Mana

Mana (魔力) is the energy required by magicians to operate the supernatural mechanisms of magic.

Principles
Mana is a form of energy that exists in all forms of life; magicians however, are the only entities able to harness it for practical purposes. A method stated for producing mana is to convert the life force (生命力) 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; each require particular 'kinds' of mana for casting that other styles might not use.

There exists mana generated by the earth itself; it is available for use by certain magicians and magical objects. Once used, it is visually represented in the form of "leylines", and the leylines aid identification of the mana being a type that has been generated by the earth. Mana in general is incompatible with AIM and it is because of this that attempts of an esper to perform magic results in subjection to injury. Through use of complex rituals, mana will be inserted into objects. This leads to the existence of "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.

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 helping them determine who is a magician and who isn't.

Experts in the field of magic have hypothesised that since mana is convertable from the caster's life force, immortal beings by logic, should be able to produce an infinite supply of mana. This should thus enable their production of an infinite array 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.