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The Upanishads (Sanskrit: उपनिषद्, IAST: Upaniṣad, IPA: [upəniʂəd]) are a collection of philosophical texts which form the theoretical basis for the Hindu religion. They are also known as Vedanta, the end of the veda. The Upanishads are considered by orthodox Hindus to contain revealed truths (Sruti) concerning the nature of ultimate reality (brahman) and describing the character and form of human salvation (moksha). The Upanishads are found mostly in the concluding part of the Brahmanas and Aranyakas and have been passed down in oral tradition.
More than 200 Upanishads are known, of which the first dozen or so are the oldest and most important and are referred to as the principal or main (mukhya) Upanishads. With the Bhagavad Gita and the Brahmasutra (known collectively as the Prasthanatrayi), the mukhya Upanishads provide a foundation for several later schools of Indian philosophy (vedanta), among them, two influential monistic schools of Hinduism.[note 1][note 2][note 3]
Historians believe the chief Upanishads were composed over a wide period ranging from the Pre-Buddhist period to the early centuries BCE though minor Upanishads were still being composed in the medieval and early modern period. However, there has been considerable debate among authorities about the exact dating of individual Upanishads. The Upanishads were collectively considered amongst the 100 Most Influential Books Ever Written by the British poet Martin Seymour-Smith. Their significance has been recognized by writers and scholars such as Schopenhauer, Emerson and Thoreau, among others. Scholars also note similarity between the doctrine of Upanishads and those of Plato and Kant 
The Sanskrit term Upaniṣad derives from upa- (nearby), ni- (at the proper place, down) and ṣad (to sit) thus: "sitting down near", implying sitting near a teacher to receive instruction or, alternatively, "sitting at the foot of ..(teacher)", or "laying siege" to the teacher. Monier-Williams' late 19th century dictionary adds that, "according to native authorities Upanishad means 'setting to rest ignorance by revealing the knowledge of the supreme spirit.'" A gloss of the term Upanishad based on Shri Adi Shankara's commentary on the Kaṭha and Brihadaranyaka Upanishad equates it with Ātmavidyā, that is, "knowledge of the Self", or Brahmavidyā "knowledge of Brahma". Other dictionary meanings include "esoteric doctrine" and "secret doctrine".
There are more than 200 known Upanishads, one of which, the Muktikā, gives a list of 108 Upanishads – this number corresponding to the holy Hindu number of beads on a mala or Hindu rosary. Modern scholars recognize the first 10, 11, 12 or 13 Upanishads as principal or Mukhya Upanishads and the remainder as derived from this ancient canon. If an Upanishad has been commented upon or quoted by revered thinkers like Shankara, it is a Mukhya Upanishad, accepted as shruti by most Hindus.
The new Upanishads recorded in the Muktikā probably originated in southern India, and are grouped according to their subject as (Sāmānya) Vedānta (philosophical), Yoga, Sanyasa (of the life of renunciation), Vaishnava (dedicated to the god Vishnu), Shaiva (dedicated to Shiva) and Shakti (dedicated to the goddess). New Upanishads are often sectarian since sects have sought to legitimize their texts by claiming for them the status of Śruti.
Another way of classifying the Upanishads is to associate them with the respective Brahmanas. Of nearly the same age are the Aitareya, Kauṣītaki and Taittirīya Upanishads, while the remnant date from the time of transition from Vedic to Classical Sanskrit.
Mukhya Upanishads 
The Mukhya Upanishads can themselves be stratified into periods. Of the early periods are the Brihadaranyaka, Jaiminiya Upanisadbrahmana and the Chandogya, the most important and the oldest, of which the two former are the older of the two, though some parts were composed after the Chandogya.[note 4]
It is alleged that the Aitareya, Taittiriya, Kausitaki, Mundaka, Prasna, and Katha Upanishads show Buddha's influence, and must have been composed after the 5th century BCE, but it could just as easily have been the other way around. It is also alleged that in the first two centuries A.D., they were followed by the Kena, Mandukya and Isa Upanishads. Not much is known about the authors except for those, like Yajnavalkayva and Uddalaka, mentioned in the texts. A few women discussants, such as Gargi and Maitreyi, the wife of Yajnavalkayva, also feature occasionally.
