The Lifestream, also known as Spirit Energy, is an ethereal substance that flows in streams just beneath the surface of all worlds with the capability to sustain life. On Earth, when seen beneath the planet, it is shown as many separate bands of green-white fluid flowing as a whole. In several places, such as the hot springs bath at Hinata-Sou, Lifestream springs shoot out of the ground, creating a local spectacle.
The Lifestream contains the essence of the planet and the memories, emotions, and knowledge of all who have lived on it. Portions of the Lifestream are believed to be used to create new life on the planet, and the energy of a person returns to the planet when they die, bringing with it the emotions, memories, and knowledge they obtained during life. It is referred to many times as the life of the planet itself, and acts as an afterlife for the conscious spirits of the inhabitants of the Planet, as well as a sort of immune system for the planet, flowing to and "healing" scars in the planet. All life is said to exist within the cycle.
The Lifestream has been equated to an afterlife with a concept of Heaven and Hell. Those who led good lives join with the Lifestream and their minds fragment and join the collective, while those who were sinful remain intact and must exist with the knowledge they cannot find absolution because of their actions.
- The Lifestream is an iteration of the ancient Hellenic philosophical concept of the Absolute or the World-Soul, an infinite wellspring of spiritual energy. All human beings possess a fragment of it which returns upon death. The description of the Lifestream's afterlife is also consistent with Neo-Platonic belief. This concept of the Absolute would, over a millennium later, become part of Jewish Kabbalist belief, which factors heavily into the game.
- The Lifestream is an early part of the trend to use a supernatural substance to explain magic and other phenomena. Other instances in the franchise include Mist and aether.