This single practice itself may be an expression in concrete form of the very ancient belief that ultimate reality is one and only one –“only One Buddha Vehicle,” as the Lotus Sutra states. The attribute of suiting all people’s capacities similarly finds a doctrinal counterpart in the teaching that all beings are equally endowed with the Buddha nature, which can be traced back to the origins of Mahayana Buddhism and was well established in Heian Buddhism as the doctrine of original enlightenment (hongaku shisō). The attribute of eternal validity echoes the belief, equally old, that the absolute is changeless and imperishable. The idea of one practice including the merits of all practices may have its theoretical foundation in the doctrine that one truth encompasses all truths, a major theme of the Lotus Sutra and a teaching central to the Kegon, Shingon and Tendai doctrinal systems. The concept of attaining Buddhahood “quickly” probably also has connections to belief in the universality of the Buddha nature. The principle of “attaining Buddhahood in one’s present form” is integral to both Tendai and Shingon doctrine, though not until the Kamakura period was it welded to a universally feasible way of practice.