Title: constancy, with all its parts replaced
Rating: PG-13
Author: Serendipity
Pairing: Don/Leo
Notes: Wow, this is...very philosophical. Donatello's a bit fixated on Leonardo. Sort of existiential. Involves the Theseus' Ship paradox, musings, and jaded!Don.

(It doesn't start with anything drastic; he thought it should feel like the world tilting on its axis, or gravity increasing, or his internal organs shifting to make room for something. It starts small, like a single heartbeat, and grows.)

"The Theseus' Ship paradox," he says once, "asks whether something can be considered the same object if all of its components have been replaced. If Theseus' ship has had a board replaced, it's still considered the same ship. If it has had its sails and a piece of its bow replaced, it is considered the same. But what if it was entirely replaced, piece by piece? Would the ship that remained be considered Theseus' ship? And if so, what if the pieces of the original ship were compiled into an entirely new ship? Would that be Theseus' ship, and the redone vessel a false one? Which is the original ship of Theseus?"

Leonardo barely pauses from his kata. "Neither."

If neither is the ship, he wonders, is there anything with solidity or did things exist only in flux? If the existence of a thing is so tenuous it can be changed with a simple chip, a break, a crack, then how can anything be considered stable? But he turns back to his book and says nothing.

A little over seventeen years ago, he thinks, four infant red-eared sliders were exposed to an unknown mutagen and that was how their lives began. Not at conception but at infection. Those turtles, he wonders, were they those turtles or did they die once the mutagen touched their skin? Did the caterpillar cease to exist once it became the chrysalis?

(This only ever existed if it is composed of the same pieces from t1 as it was to t2, and here at t2 he definitely feels disassembled and reconstructed. So is it dead? Or was it never truly there?)

It doesn't start so much as peak- he thinks maybe it was a building pressure inside him and is just now showing itself. It's a sudden shift in perception. One day he is suddenly aware of his brother as a sexual being and he wants to be a part of that, and is that completely wrong or is the morality more subjective than that?

The needle is a fragile semicircle in his hand, and he stitches the skin like he was taught to- his fingers are the most deft, so he usually does the sutures. Leonardo hisses through his teeth and his muscles twitch every time the needle breaks the skin and the thread pulls tight. He's closing up the slashes in his brother's arm, pulling his skin together and making him whole again. When he breathes harshly and stiffens, twitches under his hands, Donatello lays a hand on his shoulder and steadies him.

"It's almost finished," he tells him quietly. He wants to do more in that moment, and the want makes him work faster.

Maturing is like being under constant bombardment in an earthquake zone: once you become used to the explosions above, you deal with the ground under your feet shifting and shaking apart, until everything becomes a different landscape altogether. This is just another little earthquake.

He thinks of his brother's breath later, hitching and hissing through his teeth, and his neck muscles tightening along with his jaw, and wonders if the people alongside a person shift along with them, or if a person has no impact on his family as he drags himself through the change to young adulthood. He wonders if family is the same, but they seem different through his distorted lenses, or if they are growing alongside him, or if he is shifting too quickly in such an odd direction that none of them have noticed him break away.

And if this is some kind of psychological disease, he wonders not if there's a cure or an antidote, but if he can continue on this way without breaking. He wants to study it.

(If a thing is identified by its parts and its parts identified by what they belong to, then he is at a loss to describe what this is. It has no parts that remain static, only fleeting thoughts and feelings and an ever-erratic heartbeat and respiratory system.)

At some point he makes the following observations.

One: that he might be attracted to one brother and it doesn't follow that he's attracted to them all, or that his attraction to one is completely physical. In fact, it seems nonphysical in that it's not the body he focuses on, and he's not even sure how much of his brother's personality does it to him. Is he attracted to a concept?

Two: that he is not losing his mind or even key pieces of his personality because of this. That this is almost a non-issue, and no one notices anything.

Three: that Leonardo has a strange fondness for peppermint: gum or tea, and the scent of it is on him occasionally. He takes a piece of peppermint gum and chews it while he fixes the Battleshell, the thick smell of grease and oil mixing with the cool sharpness of mint, and he can't think of anything but his brother the whole time.

Four: that emotions and attachments can change so quickly that it leaves vestiges of the original relationship behind. That 'brother' and 'leader' and this new other feeling can clash against each other and overlap at points, leaving obvious traces of where each ends and begins.

Dreaming, he thinks of connections lost and ones made again, of sparks shooting out from two quickly-moving objects brushing together, and colliding stars and how bright they burn. About forming black holes, and what could be pulled into the void and what could ever be left behind. Then he wakes up in the morning and smiles about breakfast and says good morning to Leonardo, good morning and looks into his eyes for sparks and the collision of stars.

(Maybe he can't analyze this. Maybe it's like trying to pin down water to a chart, with the river constantly renewing itself with each test he makes.)

Donatello catches him in quiet moments, sometimes.

He's meditating now, candle glow on his skin and he could swear he's breathing to the rhythm of the flickering flames. His eyes are closed, so there's no way he can see him there, watching on the edge of his perception like he's on the outskirts of some protective circle.

