Book Cover
Harry Potter and the Methods of Rationality

★ ★

Tiger is a self-taught software and machine learning engineer who has been working full-time in the industry since he was 15. He is currently working on a startup in Denver.

TL;DR: Adapts the story of Harry Potter by attempting to explain wizardry through the scientific method. I don’t think I was smart enough to get all of the content about rationality but I enjoyed the story.


Really long and unfortunately the book and my notes/highlights are not making an appearance on my computer clients. I have had trouble with concrete takeaways, but I found a few summaries that do a decent job if I want a quick refresher. (sources:, Maybe will come back at a later date once I can wrap my head around more of the book.

The Boy-Who-Lived was raised by Petunia Evans and her husband, Oxford professor of physics Michael Verres. He grew up in a loving home and was raised to revere the scientific method and its results. When he discovered he was a wizard, Harry Potter-Evans-Verres was shocked at the backwards world he found – a society that was almost feudal, a school hardly worthy of the name, and a distinct lack of optimization. Harry’s quest to rectify these flaws was quickly derailed by the plotting of his mentor Professor Quirrell, a Defense Against the Dark Arts teacher who was actually the Dark Lord Voldemort, and who seeks the Philosopher’s Stone.

Working with Hermione Granger and (separately and secretly) with Draco Malfoy, Harry did his best to navigate the challenges of his first year: fighting in mock battles in Defense class, inventing partial Transfiguration, denouncing the abusive Professor Snape, devising a new Patronus that can destroy Dementors, breaking an innocent out of Azkaban, and angrily pointing out the flaws of Quidditch. But his worst fears are realized when Hermione is murdered. Harry vows that he will keep to the old words of the Potter family… “the last enemy that shall be defeated is death.”

At the climax of the story, Voldemort’s plot to retrieve the Philosopher’s Stone succeeded. In the process, Dumbledore was defeated and banished beyond time, while Hermione was resurrected and imbued with the regenerative powers of a troll and a unicorn to keep her alive. Voldemort revealed that the Philosopher’s Stone’s power was actually to make any Transfiguration permanent, even a human one, and that his overriding concern was to prevent a prophecy that predicts Harry will destroy the very stars in heaven in his vain questing to optimize the world. He further revealed that the curse he cast on Harry as a child left an imprint of Voldemort’s own thinking on the child, explaining some of Harry’s mysterious “dark side.” Harry was surrounded by Death Eaters, and Voldemort demanded any secrets he might possess,

Calling upon all his ingenuity, Harry used partial Transfiguration to kill all of his enemies at a blow, except Voldemort, whom he captured. He took up the Stone of Permanency, and returned to school to take control: to discover the source of magic, to end the injustices in the world, and to destroy death.

ch 1 – Test things to see if they are true.

ch 2 – Establish the parameters of your experiment carefully before performing it. You can’t make little changes to the laws of physics without breaking everything else.

ch 3 – You can scare a much larger group into submission by killing anyone who tries to speak out. Critical thinking is the hardest when it’s most important.

ch 4 – Successful entrepreneurship requires taking advantage of a money-making opportunity before everyone else realizes it exists. Knowing how to do Fermi calculations will make you smarter.

ch 5 – The way people behave depends on circumstances just as much as personality.

ch 6 – When planning things, take into account that things usually go worse than planned.

ch 7 – You can never have too many books. When you give someone something for free and then ask for something in return, they feel obligated to reciprocate even if they wouldn’t have agreed to the trade.

ch 8 – It’s more important what a hypothesis rules out than what it allows.

ch 9 – It’s hard to explain something to someone with a completely different worldview.

ch 10 – Don’t do things that might destroy the world just to indulge your personal pride. Things have moral value if they are aware of their own awareness.

ch 11 – You can’t give Ender a wand without giving the hive queen a horcrux. Songs in fanfics are really bad. It is hard to be too pessimistic.

ch 12 – Not becoming evil requires conscious effort.

ch 13 – Sufficiently powerful magic is indistinguishable from Dada.

ch 14 – There is a perfectly reasonable explanation for everything; things only seem strange because of your own ignorance.

