muggy_mountain asked three FFX questions on her LJ, including, "What happens to the Unsent when they die? Phoenix Down works on them, but...aren't they dead?"
Disclaimer: The following essay is uber-geeking, based on the premise that the game is presenting consistent, fully-thought-out canon, not simply things that look cool or are convenient for gameplay.
Maechen: The Al Bhed have a theory, you know. They say the pyreflies are just reacting to visitors' thoughts and dreams. But only the dead appear on the Farplane. No image of the living has ever been seen. It's a great mystery! But maybe...Maybe the dead leave a bit of themselves in the hearts of the living. And that little bit borrows the pyreflies' power for their paranormal performance! Or maybe not. Who knows?
[...]They may be called "pyreflies" but they aren't really "flies," you see. They're those lights you see whenever a fiend dies. The little fellows are responsible for a few fantastic phenomena. Visions of the past, spheres, fiends-- these are all the pyreflies' doing. In fact... pyreflies have something to do with aeons, too. The dreams of the fayth reach through the spirit of the summoner... And that which is unreal becomes real for all to see! Or maybe not. Who knows?
With a Dose of Jungian Psychology
There are a number of non-living states of existence in the world of Spira, most of which seem to be sustained by magical, animate energy particles called pyreflies. Or, as I think of them, "Mitichlorians with a glue gun." They translate psychic images* into visible images, or, with a material substrate on which to build, corporeal forms.
*[Psyche means "soul". In Freudian and Jungian psychology, psyche ("mind, soul") is the sum total of one's conscious thoughts, the part that says "I think, therefore I am", and also unconscious urges, fears, irrationalities, instincts, and the "deeper" parts of your mind of which you aren't consciously aware. ALL of them constitute your "Self". In the real world, "unconscious contents" manifest visibly in dreams, poetry, and myths, whereas in FFX, pyreflies can make those "psychic images" manifest in the real world. As Maechen puts it, "that which is unreal becomes real for all to see."]
Okay. Back to those "non-living states of existence" in Spira. Here they are:
1. Zombies are dead bodies kept animate by some icky form of magic, but NOT pyreflies. The soul is gone. Healing magic works in reverse.
2. Fiends are the souls of the dead still clinging to this world. They're corporeal, can be healed, but are no longer human. To me, it looks as if the dead souls have lost almost all their self-identity, and all that's left are fragments of the unconscious self. The pyreflies are preserving them in a mutated form that visibly manifests what little is left of their souls: primal animal/vegetal urges, pain, madness, etc.
3. Unsent are the souls of the dead clinging to their self-identity through some powerful emotion or purpose: some focused bit of the psyche that death can't break down. I believe that unsent retain the original matter of their dead bodies patched, reassembled and preserved by the pyreflies, which act as magical life support systems after the body's processes give out.
My reason for thinking this is that the unsent usually retain their original shapes, although they are susceptible to wounds, signs of strain/aging (see Auron), and healing magic.
Evidence that they retain their original bodies (or some form of biological matter):
–Seymour's body is hauled away by the Guado before Yuna can send him.
–Before turning into his first "Boss" form, Seymour absorbs the dead bodies of Kinoc and three of his guards. His first "boss" form is larger than he was by about the same amount of matter as those he absorbed.
–in the Mt. Gagazet battle, after killing a bunch of Ronso, Seymour appears in a third, larger form with a huge bony exoskeleton. I suspect he absorbed the Ronso as he had Kinoc. (Possibly, Yunalesca's creepy forms are made of all the summoners and guardians she's killed).
Evidence that maybe they don't retain their original bodies:
–In Zanarkand, Auron kneels and briefly dissolves into pyreflies to show Tidus his memories.
I've waffled back and forth on whether the pyreflies are patching and repairing the original body, simply acting as artificial life support for it, or whether they've dissolved the body into basic building blocks/atoms/energy (as when Seymour dissolves Kinoc and his guards) and reassembling it like a transporter beam using the blueprint of the Unsent's self-image and (possibly) others' memories of him/her. Because Auron disappears into a puff of pyreflies, I'mn now leaning towards "reassembled matter" as opposed to "body on magical life support".
Either way, I think there's biological matter being held together by the pyreflies based on a mental memory of the person. Seymour apparently has learned to manipulate the pyreflies into making new shapes that seem to manifest the inner twistedness of his psyche, the way fiends seem to reflect the unconscious, "primitive" parts of the soul.
Fans debate whether Auron's white hair and signs of age are a conscious attempt at camouflage, a choice, since other unsent don't seem to age. I think it's not so much physical aging as premature aging of his soul reflected in visible form. As camouflage, it's lousy: he looks far older than 35. Also, if he were able to manipulate his pyrefly-constructed form, he would have kept his eye.
I'm also not convinced Seymour consciously designed his mutated forms. I think he simply stuffed an excess of pyreflies, magic, and dead bodies together and said "Go!" The first time we see him shapeshift, he's clearly in pain and not in control of his body as the shape change occurs.
Lulu: The fayth are people who gave their lives to battle Sin. Yevon took their souls, willingly given from their still-living bodies.
Lulu: Now they live forever, trapped in statues. But when a summoner beckons, the souls of the fayth emerge once again. That's what we call an aeon.