Leonardo is like a magnet drawing him in, and Donatello is all iron and no will to oppose this. If he moves the slightest bit closer, he could reach out and touch his shoulder with the brush of a hand, but he leaves it at this impasse- one side unaware and the other too aware, and he's aching with the power of that knowledge.

Something creaks, either Raphael toying with a motorcycle part or Michelangelo opening a door, but it's enough to make Leonardo stiffen, muscles drawing tense, and open his eyes.

"It's nothing," Donatello tells him, like he was just on the way to his computer, just passing through, and walks the other way. He feels the pull as he goes, inextricably bound to his heartbeat, tugging with every footstep.

This is nothing like he never thought it wouldn't be.

Leonardo won't push him to speak about a problem, but is quietly there nonetheless. "Don," he says, then falls silent because really, there's nothing different about his habits or even his attitude lately. He's made sure of it. He supposes Leonardo simply intuits a difference, or catches one in the air. He wonders if this is possible- is it like a strange scent? Pleasant, or off-putting, or simply out-of-place, like peppermint on the tongue while breathing in the smell of oil and grease and rubber?

Donatello waits quietly, even patiently, for the question he knows will never come.

(And this is like and unlike how it's always been. He turns to his brother to make decisions, supports him as the other levelheaded member of their family, joins him in philosophical debate- but then the little changes make the difference, and the entirety of it is thrown askew. It might right itself, it might not- will he live his life without equilibrium? )

If this isn't the worst thing he could have done to his brother, it certainly ranks among the highest.

Or so he thinks, when he is still at the computer at three AM mainly because he knows that if he stays here too long, Leonardo will come and check up on him. He's done this for at least two weeks now- not just staying long at the computer, but other more subtle ways of getting time alone with Leonardo. He's spoken to him more, or joined him in more training, requested him for games of chess or go.

Very slowly, he's built this up layer-by layer, and what it's meant to become at the end is beyond his comprehension. Sometimes he doesn't know if it's even meant to, if he's trying to create a perpetual balance between acknowledging this and keeping it hidden. Maybe he's been subconsciously sabotaging it from the beginning, and there's no way to tell but with the hindsight that comes from years after the fallout.

He tells himself it won't harm anyone because the other party is unaware of the root of his behavior, and in fact it probably makes his brother happier, since he's been acting much more interested in the whole business of ninjitsu lately, along with anything and everything Leo-centric that could make up a conversation. Therefore, he's decided, he's actually strengthening the sibling bond.

Or at least he would have been if the entire foundation wasn't there with ulterior motives. He won't lie to himself about wanting his brother, but everyone else is fair game. This is a temporary phase, he's decided, and he's got enough self control to keep it from going too far.

There's no need to cause any alarm, there's no need to panic, because he can take care of it entirely on his own.

Donatello thinks of it something like a drug, despite the inherent dangers with that analogy. Maybe not a drug, he revises, more like alcohol. It could be intoxicating, addictive, even dangerous when taken too far, but in moderation it is harmless.

He takes very small doses of interaction over a long period of time and checks the effects. Analyzes what a talk with Leonardo on the subject of the book he's reading does to his heartbeat, decides the quantity, the degree of his emotions. It's still not at a dangerous level, he decides, and his brother is not affected by it.

There's something intrinsically wrong with this entire situation. He knows it, but continues anyway. It's a careful balancing act, everything carefully chosen and set into motion with the consequences fully considered. He's barely touched Leo yet. He wonders what might happen if he did.

(And he doesn't take it further than he has; touches that last a little longer than necessary, with a sense of impact they shouldn't carry. He wonders if it has to be Leonardo, or if could have been Michelangelo, or Raphael, or even his own reflection. If it had to be a person. He thinks it did, he thinks it's something unique about Leonardo that causes this in him, but can't pin down the root of the issue. Maybe it's a myriad of tiny pieces, disassemble them and come up with a different brother, or a woman, or maybe, poetically, the moon. Or maybe all are one and the same when it comes to this. Maybe the emotion is as fleeting and replaceable as boards or sails.)

And then he does touch him, making it linger. Leonardo looks at him strangely, but he shakes his head like it's nothing. Just an adjustment, or his injury needed extra tending. Donatello continues stitching blithely, remembering. He could be one in an unlimited number of moments- thousands of reflections of him playing physician to an injured brother, cast out and reflected endlessly in a single mirror. It's that familiar, except now there's the heat that shouldn't be there and the lingering feeling of skin against his fingertips.

This isn't wrong, he thinks, but it also isn't right.

The stitches go in and he thinks about the two ships, and of Theseus with neither or both of them, and how one could continue as the other and how the other could have been destroyed in favor of the first and then resurrected as something new.

"Sometimes I'd like to know what you're thinking," Leonardo says when the wound is bandaged. "You have such a distant look in your eyes lately."

"Constancy," he says. "And how even stability is in flux."

He could explain this, he thinks. He doesn't.