ch 15 – Competition is a powerful motivator.

ch 16 – Talking about other people as objects to be manipulated makes you look like a sociopath.

ch 17 – Sometimes the hardest part of finding answers is figuring out where to look.

ch 18 – Status hierarchies seem very important when you happen to be on top.

ch 19 – It’s easy to fixate on complicated explanations when you have no idea what’s really going on. Sometimes it’s wise to swallow your pride and let someone else (seem to) win.

ch 20 – For something to act as evidence for a proposition, it has to be more likely if the proposition is true than if the proposition is false. Hints that seems obvious to a writer may be missed completely by readers.

ch 21 – Sometimes goals are just stories people tell to explain their own behaviour. There is no point making up complex arguments if you’ve already decided on your conclusion.

ch 22 – The trouble with science is you find out when you’re wrong.

ch 23 – Don’t try to make yourself believe something if you know it isn’t true.

ch 24 – The reason people are so smart is so they can outsmart other people.

ch 25 – It’s a good idea to discuss a problem thoroughly before suggesting solutions, so you don’t become too attached to your first idea.

ch 26 – When two pieces of data are contradictory, one of them must be false.

ch 27 – To outsmart someone, you have to be slightly trickier than they think you are. Don’t underestimate or trivialize the pain of others.

ch 28 – Complex systems are entirely explained by the behaviour of low-level interactions.

ch 29 – Everyone thinks they are the hero of their own story.

ch 30 – People will do anything for the sake of military simulation.

ch 31 – Don’t try to do everything by yourself.

ch 32 – To win people over, word things in a way they find appealing.

ch 33 – The current release of the human brain does not correctly implement Hofstadterian superrationality.

ch 34 – People are so bad at working together that sometimes dictatorship works better than democracy.

ch 35 – People are so evil that that democracy is usually preferable to dictatorship.

ch 36 – Relationships are hard because not everyone involved sees them the same way. Personal interactions get messy when everyone is trying to assert their dominance.

ch 37 – It’s nice when two people are willing to put pride and pretense aside and speak openly.

ch 38 – Don’t go making stuff up when you have no idea what you’re talking about.

ch 39 – Just because something sounds deep doesn’t mean it’s true.

ch 40 – If you really believed something, you’d act like it were were true.

ch 41 – But sometimes it’s really hard to convince your animal brain to act on intellectual beliefs.

ch 42 – Then again, when your brain tells you not to jump off a cliff, it might be right.

ch 43 – Being good means valuing human life.

ch 44 – Only true love can thaw a frozen heart.

ch 45 – To value life should mean to defy the necessity of death.

ch 46 – If knowing something is likely to hurt someone, just don’t tell them.

ch 47 – If you can’t understand how someone could believe something, you might be missing something important. For the sake of humanity, put aside your tribal allegiances and accept the moral worth of every human being.

ch 48 – Accepting the moral worth of others is a bit of a slippery slope. Ethics is fine, but don’t forget to be nice to people as well.

ch 49 – If you want to look smart, it helps to figure out the answers in advance.

ch 50 – People don’t always think about how their actions hurt others until they themselves are hurt.

ch 51 – Deciding to trust someone is a large and dangerous decision.

ch 52 – Prisons sound pretty awful when you think about it. The Bayesian Conspiracy offers its support to Amanda Knox.

ch 53 – Perfect crimes could happen all the time and nobody would know.

ch 54 – A plan may fail if it doesn’t take into account the emotional states of the human beings expected to carry it out.

ch 55 – Depression is dangerous because it makes you unable to remember why happiness is worthwhile.

ch 56 – The great superpower of the human brain is its ability to generate solutions to ridiculously constrained problems.

ch 57 – It’s hard to think clearly if you stop yourself thinking certain thoughts.

ch 58 – When trapped in a dilemma, select a preferred option and then search for a way out.

ch 59 – Try to find out how things work rather than just accepting them because they seem normal. When you suspect something, everything looks like evidence of it.

ch 60 – People allow many terrible things to happen because they don’t expect to be personally affected. Don’t point a gun at someone if you don’t want them dead.