Each of the principal Upanishads can be associated with one of the schools of exegesis of the four Vedas (shakhas). Many Shakhas are said to have existed, of which only a few remain. The new Upanishads often have little relation to the Vedic corpus and have not been cited or commented upon by any great Vedanta philosopher: their language differs from that of the classic Upanishads, being less subtle and more formalized. As a result, they are not difficult to comprehend for the modern reader.
|Rig Veda||Only one recension||Shakala||Aitareya|
|Sama Veda||Only one recension||Kauthuma||Chāndogya|
|Yajur Veda||Krishna Yajur Veda||Katha||Kaṭha|
|Taittiriya||Taittirīya and Śvetāśvatara|
|Shukla Yajur Veda||Vajasaneyi Madhyandina||Isha and Bṛhadāraṇyaka|
|Atharva||Two recension||Shaunaka||Māṇḍūkya and Muṇḍaka|
New Upanishads 
There is no fixed list of the Upanishads as newer ones have continued to be composed. On many occasions, when older Upanishads have not suited the founders of new sects, they have composed new ones of their own. 1908 marked the discovery of four new Upanishads, named Bashkala, Chhagaleya, Arsheya and Saunaka, by Friedrich Schrader, who attributed them to the first prose period of the Upanishads. The text of three, the Chhagaleya, Arsheya and Saunaka, was reportedly corrupt and neglected but possibly re-constructable with the help of their Perso-Latin translations. Texts called "Upanishads" continued to appear up to the end of British rule in 1947.
The main Shakta Upanishads mostly discuss doctrinal and interpretative differences between the two principal sects of a major Tantric form of Shaktism called Shri Vidya upasana. The many extant lists of authentic Shakta Upaniṣads vary, reflecting the sect of their compilers, so that they yield no evidence of their "location" in Tantric tradition, impeding correct interpretation. The Tantra content of these texts also weaken its identity as an Upaniṣad for non-Tantrikas and therefore, its status as shruti and thus its authority.
The text composed by Vaishnava saint Namalvar (Satkopa) is also known as the Dravidopanisatsangati.
Two words that are of paramount importance in grasping the Upanishads are Brahman and Atman. The Brahman is the universal spirit and the Atman is the individual Self. Differing opinions exist amongst scholars regarding the etymology of these words. Brahman probably comes from the root brh, which means "The Biggest ~ The Greatest ~ The ALL". Brahman is "the infinite Spirit Source and fabric and core and destiny of all existence, both manifested and unmanifested and the formless infinite substratum and from whom the universe has grown". Brahman is the ultimate, both transcendent and immanent, the absolute infinite existence, the sum total of all that ever is, was, or shall be. The word Atman means the immortal perfect Spirit of any living creature, being, including trees etc. The idea put forth by the Upanishadic seers that Atman and Brahman are One and the same is one of the greatest contributions made to the thought of the world.
The Brihadaranyaka and the Chandogya are the most important of the mukhya Upanishads. They represent two main schools of thought within the Upanishads. The Brihadaranyaka deals with acosmic or nis-prapancha, whereas the Chandogya deals with the cosmic or sa-prapancha. Between the two, the Brihadaranyaka is considered more original.
The Upanishads also contain the first and most definitive explications of the divine syllable Aum, the cosmic vibration that underlies all existence. The mantra Aum Shānti Shānti Shānti, translated as "the soundless sound, peace, peace, peace", is often found in the Upanishads. The path of bhakti or "Devotion to God" is foreshadowed in Upanishadic literature, and was later realized by texts such as the Bhagavad Gita.
|Sanskrit quote||English meaning||Upanishad|
|Prajñānam brahma||"Consciousness is Brahman"||Aitareya Upanishad
|Aham brahmāsmi||"I am Brahman"||Brihadaranyaka
|Tat tvam asi||"That Thou art"||Chandogya
|Ayamātmā brahmā||"This Atman is Brahman"||Mandukya
Sarvepalli Radhakrishnan notes that the Upanishads are primarily presented as conversations between two persons or animals rather than expository statements of philosophy or ideology. He contends that the frog's metaphorical speech the Mandukya Upanishad (manduka means frog in Sanskrit) is a common source of confusion.
