Harry Potter and the Methods of Rationality

Harry dropped the bag of gold into the mokeskin pouch with his left hand; his right hand came out of his pocket tightly holding some of the gold coins, reached into the mokeskin pouch, dropped the loose Galleons, and (with a whisper of "bag of gold") retrieved the original bag. Then the bag went back into his left hand, to be dropped in again, and Harry's right hand went back into his pocket...

Professor McGonagall looked back at him once, but Harry managed to avoid freezing or flinching, and she didn't seem to notice anything. Though you never did quite know, with the adults that had a sense of humour. It took three iterations to get the job done, and Harry guessed he'd managed to steal maybe thirty Galleons from himself.

Harry reached up, wiped a bit of sweat from his forehead, and exhaled. "I'd like this one, please."

Fifteen Galleons lighter (twice the price of a wizard's wand, apparently) and one Moke Super Pouch QX31 heavier, Harry and Professor McGonagall pushed their way out of the door. The door formed a hand and waved goodbye to them as they left, extruding its arm in a way that made Harry feel a bit queasy.

And then, unfortunately...

"Are you really Harry Potter?" whispered the old man, one huge tear sliding down his cheek. "You wouldn't lie about that, would you? Only I'd heard rumours that you didn't really survive the Killing Curse and that's why no one ever heard from you again."

...it seemed that Professor McGonagall's disguise spell was less than perfectly effective against more experienced magical practitioners.

Professor McGonagall had laid a hand on Harry's shoulder and yanked him into the nearest alleyway the moment she'd heard "Harry Potter?" The old man had followed, but at least it looked like no one else had heard.

Harry considered the question. Was he really Harry Potter? "I only know what other people have told me," Harry said. "It's not like I remember being born." His hand brushed his forehead. "I've had this scar as long as I remember, and I've been told my name was Harry Potter as long as I remember. But," Harry said thoughtfully, "if there's already sufficient cause to postulate a conspiracy, there's no reason why they wouldn't just find another orphan and raise him to believe that he was Harry Potter -"

Professor McGonagall drew her hand over her face in exasperation. "You look just about exactly like your father, James, the year he first attended Hogwarts. And I can attest on the basis of personality alone that you are related to the Scourge of Gryffindor."

"She could be in on it too," Harry observed.

"No," quavered the old man. "She's right. You have your mother's eyes."

"Hmm," Harry frowned. "I suppose you could be in on it too -"

"Enough, Mr. Potter."

The old man raised up a hand as if to touch Harry, but then let it fall. "I'm just glad that you're alive," he murmured. "Thank you, Harry Potter. Thank you for what you did... I'll leave you alone now."

And his cane slowly tapped away, out the alley and down the main street of Diagon Alley.

The Professor looked around, her expression tense and grim. Harry automatically looked around himself. But the alley seemed empty of all but old leaves, and from the mouth leading out into Diagon Alley, only swiftly striding passersby could be seen.

Finally Professor McGonagall seemed to relax. "That was not well done," she said in a low voice. "I know you're not used to this, Mr. Potter, but people do care about you. Please be kind to them."

Harry looked down at his shoes. "They shouldn't," he said with a tinge of bitterness. "Care about me, I mean."

"You saved them from You-Know-Who," said Professor McGonagall. "How should they not care?"

Harry looked up at the witch-lady's strict expression beneath her pointed hat, and sighed. "I suppose there's no chance that if I said fundamental attribution error you'd have any idea what that meant."

"No," said the Professor in her precise Scottish accent, "but please explain, Mr. Potter, if you would be so kind."