- “The Mantle. You still hold to that [belief] after all that has happened? After this thing has consumed a million worlds?
Can't you see? Belief in the Mantle sealed our doom! Weakened our [protectorates], bred dependence and sloth. Our [so-called Guardianship] has stripped those we would keep safe of any capacity for self-defense!
Were we such noble [Guardians] when we drew our line and abandoned billions to the parasite?”— The Librarian
- “The Mantle of Responsibility, for all things, belongs to Forerunners alone!”
- “The mantle accepts all who live fiercely, who defend their young, build, and struggle and grow. And even those who dominate, as humans have dominated. Cruelly. Without wisdom.”
The Mantle of Responsibility was a Forerunner philosophy. Although no precise definition of it exists, it is heavily implied to have been the source of both the Forerunners' "authority" and their apparent duty to preserve the galaxy's biological diversity. Bridging the social strata between an interstellar Marshall Plan and a benevolent, religious stewardship, the Forerunners took responsibility for the protection and cultivation of the species and planetary systems within their domain.
Forerunner mythology held that the Mantle was passed down to them by an even earlier and more advanced race, the Precursors; the versimilitude and commonality of this belief are unknown. The Forerunners seem to have handed their Mantle down to humanity; it may play a significant part in humanity's Reclaimer status. After the Forerunners and humans had reached Tier 1 technology, the Precursors had intended that the humans take on the role of inheriting the Mantle instead of the Forerunners. The Forerunners discovered the Precursors' plot and rose to seize the Mantle by killing almost every Precursor.
Some Forerunners doubted the purpose and validity of the Mantle and dismissed it as superstition and fairy tales. The Librarian especially had such doubts and even held it responsible for the downfall of the Forerunners at the hands of the Flood. The rampant AI 032 Mendicant Bias believed that the Forerunners used the Mantle to obstruct even beneficial change.
The Mantle and religious beliefs of individual Forerunners entirely dictated the governance of the Forerunner Ecumene. However, their approach of inspiring peace throughout their domain by minimizing their military capacity and disarming their weapons cache proved costly when the new alien threat emerged.
Rather than allow species to rise and fall, the Forerunners, following their Mantle, protected them against any threats that might have forced technological or cultural progression. An unforeseen side-effect was that only the Forerunners were able to hold their own against the Flood (at least once they rearmed) while their charges were defenseless; the Forerunners had unintentionally left the galaxy vulnerable to Flood infection.
The novel Halo: Cryptum, through its Forerunner main character, provides some such insight on the Mantle as its effectively requiring a strict vegetarian diet by forbidding "the eating of the flesh of unfortunates." The novel also states that suicide contradicts the greatest and most solemn instructions of the Mantle.
In Halo: Silentium, this idea is taken further by more closely detailing how both the firing of the Halo Array and the Ur-Didact's composition of human specimens contradicts the Mantle.