Mindstream in Buddhist philosophy is the moment-to-moment "continuum" (Sanskrit: saṃtāna) of awareness. There are a number of terms in the Buddhist literature that may well be rendered "mindstream". The mindstream doctrine, like most Buddhist doctrines, is not homogeneous and shows historical development, different applications according to context and varied definitions employed by different Buddhist traditions.
Most Buddhist schools are committed doctrinally to anātman (Pali: anattā), "non-self," the teaching that none of the things perceived by the senses constitute a "self." As Thanissaro Bhikkhu explains, "...the Buddha was asked point-blank whether or not there was a self, he refused to answer. When later asked why, he said that to hold either that there is a self or that there is no self is to fall into extreme forms of wrong view that make the path of Buddhist practice impossible." Scholar Herbert V. Gunther further explains, "an individual, which in other systems is imagined as a combination of matter and a permanent mental principle (ātman), is in reality a continuously changing stream of that which from one viewpoint is believed to be matter and from another a mind. However, what we call the mental and the material occurs in a unity of organization. Organization is something dynamic."
In discussing the continuity of mind or awareness in the absence of a self, various words and concepts have been employed. "Mindstream" is often used both colloquially and in more scholarly discourse, as when Dzogchen Rinpoche writes "[t]he Buddhadharma is a process, one through which we train and tame our own mindstreams. One approach is to go to the root of what we mean by 'I,' our sense of self or individual self-identity. According to scholar Wiliam Waldron, "Indian Buddhists see the 'evolution' of mind in terms of the continuity of individual mind-streams from one lifetime to the next, with karma as the basic causal mechanism whereby transformations are transmitted from one life to the next."
Scholar Caroline Augusta Foley Rhys Davids (1903: pp. 587–588) assesses Louis de La Vallée-Poussin's work on "mindstream" in Buddhism:
Professor de la Vallee Poussin finds a very positive evolution of vijnana-theory in certain Sanskrit-Buddhist texts. The term samtana is joined to or substituted for it — a term which seems to approximate to our own neopsychological concept of mind as a 'continuum' or flux. And he infers from certain contexts that this vijnana-samtana was regarded, not as one permanent, unchanging, transmigrating entity, as the soul was in the atman-theory, but as an "essential series of individual and momentary consciousnesses," forming a "procession vivace et autonome." By autonomous he means independent of physical processes. According to this view the upspringing of a new vijnana at conception, as the effect of the preceding last vijnana of some expiring person, represents no change in kind, but only, to put it so, of degree. The vijnana is but a recurring series, not a transferred entity or principle. Hence it is more correct, if less convenient, to speak, not of vijnana, but of the samtana of pravrtti-vijnanani.
In Vajrayāna (tantric Buddhism) "mindstream" may be understood as an upāya metaphor for the nonlocal, atemporal stream of moments (Tibetan: bkod-pa thig-le) or "quanta of consciousness" (Tibetan: thig-le; Sanskrit: bindu). It proceeds endlessly in a lifetime, in between lifetimes (Tibetan: bardo), from lifetime to lifetime, prior to engagement in the wheel of life, through samsara and beyond. It does so as an inclusive "continuum" (Tibetan: rgyud) rather than an individuated, separate, or discrete perceptual, cognitive, or experiential entity, as in the Buddhadharma conception of the ātman which is diametrically opposed to the Atman of the Upanishads.
A yogin who has recognized the inseparability of samsara and nirvana is said to dwell in (Wylie: zung-'jug; Sanskrit: yuganaddha). In the entwined Dzogchen traditions of Bönpo and Nyingmapa, the mindstream constitutes a continuum of gankyil composed of the five pure lights of the five wisdoms which unite the trikāya. These tantric correlations (or "twilight language") are evident in the iconographic representation of the five Jīnas and the saṃpanna-krama of the gankyil and mandala in Dzogchen practice. The "supreme siddhi" or "absolute bodhicitta" of the Dzogchenpa is when the stream of their bodymind (namarupa) is "released" (in nirvana) as the rainbow body.
The metaphorical use of "stream" to describe mentality is characteristic of but not unique to the Buddhist literature and worldview. In English for example, "stream of consciousness" is more familiar than "mindstream". This term, "stream of consciousness" was coined by the philosopher Alexander Bain in 1855 and later popularized by the psychologist, William James. Bain (1855: p. 380) wrote, "The concurrence of Sensations in one common stream of consciousness, — on the same cerebral highway, — enables those of different senses to be associated as readily as the sensations of the same sense." After originating in psychological theory, the "stream of consciousness" metaphor became more common in English usage, and was adapted into different contexts, for instance, the stream of consciousness (narrative mode) in literary criticism.