Harry Potter and the Methods of Rationality

"Harry! What are you -"

- and poured everything down his arm into his hand -

"Frigideiro!"

"- doing?"

"Hypothermia," Harry said distantly, as he staggered. It'd been one of the spells he and Hermione had experimented on, a lifetime ago, so he was able to control it precisely, though it had taken a lot of power to affect that much mass. Hermione's body should now be at almost exactly five degrees Celsius. "People have been revived from cold water after more than thirty minutes without breathing. The cold protects you from brain damage, you see, it slows everything down. There's a saying Muggle doctors have, you're not dead until you're warm and dead - I think they even cool down the patient during some surgeries, if they have to stop someone's heart for a while."

Fred and George started sobbing.

Dumbledore's face was already streaked with tears. "I'm sorry," he whispered. "Harry, I'm so sorry, but you have to stop this." The Headmaster took Harry by the shoulders and pulled on him.

Harry allowed himself to be turned away from Hermione's body, walked forward as the Headmaster pushed him away from the blood. The Cooling Charm would buy him time. Hours at least, maybe days if he could manage to keep casting the spell on Hermione or if they stored her body somewhere cold.

Now there was time to think.

Minerva had seen Albus's face and she'd known something was wrong; there had been time for her to wonder what had happened, and even who had died; her mind flashing to Alastor, to Augusta, to Arthur and Molly, all the most likely targets at the start of Voldemort's second rise. She had thought that she had steeled herself, she had thought herself ready for the worst.

Then Albus spoke, and all the steel left her.

Not Hermione - no -

Albus gave her a brief space to weep; and then told her that Harry Potter, who had watched Miss Granger die, had seated himself outside the infirmary storeroom where Miss Granger's remains were being kept, refusing to move from the spot, and telling anyone who spoke to him to go away so he could think.

The only thing that had elicited any reaction from the boy was when Fawkes had tried to sing to him; Harry Potter had shrieked at the phoenix not to do that, his feelings were real, he didn't want magic trying to heal them like they were a disease. After that Fawkes had refused to sing again.

Albus thought that she might have the best chance of reaching Harry Potter now.

So she had to pull herself together, and clean up her face; there would be time later for private grief, when her surviving children no longer needed her.

Minerva McGonagall pulled together the dislocated pieces of herself, wiped her eyes a final time, and laid her hand on the doorknob of the infirmary section whose back storeroom was now being used, for the second time this century and for the fifth time since the castle of Hogwarts had been raised, as the resting place of a promising young student.

She opened the door.

Harry Potter's eyes gazed at her. The boy was sitting on the floor in front of the door to the back storeroom, and holding his wand in his lap. If those eyes were grieving, if they were empty, if they were even broken, it couldn't be seen from looking at the boy's face. There were no dried tears on those cheeks.

"Why are you here, Professor McGonagall?" Harry Potter said. "I told the Headmaster I'd like to be left alone for a while."

She couldn't think of anything to say. To help you - you're not all right - but she didn't know what to say, there was nothing she could imagine saying that would make things better. She hadn't planned ahead before she'd walked into the room, having not been at her best.

"What are you thinking about?" Minerva said. It was the only sentence that came into her mind. Albus had told her that Harry had been saying, over and over, that he was thinking; and she had to get Harry talking, somehow.

Harry stared half at her and half past her, a tension coming into his face, as she held her breath.

It took a while before Harry spoke.

"I'm trying to think if there's anything I should be doing right now," said Harry Potter. "It's hard, though. My mind keeps on imagining ways the past could have gone differently if I'd thought faster, and I can't rule out that there might be a key insight in there somewhere."

"Mr. Potter -" she said falteringly. "Harry, I don't think it's healthy for you to be - thinking like that -"

"I disagree. It's not thinking that gets people killed." The words were spoken in a level monotone, as though reciting lines from a book.

"Harry," she said, hardly even thinking as she said it, "there's nothing you could have done -"

Something flickered in Harry's expression. His eyes seemed to focus on her for the first time.

"Nothing I could have done?" Harry's voice rose on the last word. "Nothing I could have DONE? I've lost track of how many different ways I could've saved her! If I'd asked to have us all given communications mirrors! If I'd insisted on Hermione being taken out of Hogwarts and put in a school that isn't insane! If I'd snuck out immediately instead of trying to argue with normal people! If I'd remembered the Patronus earlier! If I'd thought through possible emergencies and trained myself to think about Patronuses earlier! Even at the very last minute it might not have been too late! I killed the troll and turned to her and she was still ALIVE and I just knelt next to her listening to her last words like an IDIOT instead of casting the Patronus again and calling Dumbledore to send Fawkes! Or if I'd just approached the whole problem from a different angle - if I'd looked for a student with a Time-Turner to send a message back in time before I found out about anything happening to her, instead of ending up with an outcome that can't be altered - I asked the Headmaster to go back and save Hermione and then fake everything, fake the dead body, edit everyone's memories, but Dumbledore said that he tried something like