Harry Potter and the Methods of Rationality

"You used me," said Draco Malfoy.

"I only used you in ways that made you stronger. That's what it means to be used by a friend."

"Even I know that's not what friendship is!"

Now Lucius Malfoy spoke again. "For what purpose? To what end?" Even the elder Malfoy's voice was not quite steady. "Why?"

Harry regarded him for a moment, and then turned to Draco. "Your father's probably not going to believe this," Harry said. "But you, Draco, should be able to see that everything which has happened is compatible with this hypothesis. And that any more cynical hypothesis wouldn't explain why I didn't press you harder when you thought I had leverage, or why I taught you so much. I thought that the heir of House Malfoy, who'd been publicly seen to grab a Muggleborn girl to stop her falling off the roof of Hogwarts, would be a good compromise candidate to lead magical Britain after the reformation."

"So you would have me believe," Lucius Malfoy said in a thin voice, "that you are claiming to be mad. Well, let us leave all that aside. Tell me who set that troll on Hogwarts."

"I don't know," Harry said.

"Tell me who you suspect, Harry Potter."

"I have four suspects. One of them is Professor Snape -"

"Snape?" Draco burst out.

"The second, of course, is the Defense Professor of Hogwarts, just because he's the Defense Professor." Harry would have left him out, not wanting to bring Professor Quirrell to the Malfoys' attention if he was innocent, but Draco might have called him on that. "The third, you wouldn't believe me about. The fourth is a catchall category called Everything Else." And the fifth, Lord Voldemort, I do not think I should name to you.

Lucius Malfoy's face contorted in a snarl. "Do you think I cannot recognize bait upon your hook? Tell me about this third possibility, Potter, the one you wish me to believe is the true answer, and leave aside games."

Harry regarded Lord Malfoy steadily. "I once read a book I wasn't supposed to read, and it told me this: Communication is an event that takes place between equals. Employees lie to their bosses, who, in turn, expect to be lied to. I'm not playing coy, I'm observing that it's simply not possible, in our present situation, for me to tell you about the third suspect, and have you believe that my story was anything but a lure."

Draco spoke then. "It's Father, isn't it?"

Harry gave Draco a startled look.

Draco spoke evenly. "You suspect that Father sent the troll into Hogwarts to get at Granger, don't you? That's what you're thinking, isn't it!"

Harry opened his mouth to say, Actually, no, and then managed to think ahead and stop himself for once in his life.

"I see..." Harry said slowly. "That's what this is about. Lucius Malfoy publicly says that Hermione won't get away with what she's done, and lo and behold, a troll kills her." Harry smiled then, in a way that bared his teeth. "And if I deny that here, then Draco, who isn't an Occlumens, can then testify under Veritaserum that the Boy-Who-Lived does not suspect Lucius Malfoy of having sent a troll into Hogwarts to kill Hermione Granger, sworn to the Noble House of Potter, whose blood debt was recently purchased for a hundred thousand Galleons et cetera." Harry leaned back slightly, though his wooden stool had no back with which to do it properly. "But now that it's been pointed out, I see that it's very reasonable. Obviously you killed Hermione Granger, just like you threatened to do in front of the whole Wizengamot."

"I did not," Lucius Malfoy said, expressionless once more.

Harry bared his teeth again in that non-smile. "Well then, in that case, there must be someone else out there who killed Hermione and messed with the Hogwarts wards, the same person who earlier tried to frame Hermione for Draco Malfoy's murder. Either you killed Hermione Granger after being paid for her life, or you blamed your son's attempted murder on an innocent girl and took all my family's money under false pretenses, one of those two things must be true."

"Perhaps you killed her in hopes of your money being returned." Lucius Malfoy had leaned forward, and was staring hard at Harry.

"Then I would not have given away my money for her in the first place. As you already know. Don't insult my intelligence, Lord Malfoy - no, wait, sorry, you just had to say that in case Draco had to testify to it, never mind."

Lucius Malfoy sat back in his chair and stared.

"I tried to tell you, Father," Draco said under his breath, "but nobody can imagine Harry Potter until they've actually met him..."

Harry tapped a finger on his cheek. "So people are starting to figure out the blatantly obvious? I'm surprised, actually. I wouldn't have predicted that would happen." Harry had by now caught the general rhythm of Professor Quirrell's cynicism and was able to generate it independently. "I wouldn't think a newspaper would be able to report on a concept like 'Either X or Y must be true, but we don't know which.' I would only expect journalists to report stories consisting of series of atomic propositions, like 'X is true', 'Y is false', or 'X is true and Y is false'. Not more complex logical connectors like 'If X is true then Y is true, but we don't know whether X is true'. And all your supporters ought to be rapidly switching between 'You can't prove that Lord Malfoy killed Granger, it could've been someone else' and 'You can't prove there was someone else to frame Granger', so long as it's uncertain they should be trying to have it both ways at once... wait, don't you own the Daily Prophet?"

"The Daily Prophet," Lucius Malfoy said thinly, "which I certainly do not own, is far too respectable to publish any such scurrilous nonsense. Unfortunately, not all wizards of influence are so reasonable."

"Ah. Got it." Harry nodded.

Lucius glanced at Draco. "The rest of what he said - was any of it important?"