AN: I don't particularly like Descartes' philosophy. Nor did I like having to write an essay on him, and this was originally some weird sort of revenge but then it turned into... something else completely. This is severely AU, of course - no witches, not only girls need to contract - and you'll have to endure my vague explanations on entropy. Madoka physics don't really make sense anyways, so perhaps my slip-ups can be forgiven.

"The world isn't fair, is it?"

It was a dissonant voice in a day full of tiny disarrays, all leading up to the fact that there was a small, still body in a coffin. René could not bring himself to ignore it. The voice was that of a child, perhaps, young and cheerful despite the words. He closed the door to his room, gently, and turned around to face the window, and the red eyes of the creature that sat upon it.

- time unwinds -

It was a fine day, the sun shining down on the roofs of the houses around him. The sunlight, or the greenery that was moved by the ceaseless wind, did not interest him, though. The letter – dripping with seals and official proclamations – was the only thing he could bring himself to care about, and he retraced the words again, frowning. Galileo's condemnation was a weighty matter, and if the Church took such a stance against him...

Well, his own publications were doomed. He would have to write a letter himself, and request his work be withdrawn before the Church turned its sharp gaze on him. He had so many things left to write, so many ideas, half-drawn out arguments that only needed to be finished and spread through Holland like wildfire.

The world was on the cusp of a new age, old traditions being transformed and purified by reason. It was heady enough to drown in, the thoughts of new systems, rational thinking sweeping throughout all of Europe – so when a cat sat down on the windowsill, he ignored it easily.

Of course, even the heaviest of arguments on the nature of reality could be driven out of one's head if the distraction was potent enough.

"So you're René Descartes!" He looked down, expecting a child, and met the eyes of... a cat. No, not a cat – it was larger, something like a fox or a weasel... but a cat as well. The red eyes were weasel-like, at least, but no animal he had seen looked so... angelic. It had strange wing-like ears – or perhaps wings attached to its ears, it was difficult to say – and it tilted its head as he stared down, its tail moving in a hypnotic, boneless motion. "You're becoming quite renowned, aren't you!"

The mouth didn't move but it was talking to him. He was certain of it – or at least, fairly certain that his senses were not deceiving him in this matter.

"What manner of creature are you?" he asked, and the head tilted again, ears twitching.

"I am Kyubey!"

"This is... all of what you're telling me, of the world and worlds beyond it decaying – can it possibly be true?"

"Can it?" the creature parroted back at him, curling its tail neatly around its paws. "You are well known for your deductions and clear reasoning among the philosophers of this time."

He cast his mind back, cleared his thoughts as best as he could, pushing away the whirlwind of demons and strange pacts, the very concept that the universe he knew was constantly in danger. "I cannot simply rely on what you are telling me. There must be some proof of this." Kyubey twitched its – no, his, for he clearly had intelligence – ears and leaped up from his spot on the floor, landing on the low table and pacing its length soundlessly.

"Do you drink tea, René?" he asked, waiting for his nod before continuing. "I will give you your proof using it, then. You have a cup of tea. This is the universe, and the heat is its energy and life. As long as you continue to pour in hot water, it remains hot! But once you stop, the tea begins to cool, until it is completely cold." He did that strange wave of his tail, back and forth like a pendulum but with all the fluidity of water.

"That does not explain anything," he pointed out, not bothering to wonder how Kyubey could speak perfect French when his mouth was closed all the time. Or how he could speak French. Or speak if he really felt like measuring the absurdity of the situation. "The world has the sun. Why would it suddenly grow cold and die?"

"The sun is a ball of heat and gas. It will eventually flare up and then burn itself out. All things have their ends! The universe subsists on energy. However, that energy always costs more than it produces in turn, and rather than being a cup of tea with water constantly added, it is a cup left alone. Eventually, the energy will dwindle to nothing, and the universe will grow cold. All life will wither and die!" It was a chilling explanation, made more so by Kyubey's utter lack of inflection. He could have been giving polite discourse on the weather.

He shook his head. "But the existence of God –"

"Oh, I was wondering if you could explain why you still cling to that. You are relatively progressive in your arguments, after all."

The previous statements had been confusing, or shocking. This was... a little more than that. "I'm sorry?"

Kyubey swished his tail at him again. "This belief in an infinite being that is the root of all knowledge and does not lie. It seems pointless to assign such empty characteristics to a supposed being outside of time and space. If one existed, why would it have any interest in your world alone?"

And perhaps that was what finally made him truly wary of the strange creature – not its tale of entropy but the epiphany that behind that flat, red gaze there was a cold, vast intellect – and one that did not see the point of the deity or religion that his own arguments was founded on.

"Why do you keep coming back, Kyubey?"

"Because you are an interesting person! Is that not reason enough?"

"I can hardly consider myself interesting when we have debated the same points a hundred times over." But all he received in return was an enigmatic look and a polite congratulations on the reception of one of his works.

"Some people have potential energy," Kyubey confided once, on those erratic visits – sometimes choosing to appear in a room with a closed door and no window, as if purely trying to baffle him. "And many acquire more as they go through their lives. Someone like yourself is constantly collecting it!"

"And what does that potential fulfil?"

"It means that you can see and hear me, when most people can't. It also means you can fight the decay of the universe!" But when he pressed, Kyubey proved to have the same skills as a weasel in getting out of a tight spot, and once again he was left with questions and doubts.

"Kyubey," he said, heavily.

"You have a wish, René." He paused, and then leaped down from the windowsill. "I can grant it."

He stared, unable to muster up surprise, or shock, or anything other than leaden grief. "Bringing back the dead cannot be done."

"If it is your wish," Kyubey said, "it can be granted. You must make a contract with me, and in return... I will bring your daughter back to life. You will be able to share in her coming years. Is that not what you want?"

And it was.

So he wished, and found himself marvelling at the strange trinket he held after the pain had receded, a strange silvery gem. There were other things to marvel at – the hammer he could wield, and the monsters he found himself fighting. His daughter, alive and full of joy and mishaps and the eternal energy of a normal five year old. His work, arguments and philosophy that should have changed the world, was left alone. There were more important things than words and reason, now.

The silver of his soul tarnished, slowly, an inexorable process no matter how many demons and strange monstrosities he slew. In the end, it killed him, and through it all Kyubey watched, witness to the harvest of a man's hopes, and reaper for what came after.

His ideas gathered dust, and history tidily collected itself and continued onward.

And no one ever had to write essays about Descartes. Ever.