In Hindu philosophy, turiya (Sanskrit?, meaning "the fourth") or chaturiya, chaturtha, is pure consciousness. Turiya is the background that underlies and pervades the three common states of consciousness. The three common states of consciousness are: waking state, dreaming state, and dreamless deep sleep.[web 1][web 2]
Turiya is discussed in Verse 7 of the Mandukya Upanishad; however, the idea is found in the oldest Upanishads. For example, Chapters 8.7 through 8.12 of Chandogya Upanishad discuss the "four states of consciousness" as awake, dream-filled sleep, deep sleep, and beyond deep sleep. Similarly, Brihadaranyaka Upanishad, in chapter 5.14 discusses Turiya state, as does Maitri Upanishad in sections 6.19 and 7.11.
Verse VII of the Mandukya Upanishad describes Turiya:
Not inwardly cognitive, nor outwardly cognitive, not both-wise cognitive,
not a cognition-mass, not cognitive, not non-cognitive,
unseen, with which there can be no dealing, ungraspable, having no distinctive mark,
non-thinkable, that cannot be designated, the essence of assurance,
of which is the state of being one with the Self
the cessation of development, tranquil, benign, without a second,
such they think is the fourth. He is the Self (Atman). He should be discerned.-- Mandukya Upanishad 7, 
Advaita posits three states of consciousness, namely waking (jagrat), dreaming (svapna), deep sleep (su?upti), which are empirically experienced by human beings, and correspond to the Three Bodies Doctrine:
Advaita also posits the fourth state of Turiya, which some describe as pure consciousness, the background that underlies and transcends these three common states of consciousness.[web 1][web 2] Turiya is the state of liberation, where according to the Advaita school, one experiences the infinite (ananta) and non-different (advaita/abheda), that is free from the dualistic experience, the state in which ajativada, non-origination, is apprehended. According to Candradhara Sarma, Turiya state is where the foundational Self is realized, it is measureless, neither cause nor effect, all pervading, without suffering, blissful, changeless, self-luminous, real, immanent in all things and transcendent. Those who have experienced the Turiya stage of self-consciousness have reached the pure awareness of their own non-dual Self as one with everyone and everything, for them the knowledge, the knower, the known becomes one, they are the Jivanmukta.
Advaita traces the foundation of this ontological theory in more ancient Sanskrit texts. For example, chapters 8.7 through 8.12 of Chandogya Upanishad discuss the "four states of consciousness" as awake, dream-filled sleep, deep sleep, and beyond deep sleep. One of the earliest mentions of Turiya, in the Hindu scriptures, occurs in verse 5.14.3 of the Brihadaranyaka Upanishad. The idea is also discussed in other early Upanishads.
Gaudapada (ca. 7th century) was an early guru in the Advaita Vedanta. Gaudapada is traditionally said to have been the grand-guru of the great teacher, Adi Shankara, one of the most important figures in Hindu philosophy. Gaudapada is believed to be the founder of Shri Gaudapadacharya Math, and the author or compiler of the Mukya K?rik?.
Gaudapada wrote or compiled the Mukya K?rik?, also known as the Gau?ap?da K?rik? and as the ?gama stra.[note 1] In this work, Gaudapada deals with perception, idealism, causality, truth, and reality. The fourth state, (tur?ya avasth?), corresponds to silence, as the other three correspond to AUM. It is the substratum of the other three states. It is, states Nakamura, atyanta-shunyata (absolute emptiness).
Michael Comans disagrees with Nakamura's thesis that "the fourth realm (caturtha) was perhaps influenced by the Sunyata of Mahayana Buddhism."[note 2] According to Comans,
It is impossible to see how the unequivocal teaching of a permanent, underlying reality, which is explicitly called the "Self", could show early Mahayana influence.
Comans further refers to Nakamura himself, who notes that later Mahayana sutras such as the La?k?vat?ra S?tra and the concept of Buddha-nature, were influenced by Vedantic thought. Comans concludes that
[T]here can be no suggestion that the teaching about the underlying Self as contained in the Mandukya contains shows any trace of Buddhist thought, as this teaching can be traced to the pre-Buddhist Brhadaranyaka Upanishad.
Isaeva states that there are differences in the teachings in the texts of Buddhism and the Mandukya Upanishad of Hinduism, because the latter asserts that citta "consciousness" is identical with the eternal and immutable atman "soul, self" of the Upanishads. In other words, Mandukya Upanishad and Gaudapada affirm the soul exists, while Buddhist schools affirm that there is no soul or self.
Adi Shankara described, on the basis of the ideas propounded in the Mandukya Upanishad, the three states of consciousness, namely waking (jågrata), dreaming (svapna), and deep sleep (susupti),[web 3][web 4] which correspond to the three bodies:
In the waking consciousness, there is a sense of 'I' (self-identity) and awareness of thoughts. In the sleep or dream state, there is no or little sense of 'I'; however, there are thoughts and the awareness of thoughts. Waking and dreaming are not true experiences of Absolute Reality and metaphysical truth, because of their dualistic natures of subject and object, self and not-self, ego, and non-ego.
Kashmir Shaivism holds the state called turya - the fourth state. It is neither wakefulness, dreaming, nor deep sleep. In reality, it exists in the junction between any of these three states, i.e. between waking and dreaming, between dreaming and deep sleep, and between deep sleep and waking. . In Kashmir Shaivism there exists a fifth state of consciousness called Turiyatita - the state beyond Turiya. Turiyatita, also called the void or shunya is the state where one attains liberation otherwise known as jivanmukti or moksha.
While turiya stages 1 - 6 are attributed to the "internal subjective sam?dhi" (nim?lan? sam?dhi), once sam?dhi becomes permanently established in the 7. turiya stage it is described to span not only the internal subjective world anymore but beyond that also the whole external objective world (unim?lan? sam?dhi).
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