This is my reasoning for how so many people were reborn after their Heartless and Nobody are destroyed. It kind of touches on science-y stuff towards the end. I like to think there is at least one, sane scientist in the Kingdom Hearts universe who is actively trying to make sense of the nonsensical world he lives in, and I think he would write about stuff like this. Everyone else who runs around hoping magic and potions will save everything? They're nutters. That's my 2₵.

XXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXX

Suppose two things are true:

1) When a Heartless is destroyed, it releases a Heart-but not a real Heart. Rather, it is a Heart of magic.

2) A Heartless and a Nobody are from the same person.

When Heartless are destroyed, the Heart is released from the bonds of a physical body and, through an unknown magical force, the Heart not only stays intact but floats up to 'Kingdom Hearts'. It does not stay in place or move randomly around the world. So we can deduce:

3) Kingdom Hearts is like a collecting jar, which attracts the magic of a Heart.

Now, since a Heartless and a Nobody come from the same person, there must be some similarity between them, connecting them. We can say this because, through experience, it is apparent that the Heart is equivalent to a soul, and regardless of what form it takes, the Heart represents one particular person. So if one person is split into two, then the Heart must also be split (although not necessarily into equal parts). Therefore I propose:

4) A little wisp of Heart-magic is still embedded in the body of a Nobody

Perhaps there is not enough magic in a Nobody to take the form of a Heart, or even be felt as the full spectrum of emotion. However, it is not an unreasonable conclusion to make, and if it true, then not only would it explain why some Nobodies can still feel (Demyx), but it could also be said that:

5) The Heart, though safely resting in Kingdom Hearts, is still anchored to the physical world by the body of its Nobody.

So in order to make the Heart whole, rescue it, and give it back its original physical form, you would first need to destroy the Nobody. After that, you need to reach the Heart.

Conclusion?

You break the 'jar'. This was the scene where Ansem/DiZ breaks Kingdom Hearts with the strange laser contraption. Maybe we'll be science-y and say that the potential energy of the Heart, due to its position in Kingdom Hearts relative to the World-That-Never-Was, has enough energy to jump-start a magic reaction (as opposed to a chemical one). And if you say that the Heart naturally wants to be whole, and the potential energy exceeds the activation energy of the reaction, then the falling of the Hearts could result in the spontaneous rebirth of its owner...

Of course, Gibbs free energy would be negative, but Entropy would be working towards order instead of disorder...

And you'd have to modify our current equation for Total Energy to include a new energy, Em, of magic (and so on with other equations) ...

In addition, we must account for why so many of the Hearts returned to being Heartless. I'd wager a guess and say that not enough Nobodies were destroyed; the ratio of Heartless dispatch to Nobody dispatch may look something more like 3:1 if you assume a fixed rate of dispatch over time (time being ~3yrs), and only by those wielding Keyblades (Sora, Roxas, not so much King Mickey and Riku). Heartless were dispatched during KH1, CoM, 358/2 days, and KH2, but the bulk of Nobody dispatch was during KH2. The ratio might even lean towards 4:1 or greater, both because of the massive battle in Hollow Bastion, and because the Heartless were actively targeting Sora, thus increasing the likelihood of their dispatch. Recall that Heartless dispatched by civilians regenerate, and their Hearts are not released; thus the rate of dispatch is closely related to the activity of our protagonists.

A flaw in my logic, if I may point it out, would center on gravity. Since gravity is related to the mass of celestial bodies, and the worlds are so peculiarly small, gravity would naturally be much less. Therefore, the potential energy would be much less for a given height when comparing a KH world and our world. The reduced gravity would explain why people can jump so high, glide, and be so skinny and light. It may even account for the gravity-defying hairstyles of many popular characters, if some sort of magic- or electrostatic charge – based styling gel exists. (Maybe free magic has an odd electrostatic affect, and lighter/shorter hair naturally sticks up!) But it would decrease the likelihood of spontaneous rebirth, because simply falling to the ground wouldn't result in enough energy to regenerate an entire physical body.

