Harry Potter and the Methods of Rationality

It wasn't like modern-day Muggles had gotten anywhere near the limits of what Muggle physics said was possible. And yet, like the ancient hunter-gatherers conceptually bound to their rock-throwers, most Muggles lived in a world defined by the limits of what you could do with cars and telephones. Even though Muggle physics explicitly permitted possibilities like molecular nanotechnology or the Penrose process for extracting energy from black holes, most people filed that away in the same section of their brain that stored fairy tales and history books, well away from their personal realities: Long ago and far away, ever so long ago. It was no surprise at all, then, that the wizarding world lived in a conceptual universe bounded - not by fundamental laws of magic that nobody even knew - but rather by the surface rules of known Charms and enchantments. You couldn't observe the way magic was practiced nowadays and not be reminded of the National Museum of Australia, once you realized what you were seeing. Even if Harry's first guess had been mistaken, one way or another it was still inconceivable that the fundamental laws of the universe contained a special case for human lips shaping the phrase 'Wingardium Leviosa'. And yet even that fumbling grasp of magic was enough to do things that Muggle physics said should be impossible: the Time-Turner, the water conjured out of nothingness by Aguamenti. What were the ultimate possibilities of invention, if the underlying laws of the universe permitted an eleven-year-old with a stick to violate almost every constraint in the Muggle version of physics?

Like a hunter-gatherer trying to look up at the Sun, and guess that the universe had to be shaped in a way that allowed for nuclear energy...

It made you wonder if maybe twenty thousand million million million meters wasn't so much distance, after all.

There was a step beyond Abstract Reasoning Harry which he could take, given time enough to compose himself and the right surroundings. Looking up at the stars, you could try to imagine what the distant descendants of humanity would think of your dilemma - in a hundred million years, when all the visible stars would have rotated through great galactic movements into entirely new positions, every constellation scattered. It was an elementary theorem of probability that if you already knew what your answer would be after updating on future evidence, you ought to adopt that answer right now. If you knew your destination, you were already there. And by analogy, if not quite by theorem, if you could guess what the descendants of humanity would think of something, you ought to go ahead and take that as your own best guess.

From that vantage point the idea of killing off two-thirds of the Wizengamot seemed a lot less appealing than it had a few hours earlier. Even if you had to do it, even if you knew for a solid fact that it would be the best thing for magical Britain and that the complete Story of Time would look worse if you didn't do it... even as a necessity, the deaths of sentient beings would still be a tragedy. One more element of the sorrows of Earth; the Most Ancient Earth from which everything had begun, long ago and far away, ever so long ago.

He is not like Grindelwald. There is nothing human left in him. Him you must destroy. Save your fury for that, and that alone -

Harry shook his head slightly, tilting the stars a little in his vision, as he lay on the stone floor looking upward and outward and forward in time. Even if Dumbledore was right, and the true enemy was utterly mad and evil... in a hundred million years the organic lifeform known as Lord Voldemort probably wouldn't seem much different from all the other bewildered children of Ancient Earth. Whatever Lord Voldemort had done to himself, whatever Dark rituals seemed so horribly irrevocable on a merely human scale, it wouldn't be beyond curing with the technology of a hundred million years. Killing him, if you didn't have to do it, would be just one more death for future sentient beings to be sad about. How could you look up at the stars, and believe anything else?

Harry stared up at the twinkling lights of Eternity and wondered what the children's children's children would think of what Dumbledore had maybe-done to Narcissa.

But even if you tried framing the question that way, asking what humanity's descendants would think, it still drew only on your own knowledge, not theirs. The answer still came from inside yourself, and it could still be mistaken. If you didn't know the tenth decimal digit of pi yourself, then you didn't know how the children's children's children would calculate it, for all that the fact was trivial.