Harry Potter and the Methods of Rationality

But Bayes's Theorem said that any reasonable hypothesis which made it more likely than a thousand-to-one that he'd end up with the brother to the Dark Lord's wand, was going to have an advantage.

Professor McGonagall had simply said how peculiar and left it at that, which had put Harry into a state of shock at the sheer, overwhelming uncuriosity of wizards and witches. In no imaginable world would Harry have just went "Hm" and walked out of the shop without even trying to come up with a hypothesis for what was going on.

His left hand rose and touched his scar.

What... exactly...

"You're a full wizard now," said Professor McGonagall. "Congratulations."

Harry nodded.

"And what do you think of the wizarding world?" said she.

"It's strange," Harry said. "I ought to be thinking about everything I've seen of magic... everything that I now know is possible, and everything I now know to be a lie, and all the work left before me to understand it. And yet I find myself distracted by relative trivialities like," Harry lowered his voice, "the whole Boy-Who-Lived thing." There didn't seem to be anyone nearby, but no point tempting fate.

Professor McGonagall ahemmed. "Really? You don't say."

Harry nodded. "Yes. It's just... odd. To find out that you were part of this grand story, the quest to defeat the great and terrible Dark Lord, and it's already done. Finished. Completely over with. Like you're Frodo Baggins and you find out that your parents took you to Mount Doom and had you toss in the Ring when you were one year old and you don't even remember it."

Professor McGonagall's smile had grown somewhat fixed.

"You know, if I were anyone else, anyone else at all, I'd probably be pretty worried about living up to that start. Gosh, Harry, what have you done since you defeated the Dark Lord? Your own bookshop? That's great! Say, did you know I named my child after you? But I have hopes that this will not prove to be a problem." Harry sighed. "Still... it's almost enough to make me wish that there were some loose ends from the quest, just so I could say that I really, you know, participated somehow."

"Oh?" said Professor McGonagall in an odd tone. "What did you have in mind?"

"Well, for example, you mentioned that my parents were betrayed. Who betrayed them?"

"Sirius Black," the witch said, almost hissing the name. "He's in Azkaban. Wizarding prison."

"How probable is it that Sirius Black will break out of prison and I'll have to track him down and defeat him in some sort of spectacular duel, or better yet put a large bounty on his head and hide out in Australia while I wait for the results?"

Professor McGonagall blinked. Twice. "Not likely. No one has ever escaped from Azkaban, and I doubt that he will be the first."

Harry was a bit sceptical of that "no one has ever escaped from Azkaban" line. Still, maybe with magic you could actually get close to a 100% perfect prison, especially if you had a wand and they did not. The best way to get out would be to not go there in the first place.

"All right then," Harry said. "Sounds like it's been nicely wrapped up." He sighed, scrubbing his palm over his head. "Or maybe the Dark Lord didn't really die that night. Not completely. His spirit lingers, whispering to people in nightmares that bleed over into the waking world, searching for a way back into the living lands he swore to destroy, and now, in accordance with the ancient prophecy, he and I are locked in a deadly duel where the winner shall lose and the loser shall win -"

Professor McGonagall's head swivelled, and her eyes darted around, as though to search the street for listeners.

"I'm joking, Professor," Harry said with some annoyance. Sheesh, why did she always take everything so seriously -

A slow sinking sensation began to dawn in the pit of Harry's stomach.

Professor McGonagall looked at Harry with a calm expression. A very, very calm expression. Then a smile was put on. "Of course you are, Mr. Potter."

Aw crap.

If Harry had needed to formalise the wordless inference that had just flashed into his mind, it would have come out something like, 'If I estimate the probability of Professor McGonagall doing what I just saw as the result of carefully controlling herself, versus the probability distribution for all the things she would do naturally if I made a bad joke, then this behavior is significant evidence for her hiding something.'

But what Harry actually thought was, Aw crap.

Harry turned his own head to scan the street. Nope, no one nearby. "He's not dead, is he," Harry sighed.

"Mr. Potter -"

"The Dark Lord is alive. Of course he's alive. It was an act of utter optimism for me to have even dreamed otherwise. I must have taken leave of my senses, I can't imagine what I was thinking. Just because someone said that his body was found burned to a crisp, I can't imagine why I would have thought he was dead. Clearly I have much left to learn about the art of proper pessimism."

"Mr. Potter -"