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<\/a><\/div>\nThere\u2019s been a fair amount of discussion on this blog lately about whether people can choose their beliefs or whether beliefs are compelled by evidence (and then some more about what this all means for free will). \u00a0Good news for me, because that was all the excuse I needed to post the following excerpt from Harry Potter \u00a0and the Methods of Rationality<\/a><\/em>, a fanfiction story written by AI research Eliezer Yudkowsky<\/a>.<\/p>\n In Yudkowsky\u2019s story, Harry was raised by a science professor instead of the Dursleys. \u00a0Some parts of the story are fairly didactic (not that I mind), but the plots that Yudowsky has invented are intricate and entertaining. \u00a0Since his Harry has more resources at his disposal, he\u2019s made Voldemort proportionately smarter \u2014 much better than the poor planner with silly goals that Harry ended up facing in canon.<\/p>\n The scene below occurs after Harry Potter and Draco Malfoy have finished\u00a0examining\u00a0evidence about why wizarding blood is getting weaker over time. \u00a0They were both studying the scientific method together, since Malfoy believed it was giving Harry advantages even in the wizarding world. \u00a0Malfoy believes any weakening of magic is the result of intermarriage with muggles (non-wizards), but it turns out that the evidence he gathered proves his theory false. \u00a0In the passage below, Yudowsky adapts Carl Sagan\u2019s Invisible Dragon<\/a> story and shows us what happens when you try to willfully change your beliefs.<\/p>\n \u201cFine. Then you and I are through. I\u2019m going to just walk away and forget any of this ever happened.\u201d<\/p>\n \u201cDraco\u2026 you can\u2019t forget. Don\u2019t you understand? That was your sacrifice.\u201d<\/p>\n Draco stopped in midstride and turned around. \u201cWhat are you talking about?\u201d<\/p>\n \u201cTo become a scientist. You questioned one of your beliefs, not just a small belief but something that had great significance to you. You did experiments, gathered data, and the outcome proved the belief was wrong. You saw the results and understood what they meant.\u201d Harry Potter\u2019s voice was faltering. \u201cRemember, Draco, you can\u2019t sacrifice a true belief that way, because the experiments will confirm it instead of falsifying it. Your sacrifice to become a scientist was your false belief that wizard blood was mixing and getting weaker.\u201d<\/p>\n \u201cThat\u2019s not true!\u201d said Draco. \u201cI didn\u2019t sacrifice the belief. I still believe that!\u201d His voice was getting louder, and the chill was getting worse.<\/p>\n Harry Potter shook his head. His voice came in a whisper. \u201cDraco\u2026 I\u2019m sorry, Draco, you don\u2019t believe it, not anymore.\u201d Harry\u2019s voice rose again. \u201cI\u2019ll prove it to you. Imagine that someone tells you they\u2019re keeping a dragon in their house. You tell them you want to see it. They say it\u2019s an invisible dragon. You say fine, you\u2019ll listen to it move. They say it\u2019s an inaudible dragon. You say you\u2019ll throw some cooking flour into the air and see the outline of the dragon. They say the dragon is permeable to flour. And the telling thing is that they know, in advance, exactly which experimental results they\u2019ll have to explain away. They know everything will come out the way it does if there\u2019s no dragon, they know in advance just which excuses they\u2019ll have to make. So maybe they say there\u2019s a dragon. Maybe they believe they believe there\u2019s a dragon, it\u2019s called belief-in-belief. But they don\u2019t actually believe it. You can be mistaken about what you believe, most people never realize there\u2019s a difference between believing something and thinking it\u2019s good to believe it.\u201d Harry Potter had risen from the desk now, and taken a few steps toward Draco. \u201cAnd Draco, you don\u2019t believe any more in blood purism, I\u2019ll show you that you don\u2019t. If blood purism is true, then Hermione Granger doesn\u2019t make sense, so what could explain her? Maybe she\u2019s a wizarding orphan raised by Muggles, just like I was? I could go to Granger and ask to see pictures of her parents, to see if she looks like them. Would you expect her to look different? Should we go perform that test?\u201d<\/p>\n \u201cThey would have put her with relatives,\u201d Draco said, his voice trembling. \u201cThey\u2019ll still look the same.\u201d<\/p>\n \u201cYou see. You already know what experimental result you\u2019ll have to excuse. If you still believed in blood purism you would say, sure, let\u2019s go take a look, I bet she won\u2019t look like her parents, she\u2019s too powerful to be a real Muggleborn -\u201d<\/p>\n \u201cThey would have put her with relatives!\u201d<\/p>\n \u201cScientists can do tests to check for sure if someone is the true child of a father. Granger would probably do it if I paid her family enough. She wouldn\u2019t be afraid of the results. So what do you expect that test to show? Tell me to run it and we will. But you already know what the test will say. You\u2019ll always know. You won\u2019t ever be able to forget. You might wish you believed in blood purism, but you\u2019ll always expect to see happen just exactly what would happen if there was only one thing that made you a wizard. That was your sacrifice to become a scientist.\u201d<\/p>\n Draco\u2019s breathing was ragged. \u201cDo you realize what you\u2019ve done?\u201d Draco surged forward and he seized Harry by the collar of his robes. His voice rose to a scream, it sounded unbearably loud in the closed classroom and the silence. \u201cDo you realize what you\u2019ve done?\u201d<\/p>\n Harry\u2019s voice was shaky. \u201cYou had a belief. The belief was false. I helped you see that. What\u2019s true is already so, owning up to it doesn\u2019t make it worse -\u201d<\/p>\n The fingers on Draco\u2019s right hand clenched into a fist and that hand dropped down and blasted up unstoppably and punched Harry Potter in the jaw so hard that his body went crashing back into a desk and then to the floor.<\/p><\/blockquote>\n