|Index: "What is this? Agathions?"|
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Mana is a form of energy that exists in all forms of life; magicians however, are capable of harnessing it for practical purposes. A method for producing mana is to convert the life force (生命力 Seimeiryoku?, lit. (one's) life force) of a living organism into it. This conversion is treated as being similar to refining crude oil into other substances, which effectively enables its use for different reasons and in different ways. There are various schools of magic that use their own forms of mana for casting. Two notable examples of such schools are the Nordic and Shinto styles, and each requires particular 'kinds' of mana for casting that other styles might not use.
There exists a form of mana that is generated by the earth; it is available for use by certain magicians and magical objects. Once used, it is visually represented by "leylines", which are used as a sign of the fact that the mana used had been generated by the earth itself. Mana can also be inserted into objects through the use of complex rituals. This results in the creation of new "magical items" that possess certain magical properties of their own. For example: at one opportunity, Index had referenced the ritual needed to add power to a holy cross. Its to note that mana is incompatible with AIM and it is because of this that injury occurs whenever an esper attempt to perform magic.
A consequence of a magician's training to develop and improve their magical performance, is the strength of their mana being greater than that of an average person. This interestingly allows a magician to detect other magicians in close range, which is useful for aiding them in their identification of other magicians.
Experts in the field of magic have hypothesised that since mana is convertable from the caster's life force, immortal beings should be able to produce an infinite supply of mana. This should thus allow them to produce an infinite amount of magical effects; effectively causing their magical capacity to be far greater than 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. Many on the Magic Side, such as alchemists, have also sought the abilities of vampires for purposes, such as the completion of Ars Magna.