Harry Potter and the Methods of Rationality

"At least tell me there's not really a prophecy..." Professor McGonagall was still giving him that bright, fixed smile. "Oh, you have got to be kidding me."

"Mr. Potter, you shouldn't go inventing things to worry about -"

"Are you actually going to tell me that? Imagine my reaction later, when I find out that there was something to worry about after all."

Her fixed smile faltered.

Harry's shoulders slumped. "I have a whole world of magic to analyse. I do not have time for this."

Then both of them shut up, as a man in flowing orange robes appeared on the street and slowly passed them by; Professor McGonagall's eyes tracked him, unobtrusively. Harry's mouth was moving as he chewed hard on his lip, and someone watching closely would have noticed a tiny spot of blood appear.

When the orange-robed man had passed into the distance, Harry spoke again, in a low murmur. "Are you going to tell me the truth now, Professor McGonagall? And don't bother trying to wave it off, I'm not stupid."

"You're eleven years old, Mr. Potter!" she said in a harsh whisper.

"And therefore subhuman. Sorry... for a moment there, I forgot."

"These are dreadful and important matters! They are secret, Mr. Potter! It is a catastrophe that you, still a child, know even this much! You must not tell anyone, do you understand? Absolutely no one!"

As sometimes happened when Harry got sufficiently angry, his blood went cold, instead of hot, and a terrible dark clarity descended over his mind, mapping out possible tactics and assessing their consequences with iron realism.

Point out that you have a right to know: Failure. Eleven-year-old children do not have rights to know anything, in McGonagall's eyes.

Say that you will not be friends any more: Failure. She does not value your friendship sufficiently.

Point out that you will be in danger if you do not know: Failure. Plans have already been made based on your ignorance. The certain inconvenience of rethinking will seem far more unpalatable than the mere uncertain prospect of your coming to harm.

Justice and reason will both fail. You must either find something you have that she wants, or find something you can do which she fears...

Ah.

"Well then, Professor," Harry said in a low, icy tone, "it sounds like I have something you want. You can, if you like, tell me the truth, the whole truth, and in return I will keep your secrets. Or you can try to keep me ignorant so you can use me as a pawn, in which case I will owe you nothing."

McGonagall stopped short in the street. Her eyes blazed and her voice descended into an outright hiss. "How dare you!"

"How dare you!" he whispered back at her.

"You would blackmail me?"

Harry's lips twisted. "I am offering you a favor. I am giving you a chance to protect your precious secret. If you refuse I will have every natural motive to make inquiries elsewhere, not to spite you, but because I have to know! Get past your pointless anger at a child who you think ought to obey you, and you'll realise that any sane adult would do the same! Look at it from my perspective! How would you feel if it was YOU?"

Harry watched McGonagall, observed her harsh breathing. It occurred to him that it was time to ease off the pressure, let her simmer for a while. "You don't have to decide right away," Harry said in a more normal tone. "I'll understand if you want time to think about my offer... but I'll warn you of one thing," Harry said, his voice going colder. "Don't try that Obliviation spell on me. Some time ago I worked out a signal, and I have already sent that signal to myself. If I find that signal and I don't remember sending it..." Harry let his voice trail off significantly.

McGonagall's face was working as her expressions shifted. "I... wasn't thinking of Obliviating you, Mr. Potter... but why would you have invented such a signal if you didn't know about -"

"I thought of it while reading a Muggle science-fiction book, and said to myself, well, just in case... And no, I won't tell you the signal, I'm not dumb."

"I hadn't planned to ask," McGonagall said. She seemed to fold in on herself, and suddenly looked very old, and very tired. "This has been an exhausting day, Mr. Potter. Can we get your trunk, and send you home? I will trust you not to speak upon this matter until I have had time to think. Keep in mind that there are only two other people in the whole world who know about this matter, and they are Headmaster Albus Dumbledore and Professor Severus Snape."

So. New information; that was a peace offering. Harry nodded in acceptance, and turned his head to look forward, and started walking again, as his blood slowly began to warm over once more.

"So now I've got to find some way to kill an immortal Dark Wizard," Harry said, and sighed in frustration. "I really wish you had told me that before I started shopping."

The trunk shop was more richly appointed than any other shop Harry had visited; the curtains were lush and delicately patterned, the floor and walls of stained and polished wood, and the trunks occupied places of honor on polished ivory platforms. The salesman was dressed in robes of finery only a cut below those of Lucius Malfoy, and spoke with exquisite, oily politeness to both Harry and Professor McGonagall.

Harry had asked his questions, and had gravitated to a trunk of heavy-looking wood, not polished but warm and solid, carved with the pattern of a guardian dragon whose eyes shifted to look at anyone nearing it. A trunk charmed to be light, to shrink on command, to sprout small clawed tentacles from its bottom and squirm after its owner. A trunk with two drawers on each of four sides that each slid out to reveal compartments as deep as the whole trunk. A lid with four locks each of which would reveal a different space inside. And - this was the important