Harry Potter and the Methods of Rationality

"Draco, let me introduce you to something I call the Litany of Tarski. It changes every time you use it. On this occasion it runs like so: If magic is fading out of the world, I want to believe that magic is fading out of the world. If magic is not fading out of the world, I want not to believe that magic is fading out of the world. Let me not become attached to beliefs I may not want. If we're living in a world where magic is fading, that's what we have to believe, we have to know what's coming, so we can stop it, or in the very worst case, be prepared to do what we can in the time we have left. Not believing it won't stop it from happening. So the only question we have to ask is whether magic is actually fading, and if that's the world we live in then that's what we want to believe. Litany of Gendlin: What's true is already so, owning up to it doesn't make it worse. Got that, Draco? I'm going to make you memorize it later. It's something you repeat to yourself any time you start wondering if it's a good idea to believe something that isn't actually true. In fact I want you to say it right now. What's true is already so, owning up to it doesn't make it worse. Say it."

"What's true is already so," repeated Draco, his voice trembling, "owning up to it doesn't make it worse."

"If magic is fading, I want to believe that magic is fading. If magic is not fading, I want not to believe that magic is fading. Say it."

Draco repeated back the words, the sickness churning in his stomach.

"Good," Harry said, "remember, it might not be happening, and then you won't have to believe it, either. First we just want to know what's actually going on, which world we actually live in." Harry turned back to his work, scribbled some more, and then turned the parchment so Draco could see it. Draco leaned over the desk and Harry brought the green light closer.

Observation:

Wizardry isn't as powerful now as it was when Hogwarts was founded.

Hypotheses:

1. Magic itself is fading.

2. Wizards are interbreeding with Muggles and Squibs.

3. Knowledge to cast powerful spells is being lost.

4. Wizards are eating the wrong foods as children, or something else besides blood is making them grow up weaker.

5. Muggle technology is interfering with magic. (Since 800 years ago?)

6. Stronger wizards are having fewer children. (Draco = only child? Check if 3 powerful wizards, Quirrell / Dumbledore / Dark Lord, had any children.)

Tests:

"All right," Harry said. His breathing sounded a little calmer. "Now when you're dealing with a confusing problem and you have no idea what's going on, the smart thing to do is figure out some really simple tests, things you can look at right away. We need fast tests that distinguish between these hypotheses. Observations that would come out a different way for at least one of them compared to all the other ones."

Draco stared at the list in shock. He was suddenly realizing that he knew an awful lot of purebloods who were only children. Himself, Vincent, Gregory, practically everyone. The two most powerful wizards everyone talked about were Dumbledore and the Dark Lord and neither had any children just like Harry had suspected...