Harry Potter and the Methods of Rationality

'Anyway,' Petunia said, her voice small, 'she gave in. She told me it was dangerous, and I said I didn't care any more, and I drank this potion and I was sick for weeks, but when I got better my skin cleared up and I finally filled out and... I was beautiful, people were nice to me,' her voice broke, 'and after that I couldn't hate my sister any more, especially when I learned what her magic brought her in the end -'

'Darling,' Michael said gently, 'you got sick, you gained some weight while resting in bed, and your skin cleared up on its own. Or being sick made you change your diet -'

'She was a witch,' Petunia repeated. 'I saw it.'

'Petunia,' Michael said. The annoyance was creeping into his voice. 'You know that can't be true. Do I really have to explain why?'

Petunia wrung her hands. She seemed to be on the verge of tears. 'My love, I know I can't win arguments with you, but please, you have to trust me on this -'

'Dad! Mum!'

The two of them stopped and looked at Harry as though they'd forgotten there was a third person in the room.

Harry took a deep breath. 'Mum, your parents didn't have magic, did they?'

'No,' Petunia said, looking puzzled.

'Then no one in your family knew about magic when Lily got her letter. How did they get convinced?'

'Ah...' Petunia said. 'They didn't just send a letter. They sent a professor from Hogwarts. He -' Petunia's eyes flicked to Michael. 'He showed us some magic.'

'Then you don't have to fight over this,' Harry said firmly. Hoping against hope that this time, just this once, they would listen to him. 'If it's true, we can just get a Hogwarts professor here and see the magic for ourselves, and Dad will admit that it's true. And if not, then Mum will admit that it's false. That's what the experimental method is for, so that we don't have to resolve things just by arguing.'

The Professor turned and looked down at him, dismissive as usual. 'Oh, come now, Harry. Really, magic? I thought you'd know better than to take this seriously, son, even if you're only ten. Magic is just about the most unscientific thing there is!'

Harry's mouth twisted bitterly. He was treated well, probably better than most genetic fathers treated their own children. Harry had been sent to the best primary schools - and when that didn't work out, he was provided with tutors from the endless pool of starving students. Always Harry had been encouraged to study whatever caught his attention, bought all the books that caught his fancy, sponsored in whatever maths or science competitions he entered. He was given anything reasonable that he wanted, except, maybe, the slightest shred of respect. A Doctor teaching biochemistry at Oxford could hardly be expected to listen to the advice of a little boy. You would listen to Show Interest, of course; that's what a Good Parent would do, and so, if you conceived of yourself as a Good Parent, you would do it. But take a ten-year-old seriously? Hardly.

Sometimes Harry wanted to scream at his father.

'Mum,' Harry said. 'If you want to win this argument with Dad, look in chapter two of the first book of the Feynman Lectures on Physics. There's a quote there about how philosophers say a great deal about what science absolutely requires, and it is all wrong, because the only rule in science is that the final arbiter is observation - that you just have to look at the world and report what you see. Um... off the top of my head I can't think of where to find something about how it's an ideal of science to settle things by experiment instead of arguments -'

His mother looked down at him and smiled. 'Thank you, Harry. But -' her head rose back up to stare at her husband. 'I don't want to win an argument with your father. I want my husband to, to listen to his wife who loves him, and trust her just this once -'

Harry closed his eyes briefly. Hopeless. Both of his parents were just hopeless.

Now his parents were getting into one of those arguments again, one where his mother tried to make his father feel guilty, and his father tried to make his mother feel stupid.

'I'm going to go to my room,' Harry announced. His voice trembled a little. 'Please try not to fight too much about this, Mum, Dad, we'll know soon enough how it comes out, right?'

'Of course, Harry,' said his father, and his mother gave him a reassuring kiss, and then they went on fighting while Harry climbed the stairs to his bedroom.

He shut the door behind him and tried to think.

The funny thing was, he should have agreed with Dad. No one had ever seen any evidence of magic, and according to Mum, there was a whole magical world out there. How could anyone keep something like that a secret? More magic? That seemed like a rather suspicious sort of excuse.

It should have been a clean case for Mum joking, lying or being insane, in ascending order of awfulness. If Mum had sent the letter herself, that would explain how it arrived at the letterbox without a stamp. A little insanity was far, far less improbable than the universe really working like that.

Except that some part of Harry was utterly convinced that magic was real, and had been since the instant he saw the putative letter from the Hogwarts School of Witchcraft and Wizardry.

Harry rubbed his forehead, grimacing. Don't believe everything you think, one of his books had said.

But this bizarre certainty... Harry was finding himself just expecting that, yes, a Hogwarts professor would show up and wave a wand and magic would come out. The strange certainty was making no effort to guard itself against falsification - wasn't making excuses in advance for why there wouldn't be a professor, or the professor would only be able to bend spoons.

Where do you come from, strange little prediction? Harry directed the thought at his brain. Why do I believe what I believe?

Usually Harry was pretty good at answering that question, but in this particular case, he had no