Eliezer Yudkowsky - Harry Potter and the Methods of Rationality

If magic had been like that, a big complex adaptation with lots of necessary genes, then a wizard mating with a Muggle would have resulted in a child with only half those parts and half the machine wouldn't do much. And so there would have been no Muggleborns, ever. Even if all the pieces had individually gotten into the Muggle gene pool, they'd never reassemble all in one place to form a wizard.

There hadn't been some genetically isolated valley of humans that had stumbled onto an evolutionary pathway leading to sophisticated magical sections of the brain. That complex genetic machinery, if wizards interbred with Muggles, would never have reassembled into Muggleborns.

So however your genes made you a wizard, it wasn't by containing the blueprints for complicated machinery.

That was the other reason Harry had guessed the Mendelian pattern would be there. If magical genes weren't complicated, why would there be more than one?

And yet magic itself seemed pretty complicated. A door-locking spell would prevent the door from opening and prevent you from Transfiguring the hinges and resist Finite Incantatem and Alohomora . Many elements all pointing in the same direction: you could call that goal-orientation, or in simpler language, purposefulness.

There were only two known causes of purposeful complexity. Natural selection, which produced things like butterflies. And intelligent engineering, which produced things like cars.

Magic didn't seem like something that had self-replicated into existence. Spells were purposefully complicated, but not, like a butterfly, complicated for the purpose of making copies of themselves. Spells were complicated for the purpose of serving their user, like a car.

Some intelligent engineer, then, had created the Source of Magic, and told it to pay attention to a particular DNA marker.

The obvious next thought was that this had something to do with "Atlantis".

Harry had asked Hermione about that earlier - on the train to Hogwarts, after hearing Draco say it - and so far as she knew, nothing more was known than the word itself.

It might have been pure legend. But it was also plausible enough that a civilization of magic-users, especially one from before the Interdict of Merlin, would have managed to blow itself up.

The line of reasoning continued: Atlantis had been an isolated civilization that had somehow brought into being the Source of Magic, and told it to serve only people with the Atlantean genetic marker, the blood of Atlantis.

And by similar logic: The words a wizard spoke, the wand movements, those weren't complicated enough of themselves to build up the spell effects from scratch - not the way that the three billion base pairs of human DNA actually were complicated enough to build a human body from scratch, not the way that computer programs took up thousands of bytes of data.

So the words and wand movements were just triggers, levers pulled on some hidden and more complex machine. Buttons, not blueprints.

And just like a computer program wouldn't compile if you made a single spelling error, the Source of Magic wouldn't respond to you unless you cast your spells in exactly the right way.

The chain of logic was inexorable.

And it led inevitably toward a single final conclusion.

The ancient forebears of the wizards, thousands of years earlier, had told the Source of Magic to only levitate things if you said...

'Wingardium Leviosa.'

Harry slumped over at the breakfast table, resting his forehead wearily on his right hand.

There was a story from the dawn days of Artificial Intelligence - back when they were just starting out and no one had yet realized the problem would be difficult - about a professor who had delegated one of his grad students to solve the problem of computer vision.

Harry was beginning to understand how that grad student must have felt.

This could take a while.

Why did it take more effort to cast the Alohomora spell, if it was just like pressing a button?

Who'd been silly enough to build in a spell for Avada Kedavra that could only be cast using hatred?

Why did wordless Transfiguration require you to make a complete mental separation between the concept of form and concept of material?

Harry might not be done with this problem by the time he graduated Hogwarts. He could still be working on this problem when he was thirty years old. Hermione had been right, Harry hadn't realized that on a gut level before. He'd just given an inspiring speech about determination.

Harry's mind briefly considered whether to get on a gut level that he might never solve the problem at all, then decided that would be taking things much too far.

Besides, so long as he could get as far as immortality in the first few decades, he'd be fine.

What method had the Dark Lord used? Come to think, the fact that the Dark Lord had somehow managed to survive the death of his first body was almost infinitely more important than the fact that he'd tried to take over magical Britain -

"Excuse me," said an expected voice from behind him in very unexpected tones. "At your convenience, Mr. Malfoy requests the favor of a conversation."

Harry did not choke on his breakfast cereal. Instead he turned around and beheld Mr. Crabbe.

"Excuse me, " said Harry. "Don't you mean 'Da boss wants ta talk wid youse?'"

Mr. Crabbe didn't look happy. "Mr. Malfoy instructed me to speak properly."

"I can't hear you," Harry said. "You're not speaking properly." He turned back to his bowl of tiny blue crystal snowflakes and deliberately ate another spoonful.

"Da boss wants to talk with youse," came a threatening voice from behind him. "Ya'd better come see him if ya know what's good for ya."

There. Now everything was going according to plan.

Act 1:

"A reason? " said the old wizard. He restrained the fury from his face. The boy before him had been the victim, and certainly did not need to be frightened any further. "There is nothing that can excuse -"

"What I did to him was worse."

The old wizard stiffened in sudden horror. "Harry, what have you done? "

"I tricked Draco into believing that I'd tricked him into participating in a ritual that sacrificed his belief in blood purism. And that meant he couldn't be a Death Eater when he grew up. He'd lost everything, Headmaster."

There was a long quiet in the office, broken only by the tiny puffs and whistles of the fiddly things, which after enough time had come to seem like silence.

"Dear me," said the old wizard, "I do feel silly. And here I was expecting you might try to redeem the heir of Malfoy by, say, showing him true friendship and kindness ."

" Ha! Yeah, like that would have worked."

The old wizard sighed. This was taking it too far. "Tell me, Harry. Did it even occur to you that there was something incongruous about setting out to redeem someone through lies and trickery?"

"I did it without telling any direct lies, and since we're talking about Draco Malfoy here, I think the word you're looking for is congruous ." The boy looked rather smug.

The old wizard shook his head in despair. "And this is the hero. We're all doomed."

Act 5:

The long, narrow tunnel of rough stone, unlit except by a child's wand, seemed to stretch on for miles.

The reason for this was simple: It did stretch on for miles.

The time was three in the morning, and Fred and George were starting the long way down the secret passage that led from a statue of a one-eyed witch in Hogwarts, to the cellar of the Honeydukes candyshop in Hogsmeade.