"Magic
Harry stopped scribbling and looked up. His face had an angry expression. "Your father never told you that life isn't fair?"
Father had said that every single time Draco used the word. "But, but, it's too awful to believe that -"
"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:
"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
"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
"It's going to be really hard to distinguish between 2 and 6," Harry said, "it's in the blood either way, you'd have to try and track the decline of wizardry and compare that to how many kids different wizards were having and measure the abilities of Muggleborns compared to purebloods..." Harry's fingers were tapping nervously on the desk. "Let's just lump 6 in with 2 and call them the blood hypothesis for now. 4 is unlikely because then everyone would notice a sudden drop when the wizards switched to new foods, it's hard to see what would've changed steadily over 800 years. 5 is unlikely for the same reason, no sudden drop, Muggles weren't doing anything 800 years back. 4 looks like 2 and 5 looks like 1 anyway. So mainly we should be trying to distinguish between 1, 2, and 3." Harry turned the parchment to himself, drew an ellipse around those three numbers, turned it back. "Magic is fading, blood is weakening, knowledge is disappearing. What test comes out differently depending on which of those is true? What could we see that would mean any one of these was false?"
"
"Draco," Harry said, a note of pleading desperation in his voice, "I only know what Muggle scientists know! You grew up in the wizarding world, I didn't! You know more magic than I do and you know more
Draco swallowed hard and stared at the paper.
Magic is fading... wizards are interbreeding with Muggles... knowledge is being lost...
"What does the world look like if magic is fading?" said Harry Potter. "You know more about magic, you should be the one guessing not me! Imagine you're telling a story about it, what happens in the story?"
Draco imagined it. "Charms that used to work stop working."
"What does the world look like if the wizarding blood gets weaker?"
"People can't do things their ancestors could do."
"What does the world look like if knowledge is being lost?"
"People don't know how to cast the Charms in the first place..." said Draco. He stopped, surprised at himself. "That's a test, isn't it?"