The Tathagata is pure. All beings share that essence.
This essence of the Tathagata,
when not free from the store of defilement,
is referred to as the Tathagatagarbha.
Luminosity is uncreated, inseparable and all-pervasive.
Its limitless qualities are more numerous
than the grains of sand on the banks of the Ganges.
Suchness and the Tathagata are of the same substance which is called Buddha Nature. The meaning of Tathagatagarbha, Sugatagarbha, and the essence of the Buddha is the same. It abides equally in the Buddha, in all phenomena, and in all sentient beings. In sentient beings, Buddha Nature is present, but covered. In the Buddhas, Buddha Nature is completely actualized.
All sentient beings desire unblemished purity, indisputable identity, uninterrupted bliss, and enduring existence. However, the experience of all beings in this phenomenal world is precisely the opposite: corruption, deception, suffering, and death. If our experience does not support such aspirations, there must be a cause for their arising, as there is no effect without a cause.
Buddha Nature is that cause. It is real and present in all sentient beings, and manifests as the four marks of ultimate reality:
True Purity is the luminous clarity of emptiness,
free from subject and object, knower and known, grasper and graspable.
True Self is pristine non-dual nature, endowed with all the qualities of enlightenment.
True Bliss is non-temporal, independent, and non-local delight.
True Permanence is non-referential, spontaneous compassion.
—Tashi Norbu Rinpoche
Thusness defiled is the Tathagatagarbha.
Thusness undefiled is Enlightenment.