Eliezer Yudkowsky - Harry Potter and the Methods of Rationality

"Voldemort, seeking to avert that very prophecy, went to his defeat at your hands," the old wizard said then. "His knowledge brought him only harm. Ponder that carefully, Harry Potter."

"Yes, Headmaster, I do understand that. My home culture also has a literary tradition of self-fulfilling and misinterpreted prophecies. I'll interpret with caution, rest assured. But I've already guessed quite a bit. Is it safer for me to work from partial guesses?"

Time passed.

"Minerva," said Albus. "If you would."

"The one..." she began. The words came falteringly to her throat; she was no actress. She couldn't imitate the deep, chilling tone of the original prophecy; and yet somehow that tone seemed to carry all the meaning. "The one with the power to vanquish the Dark Lord approaches... born to those who have thrice defied him, born as the seventh month dies..."

" And the Dark Lord shall mark him as his equal, " came Severus's voice, making her jump within her chair. The Potions Master loomed tall by the fireplace. " But he shall have power the Dark Lord knows not... and either must destroy all but a remnant of the other, for those two different spirits cannot exist in the same world. "

That last line Severus spoke with so much foreboding that it chilled her bones; it was almost like listening to Sybill Trelawney.

Harry was listening with a frown. "Can you repeat that?" said Harry.

" The one with the power to vanquish the Dark Lord approaches, born to those who have thrice defied him, born as the seventh month - "

"Actually, hold on, can you write that down? I need to analyze this carefully -"

This was done, with both Albus and Severus watching the parchment hawklike, as though to make sure that no unseen hand reached in and snatched the precious information away.

"Let's see..." Harry said. "I'm male and born on July 31st, check. I did in fact vanquish the Dark Lord, check. Ambiguous pronoun in line two... but I wasn't born yet so it's hard to see how my parents could have thrice defied me. This scar is an obvious candidate for the mark..." Harry touched his forehead. "Then there's the power the Dark Lord knows not, which probably refers to my scientific background -"

"No," said Severus.

Harry looked at the Potions Master in surprise.

Severus's eyes were closed, his face tightened in concentration. "The Dark Lord could obtain that power by studying the same books as you, Potter. But the prophecy did not say, power the Dark Lord has not. Nor even, power the Dark Lord cannot have. She spoke of power the Dark Lord knows not ... it will be something stranger to him than Muggle artifacts. Something perhaps that he cannot comprehend at all, even having seen it..."

"Science is not a bag of technological tricks," Harry said. "It's not just the Muggle version of a wand. It's not even knowledge like memorizing the periodic table. It's a different way of thinking. "

"Perhaps..." the Potions Master murmured, but his voice was skeptical.

"It is hazardous," Albus said, "to read too far into a prophecy, even if you have heard it yourself. They are things of exceeding frustration."

"So I see," Harry said. His hand rose up, rubbed the scar on his forehead. "But... okay, if this is really all we know... look, I'll just put it bluntly. How do you know that the Dark Lord actually survived?"

" What? " she cried. Albus just sighed and leaned back in the vast Headmaster's chair.

"Well," Harry said, "imagine how this prophecy sounded back when it was made. You-Know-Who learns the prophecy, and it sounds like I'm destined to grow up and overthrow him. That the two of us are meant to have a final battle where either of us must destroy all but a remnant of the other. So You-Know-Who attacks Godric's Hollow and immediately gets vanquished, leaving behind some remnant which may or may not be his disembodied soul. Maybe the Death Eaters are his remnant, or the Dark Mark. This prophecy could already be fulfilled, is what I'm saying. Don't get me wrong - I do realize that my interpretation sounds stretched. Trelawney's phrasing doesn't seem natural for describing only the events that historically happened on October 31st, 1981. Attacking a baby and having the spell bounce off, isn't something you'd normally call 'the power to vanquish'. But if you think of the prophecy as being about several possible futures, only one of which was actually realized on Halloween, then the prophecy could already be complete."

"But -" Minerva blurted. "But the raid on Azkaban -"

" If the Dark Lord survived, then sure, he's the most likely suspect for the Azkaban breakout," Harry said reasonably. "You could even say that the Azkaban breakout is Bayesian evidence for the Dark Lord surviving, because an Azkaban breakout is more likely to happen in worlds where he's alive than worlds where he's dead. But it's not strong Bayesian evidence. It's not something that can't possibly happen unless the Dark Lord is alive. Professor Quirrell, who didn't start from the assumption that You-Know-Who was still around, had no trouble thinking of his own explanation. To him, it was obvious that some powerful wizard might want Bellatrix Black because she knew a secret of the Dark Lord's, like some of his magical knowledge that he'd told to only her. The priors against anyone surviving their body's death are very low, even if it's magically possible. Most times it doesn't happen. So if it's just the Azkaban breakout... I'd have to say formally that it isn't enough Bayesian evidence. The improbability of the evidence assuming that the hypothesis is false, is not commensurate with the prior improbability of the hypothesis."

"No," Severus said flatly. "The prophecy is not yet fulfilled. I would know if it were."

"Are you sure of that?"

"Yes, Potter. If the prophecy had already come true, I would understand it! I heard Trelawney's words, I remember Trelawney's voice, and if I knew the events that matched the prophecy, I would recognize them. What has already happened... does not fit." The Potions Master spoke with certainty.

"I'm not really sure what to do with that statement," Harry said. His hand rose up, absently rubbed at his forehead. "Maybe it's just what you think happened that doesn't fit, and the true history is different..."

"Voldemort is alive," Albus said. "There are other indications."

"Such as?" Harry's reply was instant.

Albus paused. "There are terrible rituals by which wizards have returned from death," Albus said slowly. "That much, anyone can discern within history and legend. And yet those books are missing, I could not find them; it was Voldemort who removed them, I am sure -"

"So you can't find any books on immortality, and that proves that You-Know-Who has them?"

"Indeed," said Albus. "There is a certain book - I will not name it aloud - missing from the Restricted Section of the Hogwarts library. An ancient scroll which should have been at Borgin and Burkes, with only an empty place on a shelf to show where it was -" The old wizard stopped. "But I suppose," the old wizard said, as though to himself, "you will say that even if Voldemort tried to make himself immortal, it does not prove that he succeeded..."

Harry sighed. "Proof, Headmaster? There are only ever probabilities. If there are known, particular books on immortality rituals which are missing, that increases the probability that someone attempted one. Which, in turn, raises the prior probability of the Dark Lord surviving his death. This I concede, and thank you for contributing the fact. The question is whether the prior probability goes up enough. "