Harry Potter and the Methods of Rationality

'I, I...' Draco said. His voice was breaking. 'I can't do this Harry, it's too much for me.'

Harry's look was fierce. 'Yes you can, because you have to. I thought about it myself, too, after I found out about the Interdict of Merlin. Draco, is there any way of observing the strength of magic directly? Some way that doesn't have anything to do with wizards' blood or the spells we learn?'

Draco's mind was just blank.

'Anything that affects magic affects wizards,' said Harry. 'But then we can't tell if it's the wizards or the magic. What does magic affect that isn't a wizard?'

'Magical creatures, obviously,' said Draco without even thinking about it.

Harry Potter slowly smiled. 'Draco, that's brilliant.'

It's the sort of dumb question you'd only ask in the first place if you'd been raised by Muggles.

Then the sickness in Draco's stomach got even worse as he realized what it would mean if magical creatures were getting weaker. They would know for certain then that magic was fading, and there was a part of Draco that was already sure that was exactly what they would find. He didn't want to see this, he didn't want to know...

Harry Potter was already halfway to the door. 'Come on, Draco! There's a portrait not far from here, we'll just ask them to go get someone old and find out right away! We're cloaked, if someone sees us we can just run away! Let's go!'

It didn't take long after that.

It was a wide portrait, but the three people in it were looking rather crowded. There was a middle-aged man from the twelfth century, dressed in black swathes of cloth; who spoke to a sad-looking young woman from the fourteenth century, with hair that seemed to constantly frizz about her head as if she'd been charged up by a static spell; and she spoke to a dignified, wizened old man from the seventeenth century with a solid gold bowtie; and him they could understand.

They had asked about Dementors.

They had asked about phoenixes.

They had asked about dragons and trolls and house elves.

Harry had frowned, pointed out that creatures which needed the most magic could just be dying out entirely, and had asked for the most powerful magical creatures known.

There wasn't anything unfamiliar on the list, except for a species of Dark creature called mind flayers which the translator noted had finally been exterminated by Harold Shea, and those didn't sound half as scary as Dementors.

Magical creatures were as powerful now as they'd ever been, apparently.

The sickness in Draco's stomach was easing, and now he just felt confused.

'Harry,' Draco said in the middle of the old man translating a list of all eleven powers of a beholder's eyes, 'what does this mean?'

Harry held up a finger and the old man finished the list.

Then Harry thanked all the portraits for helping - Draco, pretty much on automatic, did so as well and more graciously - and they headed back to the classroom.

And Harry brought out the original parchment with the hypotheses, and began scribbling.

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:

A. Are there spells we know but can't cast (1 or 2) or are the lost spells no longer known (3)? Result: Inconclusive due to Interdict of Merlin. No known uncastable spell, but could simply have not been passed on.

B. Did ancient first-year students cast the same sort of spells, with the same power, as now? (Weak evidence for 1 over 2, but blood could also be losing powerful wizardry only.) Result: Same level of first-year spells then as now.

C. Additional test that distinguishes 1 and 2 using scientific knowledge of blood, will explain later. Result: There's only one place in the recipe that makes you a wizard, and either you have two papers saying 'magic' or you don't.

D. Are magical creatures losing their powers? Distinguishes 1 from (2 or 3). Result: Magical creatures seem to be as strong as they ever were.

'A failed,' said Harry Potter. 'B is weak evidence for 1 over 2. C falsifies 2. D falsifies 1. 4 was unlikely and B argues against 4 as well. 5 was unlikely and D argues against it. 6 is falsified along with 2. That leaves 3. Interdict of Merlin or not, I didn't actually find any known spell that couldn't be cast. So when you add it all up, it looks like knowledge is being lost.'

And the trap snapped shut.