceptance.

"Out of curiosity, Professor Quirrell," said Harry, "if your bringing the Dementor to Hogwarts had been part of an evil plot, what would have been its goal?"

"Assassinate Dumbledore while he was weakened," Professor Quirrell said without even hesitating. "Hm. The Headmaster told you he was suspicious of me?"

Harry said nothing for a second while he tried to think of a reply, and then gave up when he realized he'd already answered.

"Interesting..." Professor Quirrell said. "Mr. Potter, it is not out of the question that there was a plot at work today. Your wand ending up that close to the Dementor's cage could have been an accident. Or one of the Aurors could have been Imperiused, Confunded, or Legilimized to exert an influence. Flitwick and myself should not be excluded as suspects, in your calculation. One notes that Professor Snape canceled all his classes today, and I suspect he is powerful enough to Disillusion himself; the Aurors cast detection charms early on, but they did not repeat them immediately before your turn. But most easily of all, Mr. Potter, the deed could have been plotted by Dumbledore himself; and if he did , why, he might also take steps in advance to cast your suspicion elsewhere."

They walked on for a few steps.

"But why would he?" Harry said.

The Defense Professor stayed quiet a moment, and then said, "Mr. Potter, what steps have you taken to investigate the Headmaster's character?"

"Not many," said Harry. He'd only recently realized... "Not nearly enough."

"Then I will observe," said Professor Quirrell, "that you do not find out all there is to know about a man by asking only his friends."

Now it was Harry's turn to walk a few steps in silence on the slightly beaten dirt path that led back to Hogwarts. He'd really been supposed to know better than that already. Confirmation bias was the technical term; it meant, among other things, that when you chose your information sources, there was a notable tendency to choose information sources that agreed with your current opinions.

"Thank you," Harry said. "Actually... I didn't say it earlier, did I? Thank you for everything . If another Dementor ever threatens you, or for that matter, slightly annoys you, just let me know and I'll introduce it to Mister Glowy Person. I don't like it when Dementors slightly annoy my friends."

That got him an indecipherable glance from Professor Quirrell. "You destroyed the Dementor because it threatened me?"

"Erm," Harry said, "I'd sort of decided on it before then, but yes, that would have been sufficient reason by itself."

"I see," said Professor Quirrell. "And what would you have done about the threat to me if your spell hadn't worked for destroying the Dementor?"

"Plan B," said Harry. "Encase the Dementor in dense metal with a high melting point, probably tungsten, drop it into an active volcano, and hope it ends up inside Earth's mantle. Ah, the whole planet is filled with molten lava under its surface -"

"Yes," said Professor Quirrell. "I know." The Defense Professor was wearing a very odd smile. "I really should have thought of that myself, all things considered. Tell me, Mr. Potter, if you wanted to lose something where no one would ever find it again, where would you put it?"

Harry considered this question. "I suppose I shouldn't ask what you've found that needs losing -"

"Quite," said Professor Quirrell, as Harry had expected; and then, "Perhaps you will be told when you are older," which Harry hadn't.

"Well," said Harry, "besides trying to get it into the molten core of the planet, you could bury it in solid rock a kilometer underground in a randomly selected location - maybe teleport it in, if there's some way to do that blindly, or drill a hole and repair the hole afterward; the important thing would be not to leave any traces leading there, so it's just an anonymous cubic meter somewhere in the Earth's crust. You could drop it into the Mariana Trench, that's the deepest depth of ocean on the planet - or just pick some random other ocean trench, to make it less obvious. If you could make it buoyant and invisible, then you could throw it into the stratosphere. Or ideally you would launch it into space, with a cloak against detection, and a randomly fluctuating acceleration factor that would take it out of the Solar System. And afterward, of course, you'd Obliviate yourself, so even you didn't know exactly where it was."