Harry Potter and the Methods of Rationality

Her wand came down and tapped her desk, which smoothly reshaped itself into a pig. A couple of Muggleborn students gave out small yelps. The pig looked around and snorted, seeming confused, and then became a desk again.

The Transfiguration Professor looked around the classroom, and then her eyes settled on one student.

"Mr. Potter," said Professor McGonagall. "You only received your schoolbooks a few days ago. Have you started reading your Transfiguration textbook?"

"No, sorry professor," Harry said.

"You needn't apologise, Mr. Potter, if you were required to read ahead you would have been told to do so." McGonagall's fingers rapped the desk in front of her. "Mr. Potter, would you care to guess whether this is a desk which I Transfigured into a pig, or if it began as a pig and I briefly removed the Transfiguration? If you had read the first chapter of your textbook, you would know."

Harry's eyebrows furrowed slightly. "I'd guess it'd be easier to start with a pig, since if it started as a desk, it might not know how to stand up."

Professor McGonagall shook her head. "No fault to you, Mr. Potter, but the correct answer is that in Transfiguration you do not care to guess. Wrong answers will be marked with extreme severity, questions left blank will be marked with great leniency. You must learn to know what you do not know. If I ask you any question, no matter how obvious or elementary, and you answer 'I'm not sure', I will not hold it against you and anyone who laughs will lose House points. Can you tell me why this rule exists, Mr. Potter?"

Because a single error in Transfiguration can be incredibly dangerous. "No."

"Correct. Transfiguration is more dangerous than Apparition, which is not taught until your sixth year. Unfortunately, Transfiguration must be learned and practised at a young age to maximise your adult ability. So this is a dangerous subject, and you should be quite scared of making any mistakes, because none of my students have ever been permanently injured and I will be extremely put out if you are the first class to spoil my record."

Several students gulped.

Professor McGonagall stood up and moved over to the wall behind her desk, which held a white wooden board. "There are many reasons why Transfiguration is dangerous, but one reason stands above all the rest." She produced a marker seemingly from thin air, and sketched letters in bright red; which she then underlined, using the same marker, in blue:

TRANSFIGURATION IS NOT PERMANENT!

"Transfiguration is not permanent!" said Professor McGonagall. "Transfiguration is not permanent! Transfiguration is not permanent! Mr. Potter, suppose a student Transfigured a block of wood into a cup of water, and you drank it. What do you imagine might happen to you when the Transfiguration wore off?" There was a pause. "Excuse me, I should not have asked that of you, Mr. Potter, I forgot that you are blessed with an unusually pessimistic imagination -"

"I'm fine," Harry said, swallowing hard. "So the first answer is that I don't know," the Professor nodded approvingly, "but I imagine there might be... wood in my stomach, and in my bloodstream, and if any of that water had gotten absorbed into my body's tissues - would it be wood pulp or solid wood or..." Harry's grasp of magic failed him. He couldn't understand how wood mapped into water in the first place, so he couldn't understand what would happen after the water molecules were scrambled by ordinary thermal motions and the magic wore off and the mapping reversed.

McGonagall's face was stiff. "As Mr. Potter has correctly reasoned, he would become extremely sick and require immediate Flooing to St. Mungo's Hospital if he was to have any chance of survival. Please turn your textbooks to page 5."

Even without any sound in the moving picture, you could tell that the woman with horribly discolored skin was screaming.

"The criminal who originally Transfigured gold into wine and gave it to this woman to drink, 'in payment of the debt' as he put it, received a sentence of ten years in Azkaban. Please turn to page 6. That is a Dementor. They are the guardians of Azkaban. They suck away at your magic, your life, and any happy thoughts you try to have. The picture on page 7 is of the criminal ten years later, on his release. You will note that he is dead - yes, Mr. Potter?"

"Professor," Harry said, "if the worst happens in a case like that, is there any way of maintaining the Transfiguration?"

"No," Professor McGonagall said flatly. "Sustaining a Transfiguration is a constant drain on your magic which scales with the size of the target form. And you would need to recontact the target every few hours, which is, in a case like this, impossible. Disasters like this are unrecoverable!"