Tathāgatagarbha sūtras
The Tathagatagarbha doctrine says that within each being is the potential to become a Buddha. The Tathagatagarbha (Sanskrit. 'Womb of the Well Gone') is known colloquially in English as Buddha-nature. Tathagata is an epithet for a Buddha. The Tathagatagarbha Doctrine is not associated with it's own school of practice as the Madhyamaka or Yogacara, it is however one of the essential Mahayana doctrines. It probably arose as one of many responses to the problem of immanence and transcendence - ie it was a way to explain how it was possible for ordinary human beings to become Buddhas. The Tathagatagarbha Doctrine is a way of talking about enlightenment as immanent.
Buddha-nature, then, is the eternal potential, present in all sentient beings, for awakening and becoming Enlightened. Buddha-nature is considered to be incorruptible, in that no matter what the outward appearance or history of a particular sentient being, buddha-nature is always latently present.
Unlike the Western concept of a soul, buddha-nature is not considered to be an isolated essence of a particular individual, but rather a single unified essence shared by all beings with Buddha-nature. However in the Mahayana version of the Mahaparinirvana Sutra Tathagatagarbha is equated with Atman in direct contradiction of earlier Buddhist doctrine. The Sutra contains many Hindu elements and is thought to have been composed during the Gupta Period which coincided with a Hindu revival in India.
In the Zen Buddhist tradition, it is considered insufficient simply to understand buddha-nature intellectually, or to achieve an intuitive understanding of it. Rather it must be experienced and felt directly, in one's entire mind and body together. Enlightenment in a certain sense consists of a direct experience of one's authentic identity, which is traditionally described as Sunyata (emptiness), the ultimate reality of Buddha-nature.
Key texts associated with this doctrine are the Tathagatagarbha Sutra which contains a series of images for what the Tathagatagarbha is, and The Lion's Roar Discourse of Queen Srimala.
The Zen tradition often uses parables to try to explain the Buddha-nature: according to one story, a monk once approached the Zen master Chao-chou and asked him, "Does a dog possess Buddha-nature or not?" Chao-chou replied with the one-word answer "mu". His response, among other things, indicated that the question could not answered with a simple "Yes" or "No." Rather through the contemplation of the question, or the absurdity of it, one may perhaps gain an experience of Buddha-nature directly.
Buddha-nature has been connected in recent decades with the developments of robotics and the possible eventual creation of artificial intelligence. In the 1970s, the Japanese roboticist Masahiro Mori popularized the idea that robots, under certain conditions, may possess Buddha-nature. Mori has since founded an institute to study the metaphysical implications of such technology.