Harry Potter and the Methods of Rationality

What do you think you're doing? Go back and get her out of there!

...kept walking...

Save her! What are you doing? She's hurting YOU HAVE TO SAVE HER!

The portkey Harry was carrying could transport two humans, only two, plus or minus a snake. If they'd had Professor Quirrell's portkey too... but they didn't, Professor Quirrell's human form was carrying that, there was no way to get it... Harry could only save one person today, and there was only one person on the lowest level of Azkaban, in the most desperate need...

"DON'T GO!" The voice came in a scream from behind the metal door. "No, no, no, don't go, don't take it away, don't don't don't -"

There was a light in the corridor and it grew brighter.

"Please," sobbed the woman's voice, "please, I can't remember my children's names any more -"

"Sit down, Bella," Harry's voice said, somehow he kept his voice in a cold whisper, "I must deal with this," the Hover Charm diminishing and switching off even as Bella obediently sat down, her skeletal form dark against the brightening air.

I'll die, thought Harry.

The air went on brightening.

After all, it wasn't a certainty that Harry would die.

It was just a probability of death, and weren't some things worth a probability of dying?

The air went on brightening, the greater Patronus was beginning to form around him, the brilliant human shape was becoming indistinct within the burning air, as Harry's life went to feed the fire.

If I wipe out the Dementors, then even if I live, they'll know it was me, that I was the one who did this... I'll lose my support, lose the war...

Yeah? said the inner voice that was urging him on. After you destroy all the Dementors in Azkaban? I'd think that'd tend to prove your credentials as a Light Lord, actually, so SAVE HER SAVE HER YOU HAVE TO SAVE HER -

The humanoid shape could no longer be seen as a separate entity.

The corridor couldn't be seen.

Harry's own body was invisible within the Cloak.

There was only a bodiless viewpoint within an infinite expanse of silver light.

Harry could feel the life leaving him, fueling the spell; far away, he could feel the shadows of Death begin to fray.

I meant to accomplish more with my life than this... I was going to fight the Dark Lord, I was going to merge the wizarding and Muggle worlds...

Lofty goals seemed very distant, very abstract, compared to one woman begging him for help, it wasn't certain that Harry would ever do anything more important than this one thing, this one thing that he could do now and here.

And with what might have been his last breath, Harry thought:

There are other Dementors, probably other Azkabans... if I'm going to do this, I should do it when I'm closer to the central pit, it will take less of my life that way, which increases the probability that I'll survive to destroy other Dementors... even assuming this is the optimal thing to do, if there's a right time and place to do this, it isn't now and here, IT ISN'T NOW AND HERE!

What? said the other part of him indignantly, as it searched for a counterargument that didn't exist -

Slowly the light died back down, as Harry concentrated on that one indisputable fact, the one obvious truth that they weren't in the optimal place, the time couldn't be now...

Slowly the light died back down.

Part of Harry's life flowed back into him.

Part had been lost as radiation.

But Harry had enough left to stay on his feet, and keep the silver human shape bright; and when his wand arm raised and his voice whispered "Wingardium Leviosa", the magic flowed obediently out of him and helped Bellatrix to her feet. (For it wasn't his magic he had expended, it had never been his magic that fueled the Patronus Charm.)

I swear, Harry thought, breathing as regularly as he could in Bellatrix's presence, while tears streamed down his invisible cheeks, I swear upon my life and my magic and my art as a rationalist, I swear by everything I hold sacred and all my happy memories, I give my oath that someday I will end this place, please, please may I be forgiven...

And the two of them walked on, as a murderess's voice screamed and begged someone to come back and save her.

There should have been more time, there should have been a ceremony, for Harry's sacrifice of that piece of himself, but Bellatrix was beside him and so Harry just had to keep on walking without a pause, saying nothing, breathing evenly.

So Harry walked on, leaving a piece of himself behind. It would dwell in this place and time forever, he knew. Even after Harry came back someday with a company of other True Patronus casters and they destroyed all the Dementors here. Even if he melted the triangular building and burned the island low enough that the sea would wash over it, leaving no trace that such a place as this had ever once existed. Even then he wouldn't get it