"I didn't know anything you didn't know," Harry said, still quietly. "I admit that I suspected. Hermione Granger was too powerful, she should have been barely magical and she wasn't, how can a Muggleborn be the best spellcaster in Hogwarts? And she's getting the best grades on her essays too, it's too much coincidence for one girl to be the strongest magically and academically unless there's a single cause. Hermione Granger's existence pointed to there being only one thing that makes you a wizard, something you either have or you don't, and the power differences coming from how much we know and how much we practice. And there weren't different classes for purebloods and Muggleborns, and so on. There were too many ways the world didn't look the way it would look if you were right. But Draco, I didn't see anything you couldn't see too. I didn't perform any tests I didn't tell you about. I didn't cheat, Draco. I wanted us to work out the answer together. And I never thought that magic might be fading out of the world until you said it. It was a scary idea for me, too."
"Whatever," Draco said. He was working very hard to control his voice and not just start screaming at Harry. "You claim you're not going to run off and tell anyone else about this."
"Not without consulting you first," Harry said. He opened his hands in a pleading gesture. "Draco, I'm being as nice as I can but the world turned out to just not be that way."
"Fine. Then you and I are through. I'm going to just walk away and forget any of this ever happened."
Draco spun around, feeling the burning sensation in his throat, the sense of betrayal, and that was when he realized he really had liked Harry Potter, and that thought didn't slow him down for a moment as he strode toward the classroom door.
And Harry Potter's voice came, now louder, and worried:
"Draco... you can't forget. Don't you understand? That was your sacrifice."
Draco stopped in midstride and turned around. "What are you talking about?"
But there was already a freezing coldness in Draco's spine.
He knew even before Harry Potter said it.
"To become a scientist. You questioned one of your beliefs, not just a small belief but something that had great significance to you. You did experiments, gathered data, and the outcome proved the belief was wrong. You saw the results and understood what they meant." Harry Potter's voice was faltering. "Remember, Draco, you can't sacrifice a true belief that way, because the experiments will confirm it instead of falsifying it. Your sacrifice to become a scientist was your false belief that wizard blood was mixing and getting weaker."
"That's not true!" said Draco. "I didn't sacrifice the belief. I still believe that!" His voice was getting louder, and the chill was getting worse.
Harry Potter shook his head. His voice came in a whisper. "Draco... I'm sorry, Draco, you don't believe it, not anymore." Harry's voice rose again. "I'll prove it to you. Imagine that someone tells you they're keeping a dragon in their house. You tell them you want to see it. They say it's an invisible dragon. You say fine, you'll listen to it move. They say it's an inaudible dragon. You say you'll throw some cooking flour into the air and see the outline of the dragon. They say the dragon is permeable to flour. And the telling thing is that they know, in advance, exactly which experimental results they'll have to explain away. They know everything will come out the way it does if there's no dragon, they know in advance just which excuses they'll have to make. So maybe they say there's a dragon. Maybe they believe they believe there's a dragon, it's called belief-in-belief. But they don't actually believe it. You can be mistaken about what you believe, most people never realize there's a difference between believing something and thinking it's good to believe it." Harry Potter had risen from the desk now, and taken a few steps toward Draco. "And Draco, you don't believe any more in blood purism, I'll show you that you don't. If blood purism is true, then Hermione Granger doesn't make sense, so what could explain her? Maybe she's a wizarding orphan raised by Muggles, just like I was? I could go to Granger and ask to see pictures of her parents, to see if she looks like them. Would you expect her to look different? Should we go perform that test?"
"They would have put her with relatives," Draco said, his voice trembling. "They'll still look the same."
"You see. You already know what experimental result you'll have to excuse. If you still believed in blood purism you would say, sure, let's go take a look, I bet she won't look like her parents, she's too powerful to be a real Muggleborn -"
"They would have put her with relatives!"
"Scientists can do tests to check for sure if someone is the true child of a father. Granger would probably do it if I paid her family enough. She wouldn't be afraid of the results. So what do you expect that test to show? Tell me to run it and we will. But you already know what the test will say. You'll always know. You won't ever be able to forget. You might wish you believed in blood purism, but you'll always expect to see happen just exactly what would happen if there was only one thing that made you a wizard. That was your sacrifice to become a scientist."
Draco's breathing was ragged. "Do you realize what you've done?" Draco surged forward and he seized Harry by the collar of his robes. His voice rose to a scream, it sounded unbearably loud in the closed classroom and the silence. "Do you realize what you've done?"
Harry's voice was shaky. "You had a belief. The belief was false. I helped you see that. What's true is already so, owning up to it doesn't make it worse -"
The fingers on Draco's right hand clenched into a fist and that hand dropped down and blasted up unstoppably and punched Harry Potter in the jaw so hard that his body went crashing back into a desk and then to the floor.
"Idiot!" screamed Draco. "Idiot! Idiot!"
"Draco," whispered Harry from the floor, "Draco, I'm sorry, I didn't think this would happen for months, I didn't expect you to awaken as a scientist this quickly, I thought I would have longer to prepare you, teach you the techniques that make it hurt less to admit you're wrong -"
"What about Father?" Draco said. His voice trembled with rage. "Were you going to prepare him or did you just not care what happened after this?"
"You can't tell him!" Harry said, his voice rising in alarm. "He's not a scientist! You promised, Draco!"
For a moment the thought of Father not knowing came as a relief.
And then the real anger started to rise.
"So you planned for me to lie to him and tell him I still believe," Draco said, voice shaking. "I'll always have to lie to him, and now when I grow up I can't be a Death Eater, and I won't even be able to tell him why not."
"If your father really loves you," whispered Harry from the floor, "he'll still love you even if you don't become a Death Eater, and it sounds like your father does really love you, Draco -"
"Your stepfather is a scientist," Draco said. The words coming out like biting knives. "If you weren't going to be a scientist, he would still love you. But you'd be a little less special to him."
Harry flinched. The boy opened his mouth, as if to say 'I'm sorry', and then closed his mouth, seeming to think better of it, which was either very smart of him or very lucky, because Draco might have tried to kill him.