The three main approaches in arriving at the solution to the problem of the Ultimate Reality have traditionally been the theological, the cosmological and the psychological approaches. The cosmological approach involves looking outward, to the world; the psychological approach meaning looking inside or to the Self; and the theological approach is looking upward or to God. Descartes takes the first and starts with the argument that the Self is the primary reality, self-consciousness the primary fact of existence, and introspection the start of the real philosophical process. According to him, we can arrive at the conception of God only through the Self because it is God who is the cause of the Self and thus, we should regard God as more perfect than the Self. Spinoza on the other hand, believed that God is the be-all and the end-all of all things, the alpha and the omega of existence. From God philosophy starts, and in God philosophy ends. The manner of approach of the Upanishadic philosophers to the problem of ultimate reality was neither the Cartesian nor Spinozistic. The Upanishadic philosophers regarded the Self as the ultimate existence and subordinated the world and God to the Self. The Self to them, is more real than either the world or God. It is only ultimately that they identify the Self with God, and thus bridge over the gulf that exists between the theological and psychological approaches to reality. They take the cosmological approach to start with, but they find that this cannot give them the solution of the ultimate reality. So, Upanishadic thinkers go back and start over by taking the psychological approach and here again, they cannot find the solution to the ultimate reality. They therefore perform yet another experiment by taking the theological approach. They find that this too is lacking in finding the solution. They give yet another try to the psychological approach, and come up with the solution to the problem of the ultimate reality. Thus, the Upanishadic thinkers follow a cosmo-theo-psychological approach. A study of the mukhya Upanishads shows that the Upanishadic thinkers progressively build on each other's ideas. They go back and forth and refute improbable approaches before arriving at the solution of the ultimate reality.
Schools of Vedanta 
The source for all schools of Vedanta are the three texts – the Upanishads, the Bhagavad Gita and the Brahmasutras. Two different types of the non-dual Brahman-Atman are presented in the Upanishads:
- The one in which the non-dual Brahman-Atman is the all inclusive ground of the universe and
- The one in which all reality in the universe is but an illusion
The later theistic (Dvaita and Visistadvaita) and absolutist (Advaita) schools of Vendanta are made possible because of the difference between these two views. The three main schools of Vedanta are Advaita, Dvaita and Vishishtadvaita. Other schools of Vedanta made possible by the Upanishads include Nimbarka's Dvaitadvaita, Vallabha's Suddhadvaita and Chaitanya's Acintya Bhedabheda. The philosopher Adi Sankara has provided commentaries on 11 mukhya Upanishads.
Advaita is considered the most influential sub-school of the Vedanta school of Hindu philosophy, though whether it represents the mainstream Hindu position has been debated. Gaudapada was the first person to expound the basic principles of the Advaita philosophy in a commentary on the apparently conflicting statements of the Upanishads. Advaita literally means non-duality, and it is a monistic system of thought. It deals with the non-dual nature of Brahman and Atman. The Advaita school is said to have been consolidated by Shankara. He was a pupil of Gaudapada's pupil. Radhakrishnan believed that Shankara's views of Advaita are straightforward developments of the Upanishads and the Brahmasutra and he offered no innovations to these, while other scholars found sharp differences between Shankara's writings and the Brahmasutra, and that there are many ideas in the Upanishads at odds with those of Shankara. Gaudapada lived in a time when Buddhism was widely prevalent in India, and he was at times conscious of the similarity between his system to some phases of Buddhist thought. His main work is infused with philosophical terminology of Buddhism, and uses Buddhist arguments and analogies. Towards the end of his commentary on the topic, he clearly said, "This was not spoken by Buddha". Although there are a wide variety of philosophical positions propounded in the Upanishads, commentators since Adi Shankara have usually followed him in seeing idealist monism as the dominant force.[note 5][note 6][note 7][note 8][note 9]
The Dvaita school was founded by Madhvacharya. Born in 1138 near Udipi, Dvaita is regarded as the best philosophic exposition of theism. Sharma points out that Dvaita, a term commonly used to designate Madhava's system of philosophy, translates as "dualism" in English. The Western understanding of dualism equates to two independent and mutually irreducible substances. The Indian equivalent of that definition would be Samkya Dvaita. Madhva's Dvaita differs from the Western definition of dualism in that while he agrees to two mutually irreducible substances that constitute reality, he regards only one – God, as being independent.
The third school of Vedanta is the Vishishtadvaita, which was founded by Ramanuja. Traditional dates of his birth and death are given as 1017 and 1137, though a shorter life span somewhere between these two dates has been suggested. Modern scholars conclude that on the whole, Ramanuja's theistic views may be closer to those of the Upanishads than are Shankara's, and Ramanuja's interpretations are in fact representative of the general trend of Hindu thought. Ramanuja strenuously refuted Shankara's works. Visistadvaita is a synthetic philosophy of love that tries to reconcile the extremes of the other two monistic and theistic systems of vedanta. It is called Sri-Vaisanavism in its religious aspect. Chari claims that has been misunderstood by its followers as well as its critics. Many, including leading modern proponents of this system, forget that jiva is a substance as well as an attribute and call this system "qualified non-dualism" or the adjectival monism. While the Dvaita insists on the difference between the Brahman and the Jiva, Visistadvaita states that God is their inner-Self as well as transcendent.
