The four essences are matter (also called body in certain contexts), spirit (sometimes called self), mind, and soul (sometimes called life). These are physical and metaphysical components which compose all of existance.
Matter (Body)
Matter is unique as it is the only physical essence. It is the fundamental building block that makes up all physical things in the universe. Matter cannot be created or destroyed; it can only be reconfigured.
Body
Body specifically refers to matter that is inhabited by a soul; literally living matter.
Spirit (Self)
The spirit (or self) is the complete embodiment of one's personality and character. An individual's spirit is typically bound to their soul, but this binding can be temporrily or permanently severed through the use of magic. Manipulation of individual spirits is the primary means of possession and the projection of one's consciousness.
The spirit is closely related to the soul; a spirit left without a soul to bind to begins to disapparate and become unstable. The individual the spirit represents slowly becomes more of a charicature of themself as less defining traits are lost first and more substaintial ones are emphasized in an effort to maintain the self.
Outside of the Inner Realms a spirit without a soul will eventaully disapparate entirely, effectively erasing the individual it represented from existance but for their memory.
Destruction of the Spirit
Because an immortal soul is fixed to the body rather than simply bound (such is the case with mortal souls) the soul is sundered when the body is destroyed. This sudden breaking apart of the soul irreparably ruptures the spirit bound to it, creating countless spirit fragments that disapparate relatively quickly.
Preservation by the Ethereal Plane
The Ethereal Plane has the unique property of partially preserving a spirit indefinitely. This is what allows ghosts and other incorporeal spirits to exist in the Mortal Realms. The Ethereal Plane does not preserve the spirit perfectly, however; the personality and characteristics of the spirit will still devolve to a point, which is why many incorporeal spirits are shadows of their living selves.
Mind
Mind (sometimes refered to as mental essence) is what allows thinking creatures to have rational thoughts, ideas, plans, logic, and memories. Even nonsapient creatures like animals possess mind.
Fragmentation of Mind
Mind is the most fragmentable essence. Every single thought, idea, plan, and memory is a tiny piece of a conglomerate mind, and can be added or removed from that conglomerate without significantly affected the other components.
Mind fragments have a finite size at which point they cannot be further divided under any means. This trait preseves information. Even when a creature's body is destroyed, it's soul sundered, and it's spirit disipated, it's thoughts and memories persist indefinitely somewhere in the universe.
Growth of the Mind
The mind grows as one learns and interacts with the world. This growth is often limited by the body; animals can not typically achieve the same mental capacity as sapient creatures.
Relation to Other Essences
While every essence can have a relationship with every other essence, mind may be the most complex.
Mind is typically a component of spirit; where the spirit goes, the mind follows. However, it's possible (typically through magic) for mind (or parts thereof) to be moved between different spirits.
Components of the mind may be bound to the body rather than the spirit—two creatures who swap bodies may find their minds jumbled as some parts travel with the spirit and others remain with the body. Additionally, the body can supress components of the mind in some cases.
Finally, while minds can be bound to souls, this never occurs naturally.
Soul (Life)
Souls are forever—they cannot be truly, permanently destroyed by any means. Rather, a soul which is sundered shatters into fragments and dissipates into the cosmos, to intermingle with the fragments of other sundered souls until they are combined into a new complete soul.
Mortal and Immortal Souls
The nature of a soul's relationship with the body and spirit, and what happens upon death, defines whether a soul is immortal or mortal.
Immortal Souls
An immortal soul is fixed to the body; the body and soul are one. If the body is destroyed, the soul is sundered, and the sundering of the soul destroys the spirit. This means that when an immortal being is killed, everything about that individual is destroyed. The fragments of the immortal soul remain and can eventaully be reconstituted into a new soul, but the body and spirit are permenantly and irreversably destroyed.
Mortal Souls
A mortal soul is separate from the body. When the body is destroyed, the soul lingers on, as does the spirit bound to it. Outside of the Inner Realms, a mortal soul lingers indefinitely. Additionally, mortal souls can be fragmented without harming the spirit, so long as the spirit remains bound to a fragment of the soul. This unique property is what facilitates an afterlife for mortals.
