Soul

A Soul is the spirit that animates and gives identity to an individual. 
In the Universe of EXA_PICO, a soul is an aggregation of Static H-Waves that vibrate at a specific frequency that no other soul has (Soulspace Address, Genometrics and FFT Spectrum), which allows the identification of individuals in a way that is impossible to falsify. However, this is not to say that sentient creatures are the only ones that possess souls: every single piece of the universe holds a soul, or as they are also known, a Will inside itself, which is what allows them to exist and exert some degree of influence over the world itself. However, possessing a Will does not necessarily mean that the D-Wave structure possessing it is sentient. 
Generally speaking, all souls exist on the H-Wave layer of the universe, which is divorced from the D-Wave (physical) layer and thus they can't directly interact with each other, through the Wave Crossover phenomenon allows them to influence each other to a limited degree, resulting in manifestations such as the poltergeist effect. 
Additionally, while Wills are incorporeal and can exist for prolonged periods of time without being bound to a physical object, souls on the other hand must be bound to a body or otherwise they will disippate among the other souls that exist throughout the world in what is known as the reincarnation cycle.Ciel nosurge: Ushinawareta Hoshi e Sasagu Uta - Chapter 8, Episode 3: "The Audience" - Cosal: "The soul goes to nowhere, and to everywhere upon death." The reason why this does not happen to a soul tied to a physical object is because of the existence of the Qualia: the walls that keep souls from mingling with each other and allow them to retain their own individuality.Ar Portal - Toukousphere Issue #109, Technical Service Center 
It is not known yet what is the cause for the Qualia dilluting itself upon the death or destruction of the body, but it could be theorized to be caused by the soul losing awareness of itself and of the (D-Wave) world surrounding it. Additionally, souls taken from other universes cannot decay or mingle with those from EXA_PICO's and will remain locked in a state of eternal unawareness and suffering until they are bound to a new body or somebody manages to Dive into them. 
However, despite establishing the existence of the H-Wave layer, so far Ar Ciel's science has been unable to explain where the soul is located in a living being: the brain only functions as a transceiver that allows to perceive and interact with the environment due to the synapses' electrical pulses oscillating at the same frequency as H-Waves, the heart is merely a pump that keeps blood circulating through the body, and it has been established that the soul controls the body remotely from somewhere, but nobody knows exactly where that "somewhere" is located. The only exception to this is the Pureblood Reyvateils due to their souls existing on an isolated artificial H-Wave layer constructed in the SH Servers: the Cosmospheres that make up their whole selves.Ar tonelico Qoga: Knell of Ar Ciel Setting Encyclopedia - Advance Wave Science Lecture, page 127. 
Souls that have polished themselves to a large extreme can also evolve into Wills and forego the need to keep depending on a body to maintain their existence, as it has been observed with the Genoms in Ra Ciela. However, Genoms that are still at the midpoint between soul and Will would still need to receive H-Wave energy somehow to sustain themselves. 
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