Number and Authorship 
New Upanishads were still composed in the medieval and early modern period: discoveries of newer Upanishads were being reported as late as 1926. One, the Muktikā Upanishad, predates 1656 and contains a list of 108 canonical Upanishads, including itself as the last. However, several texts under the title of "Upanishads" originated right up to the first half of the 20th century, some of which did not deal with subjects of Vedic philosophy. The newer Upanishads are known to be imitations of the mukhya Upanishads.
Dara Shikoh, son of the Mughal emperor Shah Jahan, translated 50 Upanishads into Persian in 1657. The first written English translation came in 1805 from Colebrooke, who was aware of 170 Upanishads. Sadhale's catalog from 1985, the Upaniṣad-vākya-mahā-kośa lists 223 Upanishads.
The Upanishads have been attributed to several authors: Yajnavalkya and Uddalaka Aruni feature prominently in the early Upanishads. Other important writers include Shvetaketu, Shandilya, Aitareya, Pippalada and Sanatkumara. Important women discussants include Yajnavalkya's wife Maitreyi, and Gargi.
Sarvepalli Radhakrishnan considers authorship claims in the text to be unreliable, believing the supposed authors to be fictional characters. An example is Shvetaketu from Chāndogya Upaniṣad for whom there are no sources or books which mention him nor any other works attributed to him.
Chronology and geography 
Scholars disagree about the exact dates of the composition of the Upanishads. Different researchers have provided different dates for the Vedic and Upanashic eras. Some authors believe the oldest of these, the Brihadaranyaka, Jaiminiya Upanisadbrahmana and the Chandogya Upanishads, were composed during the pre-Buddhist era of India,[note 10] while the Taittiriya, Aitareya and Kausitaki, which show Buddhist influence, must have been composed after the 5th century BCE. The remainder of the mukhya Upanishads are dated to the last few centuries BCE.
Ranade criticizes Deussen for assuming that the oldest Upanishads were written in prose, followed by those that were written in verse and the last few again in prose. He proposes a separate chronology based on a battery of six tests. The tables below summarize some of the prominent work:
The general area of the composition of the early Upanishads was northern India, the region bounded on the west by the Indus valley, on the east by lower Ganges river, on the north by the Himalayan foothills, and on the south by the Vindhya mountain range. There is confidence about the early Upanishads being the product of the geographical center of ancient Brahmanism, comprising the regions of Kuru-Panchala and Kosala-Videha together with the areas immediately to the south and west of these.
While significant attempts have been made recently to identify the exact locations of the individual Upanishads, the results are tentative. Witzel identifies the center of activity in the Brihadaranyaka Upanishad as the area of Videha, whose king, Janaka, features prominently in the Upanishad. Yajnavalkya is another individual who features prominently, almost as the personal theologian of Janaka. Brahmins of the central region of Kuru-Panchala rightly considered their land as the place of the best theological and literary activities, since this was the heartland of Brahmanism of the late Vedic period. The setting of the third and the fourth chapters of the Brihadaranyaka Upanishads were probably intended to show that Yajnavalkya of Videha defeated all the best theologians of the Kuru Panchala, thereby demonstrating the rise of Videha as a center of learning. The Chandogya Upanishad was probably composed in a more Western than an Eastern location, possibly somewhere in the western region of the Kuru-Panchala country. The great Kuru-Panchala theologian Uddalaka Aruni who was vilified in the Brihadaranyaka features prominently in the Chandogya Upanishad. Compared to the Principal Upanishads, the new Upanishads recorded in the Muktikā belong to an entirely different region, probably southern India, and are considerably relatively recent.
Sarvepalli Radhakrishnan claims that most of the Upanishads were kept secret for centuries, only passed on to others orally in the form of Shloka, and that it difficult to determine how much the current texts have changed from the original.
Development of thought 
While the hymns of the Vedas emphasize rituals and the Brahmanas serve as a liturgical manual for those Vedic rituals, the spirit of the Upanishads is inherently opposed to ritual. The older Upanishads launch attacks of increasing intensity on the ritual. Anyone who worships a divinity other than the Self is called a domestic animal of the gods in the Brihadaranyaka Upanishad. The Chāndogya Upanishad parodies those who indulge in the acts of sacrifice by comparing them with a procession of dogs chanting Om! Let's eat. Om! Let's drink. The Mundaka launches the most scathing attack on the ritual by comparing those who value sacrifice with an unsafe boat that is endlessly overtaken by old age and death.
