As theorized and tested by Ansem the Wise, Life is composed of a Heart, Body, and Soul.
The Heart is, contrary to popular belief, not the actual physical heart that is part of a person's body, but rather is a metaphysical property that exists within all living things. It contains a person's Light and Darkness affiliation, though being naturally a creation of Light, if a person becomes fully immersed in Darkness, the Heart will break away from the body and become a Heartless. It's affiliation with Light and Darkness is a person's most major influence in reacting to situations, and is therefore responsible for most emotions and perceptions.
World Hearts Edit
Worlds themselves also have Hearts. Little is known about the Hearts themselves, other than that the Heartless seek worlds' hearts above all others, and if they are able to reach it, the entire world will fall to Darkness, and the Heart and the Hearts of all those living on it will join Kingdom Hearts. It is the Keyblade Masters' duty to seal the "Keyholes" to worlds' Hearts in order to prevent their fall.
The Body and all its intricacies is considered nothing more than an empty shell in a metaphysical sense. It simply houses and nourishes the Heart and Soul, protecting them and allowing them to act in the physical world. Though it contains a heart and a brain, these are separate from the Heart mentioned above and the Mind mentioned below.
The Soul is a person's personality - what makes them "them." It, along with memories, is what allows a Nobody to retain their identity. Little more is known about the soul, due to studies of this nature being focused mainly on the Heart. It is known, however, that true life cannot exist without a Soul.
Loss of Life Edit
Death and Spirit Edit
When a person dies, their sick, broken, or worn Body is cast aside, and the Spirit - a combination of Heart and Soul that gives the body Life - leaves. It is unknown what happens to the Spirit after this, and all current postulation is simply theoretical, majorly various religions across the worlds.
Heartless and Nobody Edit
If a person's Heart is consumed by Darkness (either through personal corruption or theft by a Heartless), it separates from the Body and Soul. The Darkness and Heart combination becomes a new Heartless, which becomes attracted to Hearts and Darkness, and tries to consume them to make themselves more powerful and create more Heartless.
If that person had a particularly strong will, their Body and Soul (sometimes with fragments of Heart) will remain and become a Nobody. This Nobody will have no affiliation to Light or Darkness and feels little to no emotion. Due to the loss of Heart, partial memory loss is common in human-forms, and in Lesser-forms, total memory loss is almost guaranteed.
Other Loss of Heart Edit
A Heart can be lost by other means than Darkness corruption. When a Heart of pure Light is introduced to overwhelming Darkness (such as the loss of their world's Heart), their Heart does not become a Heartless, but instead simply leaves the body and finds a new vessel to carry it, even if that vessel already has its own Heart. The leftover body and soul become quite lifeless, with misted eyes and limp form.
A Heart can be removed from a person by use of a Keyblade, particularly one wielded by a Dark Keyblade Master. The released Heart can find a new vessel, can be stored, can move on to Kingdom Hearts, or can become a Heartless - based on the Heart and the means of release. If a Heartless is created, it will always be a Pureblood Heartless.
When a Light Heart is lost by means other than Darkness corruption, they can be revived through an act of love, even if they have spawned a Nobody to inhabit their body.
Other Parts Edit
There are some parts of life that were not touched on by Ansem the Wise in his studies. These include:
The Mind can be considered the workings of the brain in conjunction with the Heart and Soul. It makes decisions based on emotion and perception, and controls instincts. It could be simply an extension of the Heart and/or Soul, or simply just part of the workings of the Body. It includes a person's convictions and biases, and can theoretically act on its own.
Aeons are defined as "the spirits of inanimate objects." It is possible they are the Hearts, Souls, or Spirits (Heart+Soul) of nonliving things, or else something different altogether. It could also be an extension of a living Heart, Soul, or Spirit that is connected to the object. Some Aeons are in fact Spirits that have been sealed inside inanimate objects.