Harry Potter and the Methods of Rationality

part - a handle on the bottom which slid out a frame containing a staircase leading down into a small, lighted room that would hold, Harry estimated, around twelve bookcases.

If they made luggages like this, Harry didn't know why anyone bothered owning a house.

One hundred and eight golden Galleons. That was the price of a good trunk, lightly used. At around fifty British pounds to the Galleon, that was enough to buy a second-hand car. It would be more expensive than everything else Harry had ever bought in his whole life all put together.

Ninety-seven Galleons. That was how much was left in the bag of gold Harry had been allowed to take out of Gringotts.

Professor McGonagall wore a look of chagrin upon her face. After a long day's shopping she hadn't needed to ask Harry how much gold was left in the bag, after the salesman quoted his price, which meant the Professor could do good mental arithmetic without pen and paper. Once again, Harry reminded himself that scientifically illiterate was not at all the same thing as stupid.

"I'm sorry, young man," said Professor McGonagall. "This is entirely my fault. I would offer to take you back to Gringotts, but the bank will be closed for all but emergency services now."

Harry looked at her, wondering...

"Well," sighed Professor McGonagall, as she swung on one heel, "we may as well go, I suppose."

...she hadn't lost it completely when a child had dared defy her. She hadn't been happy, but she had thought instead of exploding in fury. It might have just been that there was an immortal Dark Lord to fight - that she had needed Harry's goodwill. But most adults wouldn't have been capable of thinking even that much; wouldn't consider future consequences at all, if someone lower in status had refused to obey them...

"Professor?" Harry said.

The witch turned back and looked at him.

Harry took a deep breath. He needed to be a little angry for what he wanted to try now, there was no way he'd have the courage to do it otherwise. She didn't listen to me, he thought to himself, I would have taken more gold but she didn't want to listen... Focusing his entire world on McGonagall and the need to bend this conversation to his will, he spoke.

"Professor, you thought one hundred Galleons would be more than enough for a trunk. That's why you didn't bother warning me before it went down to ninety-seven. Which is just the sort of thing the research studies show - that's what happens when people think they're leaving themselves a little error margin. They're not pessimistic enough. If it'd been up to me, I'd have taken two hundred Galleons just to be sure. There was plenty of money in that vault, and I could have put back any extra later. But I thought you wouldn't let me do it. I thought you'd be angry at me just for asking. Was I wrong?"

"I suppose I must confess that you are right," said Professor McGonagall. "But, young man -"

"That sort of thing is the reason why I have trouble trusting adults." Somehow Harry kept his voice steady. "Because they get angry if you even try to reason with them. To them it's defiance and insolence and a challenge to their higher tribal status. If you try to talk to them they get angry. So if I had anything really important to do, I wouldn't be able to trust you. Even if you listened with deep concern to whatever I said - because that's also part of the role of someone playing a concerned adult - you'd never change your actions, you wouldn't actually behave differently, because of anything I said."

The salesman was watching them both with unabashed fascination.

"I can understand your point of view," Professor McGonagall said eventually. "If I sometimes seem too strict, please remember that I have served as Head of Gryffindor House for what feels like several thousand years."

Harry nodded and continued. "So - suppose I had a way to get more Galleons from my vault without us going back to Gringotts, but it involved me violating the role of an obedient child. Would I be able to trust you with that, even though you'd have to step outside your own role as Professor McGonagall to take advantage of it?"

"What?" said Professor McGonagall.

"To put it another way, if I could make today have happened differently, so that we didn't take too little money with us, would that be all right even though it would involve a child being insolent to an adult in retrospect?"

"I... suppose..." the witch said, looking quite puzzled.

Harry took out the mokeskin pouch, and said, "Eleven Galleons originally from my family vault."

And there was gold in Harry's hand.

For a moment Professor McGonagall's mouth gaped wide, then her jaw snapped shut and her eyes narrowed and the witch bit out, "Where did you get that -"

"From my family vault, like I said."

"How?"

"Magic."

"That's hardly an answer!" snapped Professor McGonagall, and then stopped, blinking.

"No, it isn't, is it? I ought to claim that it's because I experimentally discovered the true secrets of how the pouch works and that it can actually retrieve objects from anywhere, not just its own inside, if you phrase the request correctly. But actually it's from when I fell into that pile of gold before and I shoved some Galleons into my pocket. Anyone who understands pessimism knows that money is something you might need quickly and without much warning. So now are you angry at me for defying your authority? Or glad that we succeeded in our important mission?"