Harry Potter and the Methods of Rationality

you say Expecto Patronum. For the honor of House Malfoy, and not just because you promised me a favor. Show me you didn't lie to me that day in the train station when you told me Lucius was a good father. Show me you can do what Salazar Slytherin could do."

And Harry stepped backward, behind Draco, out of Draco's field of vision, so that Draco only faced the dusty old teacher's desk and blackboard at the front of the unused classroom.

Draco cast one look behind him, that strange look still on his face, and then turned away to face forward. Harry saw the exhalation, the inhalation. The wand twitched once, twice, thrice, and four times. Draco's fingers slid along the wand, exactly the right distances -

Draco lowered his wand.

"This is too -" Draco said, "I can't think this right, while you're watching -"

Harry turned around and started walking toward the door. "I'll come back in a minute," Harry said. "Just hold to your happy thought, and the Patronus will stay."

From behind Draco came the sound of the door opening again.

Draco heard Harry's footsteps entering the classroom, but Draco didn't turn to look.

Harry didn't say anything either. The silence stretched.

Finally -

"What does this mean?" Draco said. His voice wavered a bit.

"It means you love your father," Harry's voice said. Which was just what Draco had been thinking, and trying not to cry in front of Harry. It was too right, just too right -

Before Draco, on the floor, was the shining form of a snake that Draco recognized; a Blue Krait, a snake first brought to their manor by Lord Abraxas Malfoy after a visit to some faraway land, and Father had kept a Blue Krait in the ophidiarium ever since. The thing about the Blue Krait was that the bite wouldn't hurt much. Father had said that, and told Draco that he was never allowed to pet the snake, no matter who was watching. The venom killed your nerves so fast that you didn't have time to feel pain as the poison spread. You could die of it even after using Healing Charms. It ate other snakes. It was as Slytherin as any creature could possibly be.

That was why a Blue Krait head had been forged into the handle of Father's cane.

The bright snake darted out its tongue, which was also silver; and seemed to smile somehow, in a warmer way than any reptile should.

And then Draco realized -

"But," Draco said, still staring at the beautifully radiant snake, "you can't cast the Patronus Charm." Now that Draco had cast it himself, he understood why that was important. You could be evil, like Dumbledore, and still cast the Patronus Charm, so long as you had something bright left inside you. But if Harry Potter didn't have a single thought inside him that shone like that -

"The Patronus Charm is more complicated than you think, Draco," Harry said seriously. "Not everyone who fails at casting it is a bad person, or even unhappy. But anyway, I can cast it. I did it on my second try, after I realized what I'd done wrong facing the Dementor my first time. But, well, my life gets a little peculiar sometimes, and my Patronus came out strange, and I'm keeping it a secret for now -"

"Am I supposed to just believe that?"

"You can ask Professor Quirrell if you don't believe me," said Harry. "Ask him whether Harry Potter can cast a corporeal Patronus, and tell him that I told you to ask. He'd know the request was from me, no one else would know."

Oh, and now Draco was to trust Professor Quirrell? Still, knowing Harry, it might be true; and Professor Quirrell wouldn't lie for trivial reasons.

The glowing snake turned its head back and forth, as though seeking a prey that wasn't there, and then coiled itself into a circle, as though to rest.

"I wonder," Harry said softly, "when it was, which year, which generation, that Slytherins stopped trying to learn the Patronus Charm. When it was that people started to think, that Slytherins themselves started to think, that being cunning and ambitious was the same as being cold and unhappy. And if Salazar knew that his students didn't even bother showing up to learn the Patronus Charm any more, I wonder, would he wish that he'd never been born? I wonder how it all went wrong, when Slytherin's House went wrong."

The shining creature winked out, the turmoil rising in Draco making it impossible to sustain the Charm. Draco spun on Harry, he had to control himself not to raise his wand. "What do you know about Slytherin House or Salazar Slytherin? You were never Sorted into my House, what gives you the right to -"

And that was when Draco finally realized.

"You did get Sorted into Slytherin!" Draco said. "You did, and afterwards you, you somehow, you snapped your fingers -" Draco had once asked Father if it would be cleverer to get Sorted into some other House so that everyone would trust him, and Father had smiled and said that he'd thought of that too at Draco's age, but there was no way to fool the Sorting Hat...

...not until Harry Potter came along.

How had he ever bought for one minute that Harry was a Ravenclaw?

"An interesting hypothesis," Harry said equably. "Do you know, you're the second person in Hogwarts to come up with a theory along those lines? At least you're the second that's actually said so to my face -"

"Snape," Draco said with certainty. His Head of House was no fool.

"Professor Quirrell, of course," said Harry. "Though come to think, Severus did ask me how I managed to stay out of his House, and whether I had something the Sorting Hat wanted. I suppose you could say you're number three. Oh, but Professor Quirrell's theory was a little different than yours, though. May I have your word not to repeat it?"