Harry Potter and the Methods of Rationality

"Yes," they said or muttered or whispered.

"If you break any of these rules you will not further study Transfiguration during your stay at Hogwarts. Repeat along with me. I will never Transfigure anything into a liquid or gas."

"I will never Transfigure anything into a liquid or gas," said the students in ragged chorus.

"Again! Louder! I will never Transfigure anything into a liquid or gas."

"I will never Transfigure anything into a liquid or gas."

"I will never Transfigure anything that looks like food or anything else that goes inside a human body."

"I will never Transfigure anything that is to be burned because it could make smoke."

"You will never Transfigure anything that looks like money, including Muggle money," said Professor McGonagall. "The goblins have ways of finding out who did it. As a matter of recognised law, the goblin nation is in a permanent state of war with all magical counterfeiters. They will not send Aurors. They will send an army."

"I will never Transfigure anything that looks like money," repeated the students.

"And above all," said Professor McGonagall, "you will not Transfigure any living subject, especially yourselves. It will make you very sick and possibly dead, depending on how you Transfigure yourself and how long you maintain the change." Professor McGonagall paused. "Mr. Potter is currently holding up his hand because he has seen an Animagus transformation - specifically, a human transforming into a cat and back again. But an Animagus transformation is not free Transfiguration."

Professor McGonagall took a small piece of wood out of her pocket. With a tap of her wand it became a glass ball. Then she said "Crystferrium!" and the glass ball became a steel ball. She tapped it with her wand one last time and the steel ball became a piece of wood once more. "Crystferrium transforms a subject of solid glass into a similarly shaped target of solid steel. It cannot do the reverse, nor can it transform a desk into a pig. The most general form of Transfiguration - free Transfiguration, which you will be learning here - is capable of transforming any subject into any target, at least so far as physical form is concerned. For this reason, free Transfiguration must be done wordlessly. Using Charms would require different words for every different transformation between subject and target."

Professor McGonagall gave her students a sharp look. "Some teachers begin with Transfiguration Charms and move on to free Transfiguration afterwards. Yes, that would be much easier in the beginning. But it can set you in a poor mold which impairs your abilities later. Here you will learn free Transfiguration from the very start, which requires that you cast the spell wordlessly, by holding the subject form, the target form, and the transformation within your own mind."

"And to answer Mr. Potter's question," Professor McGonagall went on, "it is free Transfiguration which you must never do to any living subject. There are Charms and potions which can safely, reversibly transform living subjects in limited ways. An Animagus with a missing limb will still be missing that limb after transforming, for example. Free Transfiguration is not safe. Your body will change while it is Transfigured - breathing, for example, results in a constant loss of the body's stuff to the surrounding air. When the Transfiguration wears off and your body tries to revert to its original form, it will not quite be able to do so. If you press your wand to your body and imagine yourself with golden hair, afterwards your hair will fall out. If you visualise yourself as someone with clearer skin, you will be taking a long stay at St. Mungo's. And if you Transfigure yourself into an adult bodily form, then, when the Transfiguration wears off, you will die."

That explained why he had seen such things as fat boys, or girls less than perfectly pretty. Or old people, for that matter. That wouldn't happen if you could just Transfigure yourself every morning... Harry raised his hand and tried to signal Professor McGonagall with his eyes.

"Yes, Mr. Potter?"

"Is it possible to Transfigure a living subject into a target that is static, such as a coin - no, excuse me, I'm terribly sorry, let's just say a steel ball."

Professor McGonagall shook her head. "Mr. Potter, even inanimate objects undergo small internal changes over time. There would be no visible changes to your body afterwards, and for the first minute, you would notice nothing wrong. But in an hour you would be sick, and in a day you would be dead."

"Erm, excuse me, so if I'd read the first chapter I could have guessed that the desk was originally a desk and not a pig," Harry said, "but only if I made the further assumption that you didn't want to kill the pig, that might seem highly probable but -"

"I can foresee that marking your tests will be an endless source of delight to me, Mr. Potter. But if you have other questions can I please ask you to wait until after class?"

"No further questions, professor."

"Now repeat after me," said Professor McGonagall. "I will never try to Transfigure any living subject, especially myself, unless specifically instructed to do so using a specialised Charm or potion."

"If I am not sure whether a Transfiguration is safe, I will not try it until I have asked Professor McGonagall or Professor Flitwick or Professor Snape or the Headmaster, who are the only recognised authorities on Transfiguration at Hogwarts. Asking another student is not acceptable, even if they say that they remember asking the same question."

"Even if the current Defence Professor at Hogwarts tells me that a Transfiguration is safe, and even if I see the Defence Professor do it and nothing bad seems to happen, I will not try it myself."

"I have the absolute right to refuse to perform any Transfiguration about which I feel the slightest bit nervous. Since not even the Headmaster of Hogwarts can order me to do otherwise, I certainly will not accept any such order from the Defence Professor, even if the Defence Professor threatens to deduct one hundred House points and have me expelled."

"If I break any of these rules I will not further study Transfiguration during my time at Hogwarts."

"We will repeat these rules at the start of every class for the first month," said Professor McGonagall. "And now, we will begin with matches as subjects and needles as targets... put away your wands, thank you, by 'begin' I