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Psions (想子, literally "Thought Particles", the English spelling is "psyon") are non-physical particles that come under the dimension of psychic phenomenon, an information element that records the result of cognizance and thought. They are similar to Pushions in that Pushions are the particle manifestations of emotions from intention and thought, while Psions are the particle manifestations of intention and thought. [Note: This is still at a hypothetical stage.]
Psions and Pushions are both observed in "Para-Psychological Phenomena" which includes Magic as well. These particles are composed of non-physical entities and are not similar to 'Fermions' (that makes the composition of matter) or 'Bosons'.
Normally, Psions are used in Magic and in the technology systems of modern magic, emphasis is placed on the control of Psion. Magicians first started from learning how to manipulate Psion.
Eidos, that is the theoretical basis of modern magic and activation rituals, and Magic Rituals, that are techniques that support the foundation of modern magic, are forms of information constructed with Psions. Most of the time, Psions travel through the information dimension and so are not limited by distance or obstacles. Despite this, Magicians are limited by their awareness of their surroundings, so they are unable to bring about changes in the Eidos of targets far away or out of sight. The exception is Non-Systematic Magic, where Psions travel through space and therefore are affected by distance.
CADs absorb Psions, which the Psion Information Aide transmits into the chosen Activation Sequence. The Activation Sequence will then be fed back to the Magician who will construct a Magic Sequence from the Activation Sequence, which will then affect the Eidos of the target.
Highly skilled Magicians generally do not leak Psions when casting spells. This comes from leftover Psions from activation and invocation of the magic. The leaked Psions interact with photons, leaving behind Psion light which only Magicians can perceive. When a lot of light is produced, the Magician is wasting Psions and therefore is unskilled.
In Ancient Magic, these particles are referred to as chi.
Illustrations from the Anime series