The Precursors believed that the Mantle was intimately connected with the fabric of the universe - matter, energy and life itself are all connected. The Precursors saw that the entire universe was living, but in a way that only they could comprehend, and the principle of the Mantle was to preserve life in all of its diversity. The Forerunners called this 'Living Time', their own take on the Precursors' philosophy, and it's why the Forerunners ultimately failed to uphold the Mantle when they took it by force, as the upper-echelon of their society was run by a corrupt group of Builders who sought only power and profit.