From Halopedia, the Halo wiki
Living Time was a Forerunner philosophical concept relating to the flow of time and "the joy of life's interaction with the Cosmos." It was the founding concept behind the Mantle, and seemingly drew from the Precursor philosophy of neural physics which postulated that the entire universe was a living entity enriched by the collective experience of ever-changing life. According to the premise of Living Time, struggle, competition and even war were not at odds with the harmony of the cosmos, as they were regarded as a natural part of life. However, excessive and needless cruelty and destruction were seen as introducing an imbalance to the harmony of Living Time.
The philosophy of Living Time viewed time as an infinite series of twisting and entangling streams—known as world-lines—intrinsically tied to the notion of fate. Metaphysical junctures of Living Time where the world-lines of individual entities connected and were bound together were known as synchrons. While it was common for these points in time to go entirely unnoticed, a particularly potent synchron could be felt by a Forerunner as a momentary and very concrete realization of the linkages of world-lines between themselves and other individuals sharing their fate.
The Forerunners considered themselves to be only a stage in the stewardship of Living Time, with only a temporary place in the Mantle before another civilization that would reclaim their place as stewards of the galaxy. The Forerunners also believed that individual lives would be summed and judged at the end of Living Time.
In 2558, the Librarian tells Rion Forge that Living Time was sent into an imbalance by the Forerunners wiping out their creators with the Flood continuing this imbalance, putting it at a risk of collapse. With the Flood spreading, humanity had to survive in order to assume the Mantle of Responsibility, care for the galaxy, adhere to its laws and in doing so help tend the flow of Living Time. According to the Librarian, long down the world-line there will be another Librarian, Primordial, Didact, Chakas and Rion as Living Time needs its champions and villains to keep the balance.
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