In Hindu philosophy, turiya (Sanskrit: तुरीय, meaning "the fourth") or caturiya, chaturtha, is pure consciousness. Turiya is the background that underlies and transcends the three common states of consciousness. The states of consciousness are: waking consciousness, dreaming, and dreamless sleep.
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Mandukya Upanishad
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:
The insight during meditation of Turiya is known as amātra, the 'immeasurable' or 'measureless' in the Mandukya Upanishad, being synonymous to samādhi in Yoga terminology.
Gaudapada
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 Māṇḍukya Kārikā.
Gaudapada wrote or compiled the Māṇḍukya Kārikā, also known as the Gauḍapāda Kārikā and as the Āgama Śāstra. In this work, Gaudapada deals with perception, idealism, causality, truth, and reality. The fourth state, (turīya avasthā), 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." 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 Laṅkāvatāra Sūtra 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.
Shankara
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), 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/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
Kashmir Shaivism is a school of Śaivism consisting of Trika, the three goddesses Parā, Parāparā and Aparā, and its philosophical articulation in Pratyabhijña, a branch of Kashmir Shaivism. It is described by Abhinavaguptaas "paradvaita", meaning "the supreme and absolute non-dualism".
The philosophy of Kashmir Shaivism can be seen in contrast to Shankara's Advaita. Advaita Vedanta holds that Brahman is inactive (niṣkriya) and the phenomenal world is an illusion (māyā). In Kashmir Shaivisim, all things are a manifestation of the Universal Consciousness, Chit or Brahman. Kashmir Shaivisim sees the phenomenal world (Śakti) as real: it exists, and has its being in Consciousness (Chit).
The goal of Kashmir Shaivism is to merge in Shiva or Universal Consciousness, or realise one's already existing identity with Shiva, by means of wisdom, yoga and grace.
Shiva Sutras of Vasugupta
According to the Shiva Sutras, Turiya is the fourth state of consciousness beyond the states of waking, dreaming and deep sleep. Turiya strings together those three states. It is the Metaphysical Consciousness distinct from the psychological or empirical self. It is the Saksi or witnessing consciousness. And it is the transcendental Self.