Gaia Theory

The Gaia Theory (ガイア理論) proposes that all organisms and their inorganic surroundings on Earth are closely integrated to form a single and self-regulating complex system, maintaining the conditions for life on the planet, making it as a form lifeform. It is a derivative of Holism.

In Toaru Majutsu no Index, the Gaia theory is mainly invoked when discussing the power that dwells in the earth and being used magicians.

Principles
The Gaia Theory is predominantly related to the energy that flows within the world as evidence that the world is truly a living organism. The power is referenced as a pulse that forms all forces and flows in the world like the bloodstream in humans. It is also referenced as being similar to mana of humans when refined using Feng Shui or using a temple or shrine to draw out the planetary force.

Document of Constantine Arc
Tsuchimikado Motoharu references the Gaia Theory to Kamijou Touma, when he explains Feng Shui to him, which he calls "a theory of treating the Earth as a lifeform."

Baggage City Arc
The Gaia Theory is referenced during the explanation on holism, stating its flaws, since the world is still affected by the influence of large phenomena, and cannot wholly be regarded as a singular self-sustaining complex system as it is part of the effects of the universe.