Harry Potter and the Methods of Rationality

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Harry Potter and the Methods of Rationality is a book by Eliezer Yudkowsky based on the series of "Harry Potter" books. The book was published in sequels in the years 2010-2015.


  • Harry didn't see why Hermione had been so tense about it. In what weird alternative universe would that girl not be Sorted into Ravenclaw? If Hermione Granger didn't go to Ravenclaw then there was no good reason for Ravenclaw House to exist.
  • When there was once more silence in the room, Harry sat on the stool and carefully placed onto his head the 800-year-old telepathic artefact of forgotten magic.
Thinking, just as hard as he could: Don't Sort me yet! I have questions I need to ask you! Have I ever been Obliviated? Did you Sort the Dark Lord when he was a child and can you tell me about his weaknesses? Can you tell me why I got the brother wand to the Dark Lord's? Is the Dark Lord's ghost bound to my scar and is that why I get so angry sometimes? Those are the most important questions, but if you've got another moment can you tell me anything about how to rediscover the lost magics that created you?
Into the silence of Harry's spirit, where before there had never been any voice but one, there came a second and unfamiliar voice, sounding distinctly worried:
"Oh, dear. This has never happened before..."

Harry James Potter-Evans-Verres

  • When you walk past a bookshop you haven't visited before, you have to go in and look around. That's the family rule.
  • World domination is such an ugly phrase. I prefer to call it world optimisation.
  • 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
  • I am not going to be in Gryffindor

Professor McGonagall

  • I would be doing a great service to wizarding Britain, Mr. Potter, if I locked you in this vault and left you here.
  • "So here we part ways, for a time. This has been the strangest day of my life for... many a year. Since the day I learned that a child had defeated You-Know-Who. I wonder now, looking back, if that was the last reasonable day of the world."

hermione granger


"I..." Hermione's mind went blank for a moment. She loved tests but she'd never had a test like this before. Frantically, she tried to cast back for anything she'd read about what scientists were supposed to do. Her mind skipped gears, ground against itself, and spat back the instructions for doing a science investigation project:

Step 1: Form a hypothesis. Step 2: Do an experiment to test your hypothesis. Step 3: Measure the results. Step 4: Make a cardboard poster.


  • "Before we begin our banquet, I would like to say a few words. And here they are: Happy happy boom boom swamp swamp swamp! Thank you!"

Harry explains


Suppose you come into work and see your colleague kicking his desk. You think, 'what an angry person he must be'. Your colleague is thinking about how someone bumped him into a wall on the way to work and then shouted at him. Anyone would be angry at that, he thinks. When we look at others we see personality traits that explain their behaviour, but when we look at ourselves we see circumstances that explain our behaviour. People's stories make internal sense to them, from the inside, but we don't see people's histories trailing behind them in the air. We only see them in one situation, and we don't see what they would be like in a different situation. So the fundamental attribution error is that we explain by permanent, enduring traits what would be better explained by circumstance and context.

Muggle researchers have found that people are always very optimistic, compared to reality. Like they say something will take two days and it takes ten days, or they say it'll take two months and it takes over thirty-five years. For example, in one experiment, they asked students for times by which they were 50% sure, 75% sure, and 99% sure they'd complete their homework, and only 13%, 19%, and 45% of the students finished by those times. And they found that the reason was that when they asked one group for their best-case estimates if everything went as well as possible, and another group for their average-case estimates if everything went as usual, they got back answers that were statistically indistinguishable. See, if you ask someone what they expect in the normal case, they visualise what looks like the line of maximum probability at each step along the way - everything going according to plan, with no surprises. But actually, since more than half the students didn't finish by the time they were 99% sure they'd be done, reality usually delivers results a little worse than the 'worst-case scenario'

"What you've just discovered is called 'positive bias'," said the boy. "You had a rule in your mind, and you kept on thinking of triplets that should make the rule say 'Yes'. But you didn't try to test any triplets that should make the rule say 'No'. In fact you didn't get a single 'No', so 'any three numbers' could have just as easily been the rule. It's sort of like how people imagine experiments that could confirm their hypotheses instead of trying to imagine experiments that could falsify them - that's not quite exactly the same mistake but it's close. You have to learn to look on the negative side of things, stare into the darkness. When this experiment is performed, only 20% of grownups get the answer right. And many of the others invent fantastically complicated hypotheses and put great confidence in their wrong answers since they've done so many experiments and everything came out like they expected."


  • Slowly, Draco nodded. "You think you can master both arts, add the powers together, and..." Draco stared at Harry. "Make yourself Lord of the two worlds?"
Harry gave an evil laugh, it just seemed to come naturally at that point. "You have to realise, Draco, that the whole world you know, all of magical Britain, is just one square on a much larger gameboard. The gameboard that includes places like the Moon, and the stars in the night sky, which are lights just like the Sun only unimaginably far away, and things like galaxies that are vastly huger than the Earth and Sun, things so large that only scientists can see them and you don't even know they exist. But I really am Ravenclaw, you know, not Slytherin. I don't want to rule the universe. I just think it could be more sensibly organised."
  • Dumbledore got himself under control again with a visible effort. "Ah, Harry, one symptom of the disease called wisdom is that you begin laughing at things that no one else thinks is funny, because when you're wise, Harry, you start getting the jokes!" The old wizard wiped tears away from his eyes. "Ah, me. Ah, me. Oft evil will shall evil mar indeed, in very deed."
Harry's brain took a moment to place the familiar words... "Hey, that's a Tolkien quote! Gandalf says that!"
"Theoden, actually," said Dumbledore.
"You're Muggleborn? " Harry said in shock.
"I'm afraid not," said Dumbledore, smiling again. "I was born seventy years before that book was published, dear child. But it seems that my Muggleborn students tend to think alike in certain ways. I have accumulated no fewer than twenty copies of The Lord of the Rings and three sets of Tolkien's entire collected works, and I treasure every one of them." Dumbledore drew his wand and held it up and struck a pose. "You cannot pass! How does that look?"
"Ah," Harry said in something approaching complete brain shutdown, "I think you're missing a Balrog." And the pink pyjamas and squashed mushroom hat were not helping in the slightest.