The opposition to the ritual is not explicit all the time. On several occasions the Upanishads extend the task of the Aranyakas by making the ritual allegorical and giving it a philosophical meaning. For example, the Brihadaranyaka interprets the practice of horse-sacrifice or ashvamedha allegorically. It states that the over-lordship of the earth may be acquired by sacrificing a horse. It then goes on to say that spiritual autonomy can only be achieved by renouncing the universe which is conceived in the image of a horse.
In similar fashion, the pattern of reducing the number of gods in the Vedas becomes more emphatic in the Upanishads. When Yajnavalkaya is asked how many gods exist, he decrements the number successively by answering thirty-three, six, three, two, one and a half and finally one. Vedic gods such as the Rudras, Visnu, Brahma are gradually subordinated to the supreme, immortal and incorporeal Brahman of the Upanishads. In fact Indra and the supreme deity of the Brahamanas, Prajapati, are made door keepers to the Brahman's residence in the Kausitaki Upanishad.
In short, the one reality or ekam sat of the Vedas becomes the ekam eva a-dvitiyam or "the one and only and sans a second" in the Upanishads.
Worldwide transmission 
Sarvepalli Radhakrishnan claims that translation often requires difficult research and subjective choices because most of the Upanishads were written in an old Sanskrit (comparable to Old Latin), which is no longer spoken and has a complicated sentence structure.
However, the Upanishads have still influenced world culture in part through later Hindu texts, such as the Bhagavad Gītā, which Radhakrishnan says conveyed a "message based on the ancient wisdom, prajñā purāņī, of the Upaniṣads.":13 The Gītā Dhyānam, a 9-verse poetic invocation that is often published with the Gītā, celebrates the purported Upanishadic influence in a famous verse stating that "The Upaniṣads are the cows... and the nectar-like gitā is the excellent milk.":13
Given that Indian Brahmin seers are reputed to have visited Greece, it may be that the Upanishadic sages influenced Ancient Greek philosophy. Many ideas in Plato's Dialogues, particularly, have Indian analogues – several concepts in the Platonic psychology of reason bear resemblance to the gunas of Indian philosophy. Professor Edward Johns Urwick conjectures that The Republic owes several central concepts to Indian influence. Garb and West have also concluded that this was due to Indian influence.
A. R. Wadia dissents in that Plato's metaphysics were rooted in this life, the primary aim being an ideal state. He later proposed a state less ordered but more practicable and conducive to human happiness. As for the Upanishadic thinkers, their goal was not an ideal state or society, but moksha or deliverance from the endless cycle of birth and death. Wadia concludes that there was no exchange of information and ideas between Plato and the Upanishadic thinkers: Plato remains Greek and the Indian sages remain Indian.
The Upanishads were a part of an oral tradition. Their study was confined to the higher castes of Indian society. Sudras and women were not given access to them soon after their composition. The Upanishads have been translated into various languages including Persian, Italian, Urdu, French, Latin, German, English, Dutch, Polish, Japanese and Russian. The Moghul Emperor Akbar's reign (1556–1586) saw the first translations of the Upanishads into Persian, and his great-grandson, Dara Shikoh, produced a collection called Sirr-e-Akbar (The Greatest Mysteries) in 1657, with the help of Sanskrit Pandits of Varanasi. Its introduction stated that the Upanishads constitute the Qur'an's "Kitab al-maknun" or hidden book. But Akbar's and Sikoh's translations remained unnoticed in the Western world until 1775.
Abraham Hyacinthe Anquetil-Duperron, a French Orientalist who had lived in India between 1755 and 1761, received a manuscript of the Upanishads in 1775 from M. Gentil, and translated it into French and Latin, publishing the Latin translation in two volumes in 1802–1804 as Oupneck'hat. The French translation was never published. The first German translation appeared in 1832 and Roer's English version appeared in 1853. However, Max Mueller's 1879 and 1884 editions were the first systematic English treatment to include the 12 Principal Upanishads. After this, the Upanishads were rapidly translated into Dutch, Polish, Japanese and Russian.
