HPMOR Ch. 23 said:
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.... 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?"
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