ch 61 – Time travel is confusing.

ch 62 – Don’t routinely accept non-minuscule chances of total disaster. Compromises unthinkable to an idealist may sometimes be justified.

ch 63 – Lying is dangerous, because you have to keep telling more lies to cover it up. Don’t let yourself get so swept up in a narrative that you don’t think about what you’re actually doing. People will do things they know are evil if they’re too embarrassed to protest.

ch 64 – Don’t forget to get some sleep.

ch 65 – When you don’t want to believe something that’s true, it’s easy to get into the habit of lying and thinking other people are lying. Growing up happens not as a result of age, but of being through hard adult experiences.

ch 66 – Hesitation is always easy but seldom useful. Decide right now what information you will need and what decision you will make depending on the information you get.

ch 67 – Aim for the head when fighting someone wearing body armour. Don’t keep trying strategies that aren’t working.

ch 68 – People disagree on what it means to be “who you were meant to be” but think it’s a good idea.

ch 69 – People don’t always achieve their potential because they get bad ideas from their environment. Helping people feels good, but being a true hero isn’t fun or easy.

ch 70 – It isn’t sexism if they act that way with everyone.

ch 71 – Feminism should be about women being free to be who they want to be, not trying to pretend they don’t care at all about men.

ch 72 – If you lie too much, people won’t believe things you say. If you refuse to answer questions, people won’t know what to think.

ch 73 – Making every other paragraph a flashback makes the story hard to understand. Knowledge is better than ignorance, because not knowing about a problem doesn’t make it go away.

ch 74 – You should think before you do things.

ch 75 – When you try to be sensible, your friends might not understand you. Consent is important. Being a hero means doing whatever you can to help, not worrying about whose fault it is or whether it ought to be someone else’s job.

ch 76 – Sometimes you can get good ideas by talking to well-meaning clueless people.

ch 77 – Sometimes it’s better to “lose” a small conflict than escalate it and raise the stakes. But some things are worth fighting for. Memory-wiping magic is OP.

ch 78 – Unflinchingly discard ideas that are appealing in some sense but ultimately flawed.

ch 79 – Sometimes people assess the quality of an argument based on who they usually hear using it, rather than its quality.

ch 80 – When you think of someone as “bad” for some reason, it’s easy to keep seeing more bad things about them. People are too quick to heap hate on unpopular people when they should urge restraint.

ch 81 – It’s hard to give up large amounts of money, even for a good cause.

ch 82 – Sometimes we act contrary to game theory to protect those we love.

ch 83 – Rumours aren’t always true.

ch 84 – People like to do what everyone else is doing, even if they know it’s wrong.

ch 85 – It’s really hard to say what’s good and bad, and what’s justified in pursuit of a good cause.

ch 86 – Consider multiple explanations and use Bayes’ Theorem to consider the likelihood of each.

ch 87 – A lot of the things we do are because of small rewards and punishments. Evolutionary psychology is a bad topic to bring up when someone asks you what your relationship with them means.

ch 88 – Don’t waste time thinking about things that aren’t helping you solve the problem.

ch 89 – There is no God.

ch 90 – Sometimes people act like they imagine the person others see them as would, rather than doing what is actually smart.

ch 91 – You can’t learn from books what it’s like to be Severus Snape. It sucks to live in a society where you have no rights.

ch 92 – It’s a good idea to think of doing things before it’s too late, rather than after.

ch 93 – Sometimes people will step outside their “roles” and exceed your expectations.

ch 94 – Life isn’t fair and it’s really scary to fight people who are smart.

ch 95 – Just because someone doesn’t do something that would help others doesn’t mean they don’t care about them.

ch 96 – Throughout most of history people have been unable to stop death. But maybe we can.

ch 97 – Getting a lawyer is a good idea.

ch 98 – Don’t let your real enemies trick you into fighting people who could be your friends.

ch 99 – Apparently we’re doing the forbidden forest scene.

ch 100 – Consider the worst thing that could happen and take precautions.

ch 101 – What actually happens will probably be worse.