Harry Potter and the Methods of Rationality

(From a Muggle standpoint it was just odd, a deranged version of thermodynamics invented by someone who thought life ought to be fair. From a Muggle standpoint, the heat expended in forging the Knut hadn't gone into the bronze, the heat had left and dissipated into the environment, becoming permanently less available. Energy was conserved, could be neither created nor destroyed; entropy always increased. But wizards didn't think that way: from their perspective, if you'd put some amount of work into making a Knut, it stood to reason that you could get exactly the same work back out. Harry had tried to explain why this sounded a bit odd if you'd been raised by Muggles, and Professor McGonagall had asked bemusedly why the Muggle perspective was any better than the wizarding one.)

The fundamental principle of Potions-Making had no name and no standard phrasing, since then you might be tempted to write it down.

And someone who wasn't wise enough to figure out the principle themselves might read it.

And they would start having all sorts of bright ideas for inventing new Potions.

And then they would be turned into catgirls.

It had been made very clear to Harry that he wasn't going to be sharing this particular discovery with Neville, or Hermione either after the next armies' battle. Harry had tried to say something about Hermione seeming really off lately and this being just the sort of thing that might cheer her up. Professor McGonagall had said flatly that he wasn't even to think it, and Professor Flitwick had raised his little hands and made a gesture as of snapping a wand in half.

Although the two Professors had been kind enough to suggest that if Mr. Potter thought he knew what the potion's ingredients should be, he might be able to find an already-existing recipe that did the same thing; and Professor Flitwick had mentioned several volumes in the Hogwarts library that might be useful...

The vast parchment-like screen now showed only an aerial view of the forest, from which you could barely make out the camouflaged forms of three armies, split up into two groups each, converging to fight their three-way battle.

The benches of the Quidditch stadium were now rapidly filling up with the more easily bored sort of spectator who only wanted to be there for the final battle and skip out on all the boring points along the way. (If there was anything wrong with Professor Quirrell's battles, it was widely agreed, it was that his spectacles didn't last nearly as long as Quidditch matches, once they actually started. To this Professor Quirrell had replied only, Such is real life, and that had been that.)

Within the huge window - it was all one window now, observing from a great height - the vague collections of tiny camouflaged forms grew closer.

Closer.

Almost touching -

The vast white parchment window showed the first touch of battle between Sunshine and Chaos, a screaming mass of running children with smiley-faces upon their breasts, charging forward with Contego shields held high and others shouting "Somnium!" -

Until one of their number shrieked "Prismatis!" in a terrified voice and the entire charge came to a sudden halt before the sparkling wall of force that had appeared in front of them.

Tracey Davis had walked out from behind the trees.

"That's right," said Tracey, her voice low and grim as she leveled her wand on the barrier. "You should fear me. For I am Tracey Davis, the Darke Lady! That's Darke Lady spelled D-A-R-K-E, with an E!"

(Amelia Bones, Director of the Department of Magical Law Enforcement, was sending an inquiring look at Mr. and Mrs. Davis, both of whom looked like they would have dearly preferred to die on the spot.)

Behind the Prismatic Barrier, there was some kind of hushed argument taking place among the Sunshine Soldiers, one of whom in particular seemed to be getting scolded by several of the others.

Then, a moment later, Tracey flinched.

Susan Bones had come to the front of the Sunshine contingent.

("Goodness," said Augusta Longbottom. "What do you suppose your grand-niece has been learning at Hogwarts?")

("I don't know," Amelia Bones said calmly, "but I shall owl her a Chocolate Frog and instructions to learn more of it.")

The Prismatic Barrier vanished.

The Sunshine Soldiers resumed their charge forward.

Tracey yelled, her voice high with strain, "Inflammare!" and the Sunshine charge came to another sudden halt as a line of fire blazed up between them in the half-dry grass, extending to follow the path of Tracey's wand as she pointed it; an instant later Susan Bones cried "Finite Incantatem!" and the flames dimmed, brightened, dimmed in the contest of their wills, other soldiers raising their wards to aim at Tracey; and that was when Neville Longbottom plunged shrieking out of the sky.