Harry Potter and the Methods of Rationality

Draco's feet were heavy as he walked through the corridors of Hogwarts. He should have been running but he couldn't seem to muster the energy. He kept on thinking that he didn't want to know about this, he didn't want to be involved in any of this, he didn't want this to be his responsibility, just let Harry Potter do it, if magic was fading let Harry Potter take care of it...

But Draco knew that wasn't right.

Chill the dungeons of Slytherin, gray the stone walls, Draco usually liked the atmosphere, but now it seemed too much like fading.

His hand on the doorknob, Harry Potter already inside and waiting, wearing his cowled cloak.

"The ancient first-year spells," Harry Potter said. "What did you find?"

"They're no more powerful than the spells we use now."

Harry Potter's fist struck a desk, hard. "Damn it. All right. My own experiment was a failure, Draco. There's something called the Interdict of Merlin -"

Draco hit himself on the forehead, realizing.

"- which stops anyone from getting knowledge of powerful spells out of books, even if you find and read a powerful wizard's notes they won't make sense to you, it has to go from one living mind to another. I couldn't find any powerful spells that we had the instructions for but couldn't cast. But if you can't get them out of old books, why would anyone bother passing them on by word of mouth after they stopped working? Did you get the data on the Squib couples?"

Draco started to hand the parchment over -

But Harry Potter held up a hand. "Law of science, Draco. First I tell you the theory and the prediction. Then you show me the data. That way you know I'm not just making up a theory to fit; you know that the theory actually predicted the data in advance. I have to explain this to you anyway, so I have to explain it before you show me the data. That's the rule. So put on your cloak and let's sit down."

Harry Potter sat down at a desk with torn scraps of paper arranged across its surface. Draco drew his cloak out of his bookbag, drew it on, and sat down across from Harry on the other side, giving the paper scraps a puzzled look. They were arranged in two rows and the rows were about twenty scraps long.

"The secret of blood," said Harry Potter, an intense look on his face, "is something called deoxyribose nucleic acid. You don't say that name in front of anyone who's not a scientist. Deoxyribose nucleic acid is the recipe that tells your body how to grow, two legs, two arms, short or tall, whether you have brown eyes or green. It's a material thing, you can see it if you have microscopes, which are like telescopes only they look at things that are very small instead of very far away. And that recipe has two copies of everything, always, in case one copy is broken. Imagine two long rows of pieces of paper. At each place in the row, there are two pieces of paper, and when you have children, your body selects one piece of paper at random from each place in the row, and the mother's body will do the same, and so the child also gets two pieces of paper at each place in the row. Two copies of everything, one from your mother, one from your father, and when you have children they get one piece of paper from you at random in each place."

As Harry spoke, his fingers ranged over the paired scraps of paper, pointing to one part of the pair when he said "from your mother", the other when he said "from your father". And as Harry talked about picking a piece of paper at random, his hand pulled a Knut out of his robes and flipped it; Harry looked at the coin, and then pointed to the top piece of paper. All without a pause in the speech.

"Now when it comes to something like being short or tall, there's a lot of places in the recipe that make little differences. So if a tall father marries a short mother, the child gets some pieces of paper saying 'tall' and some pieces of paper saying 'short', and usually the child ends up middle-sized. But not always. By luck, the child might get a lot of pieces saying 'tall', and not many papers saying 'short', and grow up pretty tall. You could have a tall father with five papers saying 'tall' and a tall mother with five papers saying 'tall' and by amazing luck the child gets all ten papers saying 'tall' and ends up taller than both of them. You see? Blood isn't a perfect fluid, it doesn't mix perfectly. Deoxyribose nucleic acid is made up of lots of little pieces, like a glass of pebbles instead of a glass of water. That's why a child isn't always exactly in the middle of the parents."

Draco listened with his mouth open. How in Merlin's name had the Muggles figured all this out? They could see the recipe?