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