|Index: "What is this? Agathions?"|
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Mana is a type of energy that is said to exist in all forms of life. However, it is to note that magicians are the only lifeforms capable of utilizing it for practical related tasks. It has been stated explicitly that one method of generating mana is to perform the process of converting a living organism's inner life force (生命力 Seimeiryoku?, lit. (one's) life force) into it. The process of this conversion is this is similar to how crude oil can be refined into other substances, which allows for it to be used for different things and in different ways. There are various schools of magic that each utilize their own forms of mana for work. Two notable examples of such schools are the Nordic and Shinto styles, which each require particular 'kinds' of mana for casting that other styles might not use.
There is said to exists a naturally generated form of mana that can be found within the earth itself. This form of mana can be used by certain magicians and magical objects as a power source for their magical activity. When they use this mana, its visual representation will usually appear in the form of "leylines". The leylines commonly helps the magician identify the mana they are using as being a type that is naturally generated within the earth.
Every form of mana is said to be incompatible with AIM, which is an energy emitted from an esper's body. Because of its incompatibility with AIM, an esper attempting to perform magic will suffer from severe injuries as a result. Mana is capable of being inserted into objects however, and this is done through the use of complex rituals. These rituals can give birth to "magical items" that possess magical properties of their own. For example: at one opportunity, Index had referenced the ritual needed to add power to a holy cross.
As a consequence of having trained to develop and/or improve their magical performance, the strength of a magician's mana is greater than that of an average person. This fact interestingly allows a magician to detect other magicians in close range, which is useful as it helps them determine as to who is a magician and who isn't.
Many experts in the field of magic have theorized and concluded that since the source of mana is the caster's life force, immortal beings by logic, should possess an infinite supply of mana. This should then allow them access to being capable of generating an infinite set of different magical effects; with their magical capacity being far beyond that of any ordinary magician. This is one of the many reasons as to why vampires are thought to be invincible by many within the Magic Side.