Tathāgatagarbha Sūtra

... as Alex Wayman, Michael Zimmermann, and I have noted, the original meaning of the term was that one is "already" or primordially awakened. For example, the Tathagatagarbha sutra illuminates the matter metaphorically this way: "inside a casting mold there is perfectly formed Buddha; the ignorant see the filth of the mold but the wise know that the Buddha is within."

Matsumoto [calls] attention to the similarity between the extremely positive language and causal structure of enlightenment found in the tathagatagarbha literature and that of the substantial monism found in the atman/Brahman tradition. Matsumoto, of course, is not the only one to have noted this resemblance. Takasaki Jikido, for example, the preeminent scholar of the tathagatagarbha tradition, sees monism in the doctrine of the tathagatagarbha and the Mahayana in general … Obermiller wedded this notion of a monistic Absolute to the tathagatagarbha literature in his translation and comments to the Ratnagotra, which he aptly subtitled “A Manual of Buddhist Monism” … Lamotte and Frauwallner have seen the tathagatagarbha doctrine as diametrically opposed to the Madhyamika and representing something akin to the monism of the atman/Brahman strain …

Buddhahood is thus taught to be the timeless, virtue-filled Real (although as yet unrecognised as such by the deluded being), present inside the mind of every sentient being from the beginningless beginning. Its disclosure to direct perception, however, depends on inner spiritual purification and purgation of the superficial obscurations which conceal it from view.