Harry Potter and the Methods of Rationality

promote him to their attention without having good reasons already in hand to suspect him. When there are lots of possibilities, most of the work goes into just locating the true answer - starting to pay attention to it. You don't need proof, or the sort of official evidence that scientists or courts demand, but you need some sort of hint, and that hint has to discriminate that particular possibility from the millions of others. Otherwise you can't just pluck the right answer out of thin air. You can't even pluck a possibility worth thinking about out of thin air. And there's got to be a million other things I could do besides carrying around my father's rock. Just because I'm ignorant about the universe doesn't mean that I'm unsure about how I should reason in the presence of my uncertainty. The laws for thinking with probabilities are no less iron than the laws that govern old-fashioned logic, and what you just did is not allowed." Harry paused. "Unless, of course, you have some hint you're not mentioning."

"Ah," said Dumbledore. He tapped his cheek, looking thoughtful. "An interesting argument, certainly, but doesn't it break down at the point where you make an analogy between a million potential murderers only one of whom committed the murder, and taking one out of many possible courses of action, when many possible courses of action may all be wise? I do not say that carrying your father's rock is the one best possible course of action, only that it is wiser to do than not."

Dumbledore once again reached into the same desk drawer he had accessed earlier, this time seeming to root around inside - at least his arm seemed to be moving. "I will remark," Dumbledore said while Harry was still trying to sort out how to reply to this completely unexpected rejoinder, "that it is a common misconception of Ravenclaws that all the smart children are Sorted there, leaving none for other Houses. This is not so; being Sorted to Ravenclaw indicates that you are driven by your desire to know things, which is not at all the same quality as being intelligent." The wizard was smiling as he bent over the drawer. "Nonetheless, you do seem rather intelligent. Less like an ordinary young hero and more like a young mysterious ancient wizard. I think I may have been taking the wrong approach with you, Harry, and that you may be able to understand things that few others could grasp. So I shall be daring, and offer you a certain other heirloom."

"You don't mean..." gasped Harry. "My father... owned another rock?"

"Excuse me," said Dumbledore, "I am still older and more mysterious than you and if there are any revelations to be made then I will do the revealing, thank you... oh, where is that thing!" Dumbledore reached down further into the desk drawer, and still further. His head and shoulders and whole torso disappeared inside until only his hips and legs were sticking out, as though the desk drawer was eating him.

Harry couldn't help but wonder just how much stuff was in there and what the complete inventory would look like.

Finally Dumbledore rose back up out of the drawer, holding the objective of his search, which he set down on the desk alongside the rock.

It was a used, ragged-edged, worn-spined textbook: Intermediate Potion Making by Libatius Borage. There was a picture of a smoking vial on the cover.

"This," Dumbledore intoned, "was your mother's fifth-year Potions textbook."

"Which I am to carry with me at all times," said Harry.

"Which holds a terrible secret. A secret whose revelation could prove so disastrous that I must ask you to swear - and I do require you to swear it seriously, Harry, whatever you may think of all this - never to tell anyone or anything else."

Harry considered his mother's fifth-year Potions textbook, which, apparently, held a terrible secret.

The problem was that Harry did take that oaths like that very seriously. Any vow was an Unbreakable Vow if made by the right sort of person.

And...

"I'm feeling thirsty," Harry said, "and that is not at all a good sign."

Dumbledore entirely failed to ask any questions about this cryptic statement. "Do you swear, Harry?" said Dumbledore. His eyes gazed intently into Harry's. "Otherwise I cannot tell you."

"Yes," said Harry. "I swear." That was the trouble with being a Ravenclaw. You couldn't refuse an offer like that or your curiosity would eat you alive, and everyone else knew it.

"And I swear in turn," said Dumbledore, "that what I am about to tell you is the truth."

Dumbledore opened the book, seemingly at random, and Harry leaned in to see.

"Do you see these notes," Dumbledore said in a voice so low it was almost a whisper, "written in the margins of the book?"

Harry squinted slightly. The yellowing pages seemed to be describing something called a potion of eagle's splendour, many of the ingredients being items that Harry didn't recognise at all and whose names didn't appear to derive from English. Scrawled in the margin was a handwritten annotation saying, I wonder what would happen if you used Thestral blood here instead of blueberries? and immediately beneath was a reply in different handwriting, You'd get sick for weeks and maybe die.

"I see them," said Harry. "What about them?"

Dumbledore pointed to the second scrawl. "The ones in this handwriting," he said, still in that low voice, "were written by your mother. And the ones in this handwriting," moving his finger to indicate the first scrawl, "were written by me. I would turn myself invisible and sneak into her dorm room while she was sleeping. Lily thought one of her friends was writing them and they had the most amazing fights."

That was the exact point at which Harry realised that the Headmaster of Hogwarts was, in fact, crazy.

Dumbledore was looking at him with a serious expression. "Do you understand the implications of what I have just told you, Harry?"

"Ehhh..." Harry said. His voice seemed to be stuck. "Sorry... I... not really..."

"Ah well," said Dumbledore, and sighed. "I suppose your cleverness has limits after all, then. Shall we all just pretend I didn't say anything?"