Harry Potter and the Methods of Rationality

Point two: The hypothesis was that the Defense Professor was planning to do something evil, and in that subjunctive case, Harry ought to be helping the Headmaster prevent it.

Point three...

"Headmaster," Harry said, "if Professor Quirrell is up to something, I'm not sure I can outwit him. He's got a lot more experience than I do."

The old wizard shook his head, somehow managing to appear very solemn despite his smile. "You underestimate yourself."

That was the first time anyone had ever said that to Harry.

"I remember," the old wizard continued, "a young man in this very office, cold and controlled as he faced down the Head of House Slytherin, blackmailing his own Headmaster to protect his classmates. And I believe that young man is more cunning than Professor Quirrell, more cunning than Lucius Malfoy, that he will grow to be the equal of Voldemort himself. It is he who I wish to consult."

Harry suppressed the chill that went through him at the name, frowned thoughtfully at the Headmaster.

How much does he know...?

The Headmaster had seen Harry in the grip of his mysterious dark side, as deep as Harry had ever sunk into it. Harry still remembered what it had been like to watch, invisibly Time-Turned, as his past self faced down the older Slytherins; the boy with the scar on his forehead who didn't act like the others. Of course the Headmaster would have noticed something odd about the boy in his office...

And Dumbledore had concluded that his pet hero had cunning to match his destined foe, the Dark Lord.

Which wasn't asking for very much, considering that the Dark Lord had put a clearly visible Dark Mark on all of his servants' left arms, and that he'd slaughtered the entire monastery that taught the martial art he'd wanted to learn.

Enough cunning to match Professor Quirrell would be a whole different order of problem.

But it was also clear that the Headmaster wouldn't be satisfied until Harry went all cold and darkish, and came up with some sort of answer that sounded impressively cunning... which had better not actually get in the way of Professor Quirrell's teaching Defense...

And of course Harry would go over to his dark side and think it through from that direction, just to be honest, and just in case.

"Tell me," Harry said, "everything about how the Dementor is to be brought in, and how it is to be guarded."

Dumbledore's eyebrows rose for a moment, and then the old wizard began to speak.

The Dementor would be transported to the grounds of Hogwarts by an Auror trio, all three personally known to the Headmaster, and all three able to cast a corporeal Patronus Charm. They would be met at the edge of the grounds by Dumbledore, who would pass the Dementor through the Hogwarts wards -

Harry asked if the pass was permanent or temporary - whether someone could just bring in the same Dementor again the next day.

The pass was temporary (replied the Headmaster with an approving nod), and the explanation went on: The Dementor would be in a cage of solid titanium bars, not Transfigured but true-forged; in time a Dementor's presence would corrode that metal to dust, but not in a single day.

Students awaiting their turn would stay well back of the Dementor, behind two corporeal Patronuses maintained by two of the three Aurors at any given time. Dumbledore would wait by the Dementor's cage with his Patronus. A single student would approach the Dementor; and Dumbledore would dispel his Patronus; and the student would attempt to cast their own Patronus Charm; and if they failed, Dumbledore would restore his Patronus before the student could suffer any permanent damage. Past dueling champion Professor Flitwick would also be present while there were students near, just to add safety margin.

"Why just you waiting by the Dementor?" said Harry. "I mean, shouldn't it be you plus an Auror -"

The Headmaster shook his head. "They could not withstand the repeated exposure to the Dementor, each time I dispel my Patronus."

And if Dumbledore's Patronus did fail for some reason, while one of the students was still near the Dementor, the third Auror would cast another corporeal Patronus and send it to shield the student...

Harry poked and prodded, but he couldn't see a flaw in the security.

So Harry took a deep breath, sank further into the chair, closed his eyes, and remembered:

"And that will be... five points? No, let us make it an even ten points from Ravenclaw for backchat."

The cold came more slowly now, more reluctantly, Harry hadn't been calling much on his dark side lately...

Harry had to run through that entire session in Potions in his mind, before his blood chilled into something approaching deadly crystalline clarity.

And then he thought of the Dementor.

And it was obvious.

"The Dementor is a distraction," Harry said. The coldness clear in his voice, since that was what Dumbledore wanted and expected. "A large, salient threat, but in the end straightforward, and easy to defend against. So while all your attention is focused on the Dementor, the real plot will be happening elsewhere."

Dumbledore stared at Harry for a moment, and then gave a slow nod. "Yes..." said the Headmaster. "And I do believe I know what it might be a distraction from, if Professor Quirrell means ill... thank you, Harry."