Doctrine
What buddha-nature actually is, and how it can be described accurately, is a major topic in Buddhist philosophy. In general, Buddhists employ one of two main approaches to describe reality: they use either positive language which asserts the qualities of enlightenment and buddha-nature, or they use negative language which attempts to avoid the traps of dualism. From these two approaches come two common definitions of buddha-nature: that it is (a) none other than the natural luminosity of the mind, or that it is (b) the universal truth of emptiness. "Emptiness" refers to a theory of radical selflessness, in which all phenomena are understood to lack independent existence. It also refers to a specific way of using language to avoid implying such an existence, in which reality is consistently described by what it is not. The "natural luminosity of mind" is the idea that the fundamental nature of all sentient beings is nondual wisdom, a phenomenon also known as reflexive awareness—awareness that is aware of itself, free of the duality of subject and object that infects all forms of consciousness. Advocates of this view describe buddha-nature as fundamentally real, permanent, and unchanging, and this use of positive language puts it in conflict with emptiness theory and so has led to centuries of sometimes contentious debates. You will find information about all these fascinating debates on the pages of this website.
Indian and Tibetan philosophers have also debated whether buddha-nature is a definitive teaching (one given by the Buddha to describe things as they are) or a provisional teaching (one that is not true but is valuable for encouraging a student to move in the right direction). A famous parable of this sort of teaching from the Lotus Sūtra describes a group of children unaware that the building in which they play is burning; their father, unable to convince them to leave, tells them, falsely, that a marvelous chariot is outside the house, which they run to see, thereby surviving the fire. Like the father who tells his children an untruth in order to benefit them, the Buddha is said to have taught many doctrines that are not strictly accurate, but which benefit disciples by encouraging them to get on the path. For many philosophers, the doctrine of buddha-nature is like this—the idea that we possess this spark of enlightenment inspires us to practice, but such a thing cannot be actually said to exist because of the fundamental truth of emptiness. Others, however, take the position that the mind's natural luminosity is self-evident and need not be explained as a rhetorical trick.
The methods of practice relating to buddha-nature naturally reflect the theory that is used to describe it. If buddha-nature is already complete and perfect, then one need not do anything to attain it. Mahāyāna meditation and virtuous deeds become something one does to display one's enlightenment, as in the case of Zen and Japanese Shingon rituals, or one need only rest in the natural purity of mind, as in Tibetan Dzogchen or Mahāmudrā. On the other hand, if buddha-nature is a potential that must be developed and perfected, then one has a lot of work to do, and those same practices are described as necessary in order to accumulate merit and purify the obscurations, such as are described in both Tantra and Sūtra-based path structures embraced by the Geluk and other Tibetan traditions.
Buddha-nature is taught in a wide range of scriptures, both those said to be the word of the Buddha and commentaries by Indian, Chinese, and Tibetan masters. A group of early Indian sūtras introduced the theory around the third or fourth century of the Common Era. The diverse views presented in this literature were first systematized in a treatise known as the Ratnagotravibhāga Mahāyānottaratantraśāstra, a title that roughly translates as “The Ultimate Teaching (uttaratantra)[1] of the Mahāyāna, A Treatise (śāstra) Analyzing (vibhāga) the Jewel (ratna) Disposition (gotra)." In Tibet this text is divided into root verses and commentary which are separately attributed to the bodhisattva Maitreya and the great Indian master Asaṅga, while in East Asia the entire text is believed to have been written by a north Indian named Sāramati. Popularly known in Tibet as the Gyü Lama (Uttaratantra in Sanskrit), the treatise has been widely taught and written about by all traditions of Buddhism there. The treatise was less influential in East Asia, where the Mahāparinirvāṇasūtra was of much greater importance for the spread of buddha-nature theory.