Lulu's description sounds to me like an esoteric rite to mimic the way pyreflies "naturally" preserve the unsent, using a statue instead of biological remains as a housing/anchor for the soul. "Fayth" refers both to the ghostly image of the person (e.g. Bahamut appearing as a little boy), and the portrait preserved as a fanciful statue under glass. Hymns and a temple help maintain the magic that knits soul and statue together. The Fayth is not bound within its statue the way Unsent are bound to their bodies: it can wander around, so long as the statue is preserved.
This reminds me of ancient Egyptian metaphysics. For them, the soul of the dead could exist after death only so long as there was a mummified body, even though the mummy stayed in the coffin and the soul went to the Duat (Farplane). Early on, the Egyptians started making realistic portrait-statues of themselves to serve as backup bodies, in case the original mummy was lost.
5. Aeons are the fayth manifesting an external, physical form through a summoner. The summoner's body acts like a lens, allowing the pyreflies to manifest the dead person's spirit not in terms of their former appearance, but as a fantastic, powerful representation of their inner souls. Aeons are like fiends, except that instead of manifesting only broken, primal, splintered fragments of the psyche, aeons incarnate the "better" parts: aspirations, virtues, inner character, courage, dreams, spirit. That's why, if you go back to Macalania Temple late in the game to talk to Shiva's Fayth (as opposed to aeon), what you see is not a sexy ice goddess (her inner self) but a frumpy-looking Mother Superior nun (her appearance while alive).
In psychological terms, the aeons are incarnations of the total self, conscious and unconscious, instead of mostly the conscious self (unsent) or unconscious self (fiends).
The odd thing about aeons is that, unlike fiends and unsent, they can't be built on the matter of the Fayth's body, since they appear and disappear each time they are summoned. They manifest through the summoner, anchored in some way to the foci of the statue and summoner's body.
Perhaps the physical forms of aeons are actually summoned from the summoner's surroundings: the animations seem to imply matter being pulled forth from soil, air, clouds, fire. (Yes, the aeons are a weak point in my theory that the pyreflies are assembling bodies from real matter.)
6. Final Summoning Aeons - We see two final aeons: Braska's Final Aeon for certain, and perhaps Anima. The script suggests that Seymour's mother sacrificed herself for the Final Summoning, but Seymour didn't use her because he wanted to live, and he wanted power. That explains why Yuna says, "His aeon... it was so powerful."
Braska's Final Aeon involves complications. We meet Jecht as a Fayth inside Sin -- that's the Jecht whom the party meets at the end of the game, and whom Tidus catches after he collapses. We also see his Aeon inside Sin. This is problematic, because (a) the laws of physics may be different there, and (b) Jecht may be a special case, because he's from dream-Zanarkand.
We also get two flashbacks that provide clues. In Zanarkand, Jecht says "Make me a Fayth". In the next scene, he's walking away from Yunalesca's chamber with Braska, and claps Braska on the shoulder. Either (a) the second scene's Jecht still resided in his original body, because the transformation wasn't yet complete or (b) he was already a Fayth, but unlike Bahamut and all the others, his Fayth is solid. Because Tidus catches him at the end of the game, I'm pretty sure it's (b).
My guess is that Yunalesca can convert people to Fayth by using their real body the way the lesser aeons are "housed" in a statue. In other words, her power is so great that she doesn't need the additional "battery" or "technology" of a fayth statue: she kills the person, keeps the body alive as an unsent, and then the summoner can call the aeon as usual.
Just as aeons are more powerful than ordinary unsent, Final Aeons are tremendously more powerful than ordinary aeons.
Why? Because they're doubled. Love is, essentially, carrying an image of the person you love in your soul: your love is anchored to your impressions about, memories of, and feelings for them. So, while an ordinary aeon is built on the pyschic contents of one soul, a Final Aeon is founded on a doubled soul-image: the person's self-identity, and the summoner's impression of him/her. The "Beloved" image may not be factually accurate, but it can be powerful, maybe even more powerful than the person's sense of self.
An alarming thought: the Final Summoning may build the Aeon partly from the matter of the summoner's body.
7. Sin: Sin is very odd. It's solid, but it's only a vessel; it has no soul unless there's a Fayth/Aeon inside it serving as a host for Yu Yevon. At Operation Mi'ihen, Tidus has a vision of the freshly-killed Crusaders walking around inside Sin (the same crystalline grove appears inside Sin during the endgame). I suspect that Yu Yevon has found some variant of what Seymour was doing with Kinoc and the Ronso: absorbing the bodies of the dead in order to fashion their bodies into "unholy armor". When pieces break away from Sin's carapace, they turn back into fiends, broken remnants of the person (or amalgamations of several people) they had been.
7. Tidus and Jecht. Originally they were simply figments of the Fayth's imagination, dreams. But the magic Yu Yevon used to animate Sin's carapace seems to have interacted with Dream-Zanarkand in some unique way, imbuing ordinary dreams with a vital spark such that they becomes independently self-aware. At that point the pyreflies can make them real, temporarily (like the aeons) or -- in X-2, after their "psychic images" have taken root in the memories/hearts of "real" people -- in reality.
And that, as they say, is that. Or maybe not. Who knows?