However, it is not a problem if living creatures have their own pull on magic, like the spontaneous regeneration of magic in many Final Fantasy games (which are part of the KH universe, so I find it acceptable to reference them). Therefore, a normal person would have a mini gravitational pull on magic. Since having a Heart is the common denominator in this instance, we could even go so far as to call it a specialized organ. People with stronger Hearts would conceivably pull in more magic than those with weaker Hearts. Since worlds have a Heart (their core), and because their Heart is so big, it could be said that they have not only a regular gravitational pull on objects of mass, but also a magic-based gravitational pull on magic-based objects. Now we have both gravities, Gmass and Gmagic, acting on the Hearts. The potential energy might even be greater than in our world at this point, but we cannot reason any further without knowing more about the nature of Gmagic. (Gmass might not be relevant, if a Heart is nothing but energy. Not sure about that though because the Heart energy doesn't disperse. Something's still holding the Heart together).

Supposing the aforementioned theories are correct, magic is continuously being pulled into people and the world. However, without any further laws in place, the world should theoretically be orderly and empty of free magic (ungathered magic). This is simply not true, and not only is it not true, but it contradicts the law of entropy—that the universe is trending towards disorder rather than order. How would you explain all the free magic in the KH worlds, and furthermore, how would explain phenomena like the 'Mist' in FFXII? (Which are examples of disorder). It is possible that although the Heart attracts magic, the body surrounding the Heart shields from magic. After all, magic is an energy, like light, and exposure to high amounts of magic have resulted in mutations and deformations in native flora and fauna (FFXII)—just like how long exposure to the sunlight can cause damage (UV radiation anyone?) It is already shown that an imbalance of magic in the Heart, specifically light:dark ratio, can have unforeseen and drastic consequences.

So over time, the animals, plants, and people might have naturally evolved to shield themselves from a portion of magical energy, if for no other reason than to maintain homeostasis (which is a very good reason, actually). The worlds themselves may shield magic; perhaps the elements in the soil or bedrock may deflect magical energy. Just like how a baseball that falls to the ground does not continue falling into the core of the Earth, the magic may fall toward the ground but not to the Heart of the world. Maybe some areas of the world are better suited to deflecting magic, and in that particular spot there is more magic gathered at the surface of the world because it can't pierce the crust of the earth. Maybe other parts of the world are weaker at deflecting magic, and it can piece farther into the crust, effecting crystals, underground aqueducts, the topsoil, etc. And there may be some parts of the world where magic from the Heart of the world surfaces, like how magma rises through a volcano. Although there might not be a very physical indication (no strange rocks jutting out or funny-colored grass), these permeable spots where the world casts its magic might be recognized as locations of spontaneous healing and magic regeneration—the infamous save spot.

Also contributing towards free magic is the idea that each Heart has a limit to the amount of magic it can store. For example, a physical Heart pumps a fixed amount of blood with each beat; volume doesn't change, although a healthy & fit Heart may pump more blood and do so better. The Heart of magic may also have a fixed volume, and performance and volume limits may only increase if magic is exercised often. The idea of leveling up is comparable to reaching a new level at the gym. Since each person can only retain a finite amount of magic, and magic available is infinitely larger in volume than magic which can be stored, the majority of magic is left bouncing around in the atmosphere stuck in its free form. Whether it is really 'free' or actually bound to something is debatable; magic from fallen creatures tends to gather as bubbles in KH which suggests there is something else at play. Magic also tends to gather into special crystals; the method behind that is a mystery as well.

Lastly, I suggest that free magic may move about in a fashion akin to air or water currents. This may be due to land formations and the ebb and flow of magic from the Heart of the World. This may influence weather, with areas exposed to more light-based magic having better weather and magical occurrences. Areas with more dark-based magic might spontaneously develop foul weather and be prone to bad luck. Areas of magical stagnation may be behind 'Mist' formation (in FFXII).

.

.

.

But that's just my take on things. It's certainly not spelled out in the Canon. I haven't played the more recent KH games so I might be foggy on key details. It's entirely possible that whatever applies to Xehanort's rebirth may actually contradict my own explanation for all the other rebirths. And the only Final Fantasy game I've actually played was FFXII, so that's why I keep referencing it. I actually think Sora didn't regenerate magic on his own in KH1, and in KH2 it was because of his fancy new duds. So uh, yeah. _ I also don't have an explanation for why magic falls to the ground as bubbles or anything surround those magic crystals which are present at save spots and used to power air ships or ancient machinery.

If anyone wants to point out phenomena that contradict me or which you find curious, please do so :) I would also love any proposed questions or solutions. Go ahead and dispute me :D It would make me happy.

I wish I could write my thesis on Kingdom Hearts ;)

XXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXX

I like my fantasies seasoned with logic and left to marinade :)

~TheIvoryTower