 The fundamental principles of Mahayana doctrine are based on the possibility of universal liberation from suffering for all beings, and the existence of the transcendent [[Buddha-nature]], which is the eternal Buddha essence present, but hidden and unrecognised, in all living beings.Philosophical schools of Mahayana Buddhism, such as [[zen|Chan/Zen]] and the [[vajrayana]] Tibetan and Shingon schools, explicitly teach that [[bodhisattva]]s should refrain from full liberation, allowing themselves to be reincarnated into the world until all beings achieve enlightenment.Devotional schools such as [[Pure Land]] Buddhism seek the aid of celestial buddhas—individuals who have spent lifetimes accumulating positive karma, and use that accumulation to aid all.