Idol Theory

Idol Theory or Idolatry Theory (偶像の理論(ぐうぞうのりろん)) also called Idol Worship, is the term used to refer to the phenomenon wherein an imitation of a whole or parts of a subject, the source, gains properties and attributes that are inherent to that source. It is used as an explanation on how magicians draw their power, and how they wield it. It is also known as Imitative Magic (類感魔術) and is the opposite of Infection Magic.

The theory explains and encompasses most known magic in the Toaru Majutsu no Index universe other than the thelemic magic school of thought introduced by Aleister Crowley.

Principles
With the use of imitative magic, two similar objects influence each other. Idol Theory postulates that duplicates retain the properties, attributes, and powers of the original source. For example: A cross of a church that duplicates the cross where the Son of God was crucified, it will retain the holy power of that cross. Although a duplicate can only retain a little amount of that original power.

Despite this, even the smallest percentage that is retained by the duplicate is powerful enough. As Index states that if a duplicate of the legendary Holy Manger can retain 1% of the original, then that would be enough to match the power of the twelve apostles.

Kamijou Touma compares this to the Sun providing solar power for electricity as the power of the 'original' is almost limitless even with the power split across so many duplicates.

Reversal
The reason why it remains as a theory is because of that it can also be switched around. This means that changes upon an idol (the duplicate) will also affect the original. This is shown most prominently with the Great Japanese Coastal Map, an intricate map made by Tadakata Ino, who plotted 47 portals upon the map, and making it realize upon Japan itself.

However, because of this, the belief that harming the original if carrying harm upon a duplicate persisted. An example of this would be the persecution of Christians by forcing them to step on the image of the Son of God or the Holy Virgin, known as Fumi-e.

One of the best demonstrations of this phenomenon, is during Index and Tsukuyomi Komoe's healing magic ritual. Where Index creates a replica of Komoe's apartment with a few found items. Here, it shows that the duplicate can affect the original, after Komoe accidentally hits the table, her entire apartment shakes.

It can be assumed that the overall phenomenon is a rarity.

Uses and Application
The most prominent application of Idol Theory in the universe is to draw powers from their sources, such as to draw the power of angels, as with the case when creating a magic circle. Or to apply the attributes of these idols into their magic, such as Saronia A. Irivika, applying the legend of the Leshy into her magic.

Likewise, the special properties of ether, one of the Classical elements allow it to change into one of the other elements, which carries over to its elemental weapon, the Lotus Wand.

The applications of idol theory still occurs regardless of intent. Such as dubious folk charms or occultic souvenirs still receiving some of the power of the original, as well as activation of spells, such as Angel Fall by Kamijou Touya. The greatest extent of this are Saints who are born with a body similar to the Son of God, and along with a few superhuman abilities, are also born with the vulnerabilities of Son of God, such as nails and stabbing weapons (as the Son of God was nailed upon the cross and then stabbed by a spear).

Disadvantages
Because of the very nature of using idols, magicians are in danger if a particularly knowledgeable person discovers the history or lore surrounding the original. Then one could discover the magic's weakness, as with the case with the true form of the Stab Sword, and Saronia A. Irivika hiding her magic from the Kamijou Faction all throughout the Hawaii Invasion Arc.

Differences with Personal Reality
Idol Theory is in direct contrast with Personal Reality, which allows Espers to gain powers. First off, Idol Theory requires the belief upon a subject, an idol that a wielder requires to derive their magic from, whilst espers only need to distort reality through their own will, and from there espers could gain powers. The huge advantage of magicians however is that they can wield several different types of magic derived from several sources, while espers are limited to only one power. However, to cast magic, the magician needs to consider the nature of the magic, preparations, ingredients, and other rituals necessary for the magic to take effect, whereas espers do not.

Due to the differing rules that magicians and espers follow, this may be the reason why espers cannot use any magic, lest they injure themselves, assuming that these differing laws change the "circuitry" of the human body.

Trivia

 * Idol Theory is similar to the concept of, or the Law of Similarity, that one can produce any effect just by imitating it, or more simply like things produce like things.