“Would it matter, if I was a changeling?”
Twilight stopped, carefully put down her fork, and unhurriedly finished chewing. She swallowed, then looked across the breakfast table at Starlight's (of course) guest.
“Thorax,” she said, then paused. “Are you... not a changeling?”
“No, I am.”
“I see.” Another pause. “So... have you been enjoying your visit so far?”
“Oh, yes; Starlight and I have had some very interesting conversations!”
“Of course you have,” Twilight informed her pancakes in a mutter.
“Pardon?”
“I said, friends are nice to have!” answered Twilight brightly. Took another bite. Chewed, optimistically.
“But would it matter?”
Sighed, less optimistically. Fork down. Swallow.
“Thorax, given that you are a changeling, shouldn't that be an easy question to answer?”
“How do you mean?”
“Well, can't you just look back and see if it did matter?”
“But how would I know?”
“You-- Crystal Empire, love, Chrysalis, king,” Twilight said with a vaguely waving hoof, “colors Rarity swears she likes just as much as our Rainbow Power forms, but now that I think of it, she did have this weird look on her--”
“But!” said Starlight, walking into the room and taking her seat. “Does all of that depend on him being a changeling? Would any changeling in Thorax's circumstances have done the same? If Thorax had been born a crystal pony in the old Empire, would he have taken over from Sombra?”
Thorax nodded. Twilight wanted to think that she now had some idea what was going on, but this was also clearly one of those days.
“So... this is a nature vs. nurture thing, then? Is Thorax--”
“Um, I'm actually still right here.”
“--inherently Thorax, or is he nothing more than the sum of his circumstances?”
“Right!” said Starlights in chorus. Twilight didn't blink.
“Okay. Yes, yes it would matter, because it's your exact set of past circumstances that led to you sitting here, today, as Starlight, asking me that question.”
“I asked you before I turned into Starlight, though.”
“Whatever. The point is, we would not be in this exact situation if you were not a changeling. Therefore, any universe containing this situation must also have you born a changeling, and therefore you being a changeling is fundamental to the very existence of our universe itself.”
Twilight happily took another bite of her pancakes. The Starlights looked at each other. They looked at Twilight.
“I think that's kind of dodging the question?” one of them said.
“Any other changeling--” began the other.
“Nope! Any other changeling sitting here would not be this changeling sitting here, therefore different universe.”
“But you can explain anything that way.”
“But I'm not trying to explain anything,” said Twilight. “I'm trying to explain why a specific individual's circumstances matter without also denying that individual some inherent worth and uniqueness. The only reality we personally experience is the one we're in, and therefore everything bringing about that reality matters to us, whatever hypothetical other sets of events we might talk about.”
“Or bring about through the misuse of time travel.”
“Or that, yes,” said Twilight with the casual air of one agreeing that perhaps that smudge on the weather schedule was covering the word “rain” after all. “The point is that, since you are a changeling, Thorax, whichever of you is Thorax--” One of the Starlights raised a hoof. “Thank you. --since you are a changeling, it matters that you're a changeling; if you were a pony, it would matter that you were a pony, and either way, you'd still be you, whatever “you” means in that particular reality.”
Thorax-Starlight frowned. “Wait, but about the misuse of time travel, doesn't that change the reality you're in? So that whatever new events occur, then those are the ones that matter?”
“Oh, no,” said Twilight. “You see, there's only a single correct timeline in our universe. There's actually a magical test to see whether something is part of it or an abomination of magic that shouldn't exist no matter how much it pleads that it feels real to itself and it has hopes and dreams and feelings and everything. I first learned about it from alternate-Zecora in one of those false timelines; she used it to tell me that over a decade of her and her friends' lives were transgressions against the universe that should be wiped away as soon as possible.”
Stares.
“No, really; I looked it up when I got back. Not fully understood, a few other hypotheses on how it works, but, heh, I mean, if that wasn't true, it could be argued that Starlight and I caused millions of casualties, including all our friends, over and over again, as we battled across time.”
Starlight swallowed. “Right! So... good thing we live in a universe with only one really real timeline, then!”
Thorax-as-Starlight looked between them. Cleared his-as-her throat. “So... it matters that I'm a changeling, then, because the particular person I am, and you both know, is a changeling, and any other hypotheticals are less important than the reality we live in?”
“Yes!” said Twilight, definitely not shivering in relief at the change in subject. “The question of whether you're a unique individual in circumstances or just a collection of your circumstances is a meaningless one, because the only way to separate the two is through the creation of a hypothetical in which something about you is different. Any you that is different in any way is not you, though, and any you in front of me, or in your case, that you're experiencing the world as, takes priority over even an almost-identical you who's only hypothetical.”
Starlight-Starlight shook her head. “But if everything that exists matters just _because_ it exists, doesn't that mean that nothing really--”
And then Starlight and Thorax and Twilight's other friends and Applejack burst in, and something of a scuffle ensued. From her new position on her back where her chair had fallen, Twilight sighed and pushed the pancakes off her face. “SPIKE! WHERE DID YOU PUT THE KEYS TO THE WINE CELLAR? WE'RE GOING TO NEED THEM AGAIN!”