Global scholarship and praise 
The German philosopher Arthur Schopenhauer read the Latin translation and praised the Upanishads in his main work, The World as Will and Representation (1819), as well as in his Parerga and Paralipomena (1851). He found his own philosophy was in accord with the Upanishads, which taught that the individual is a manifestation of the one basis of reality. For Schopenhauer, that fundamentally real underlying unity is what we know in ourselves as "will". Schopenhauer used to keep a copy of the Latin Oupnekhet by his side and commented, "It has been the solace of my life, it will be the solace of my death". Another German philosopher, Friedrich Wilhelm Joseph Schelling, praised the mystical and spiritual aspects of the Upanishads. Schelling and other philosophers associated with German idealism were dissatisfied with Christianity as propagated by churches. They were fascinated with the Vedas and the Upanishads. In the United States, the group known as the Transcendentalists were influenced by the German idealists. These Americans, such as Emerson and Thoreau, were not satisfied with traditional Christian mythology and therefore embraced Schelling's interpretation of Kant's Transcendental idealism, as well as his celebration of the romantic, exotic, mystical aspect of the Upanishads. As a result of the influence of these writers, the Upanishads gained renown in Western countries. One of the great English-language poets of the 20th century, T. S. Eliot, inspired by his reading of the Upanishads, based the final portion of his famous poem The Waste Land (1922) upon one of its verses. Erwin Schrödinger, the great quantum physicist said, "The multiplicity is only apparent. This is the doctrine of the Upanishads. And not of the Upanishads only. The mystical experience of the union with God regularly leads to this view, unless strong prejudices stand in the West." Eknath Easwaran, in translating the Upanishads, articulates how they "form snapshots of towering peaks of consciousness taken at various times by different observers and dispatched with just the barest kind of explanation".
John Murray Mitchell, a Western writer, asserts that by suggesting that all appearance is an illusion, the Upanishads are potentially overturning ethical distinctions. A.E. Gough, an early European orientalist, reflecting the colonial racist attitudes of that time towards India and Indians, remarked that the Upanishads were "the work of a rude age, a deteriorated race, and a barbarous and unprogressive community." About the Indian Philosophy in general, Gough continued to say, "In treating of Indian Philosophy a writer has to deal with thoughts of lower order than the thoughts of the every day life of Europe. The great difficulty lies in this, that a low order of ideas has to be expressed in a high order of terms, and that the English words suggest a wealth of analysis and association altogether foreign to thoughts that are to be reproduced. The effort is nothing less than an endeavour to revert to a ruder type of mental culture and to become for the time being barbarous." According to another writer, David Kalupahana, the Upanishadic thinkers came to consider change as a mere illusion, because it could not be reconciled with a permanent and homogeneous reality. They were therefore led to a complete denial of plurality. He states that philosophy suffered a setback because of the transcendentalism resulting from the search of the essential unity of things. Kalupahana explains further that reality was simply considered to be beyond space, time, change, and causality. This caused change to be a mere matter of words, nothing but a name and due to this, metaphysical speculation took the upper hand. As a result, the Upanishads fail to give any rational explanation of the experience of things. Paul Deussen criticized the idea of unity in the Upanishads as it excluded all plurality, and therefore, all proximity in space, all succession in time, all interdependence as cause and effect, and all opposition as subject and object.
Association with Vedas 
All Upanishads are associated with one of the four Vedas—Rigveda, Samaveda, Yajurveda (there are two primary versions or Samhitas of the Yajurveda: Shukla Yajurveda, Krishna Yajurveda), and Atharvaveda. The Muktikā Upanishad's list of 108 Upanishads groups the first 10 as mukhya, 21 as Sāmānya Vedānta, 23 as Sannyāsa, nine as Shākta, 13 as Vaishnava, 14 as Shaiva and 17 as Yoga. The 108 Upanishads as recorded in the Muktikā are shown in the table below. The mukhya Upanishads are highlighted.
|Ṛigveda||Aitareya||Kauśītāki, Ātmabodha, Mudgala||Nirvāṇa||Tripura, Saubhāgya, Bahvṛca||-||Akṣamālika (Mālika)||Nādabindu|
|Samaveda||Chāndogya, Kena||Vajrasūchi, Mahad, Sāvitrī||Āruṇeya, Maitrāyaṇi, Maitreyi, Sannyāsa, Kuṇḍika||-||Vāsudeva, Avyakta||Rudrākṣa, Jābāla||Yogachūḍāmaṇi, Darśana|
|Krishna Yajurveda||Taittirīya, Kaṭha||Sarvasāra, Śukarahasya, Skanda (Tripāḍvibhūṭi), Śārīraka, Ekākṣara, Akṣi, Prāṇāgnihotra||Brahma, Śvetāśvatara, Garbha, Tejobindu, Avadhūta, Kaṭharudra, Varāha||Sarasvatīrahasya||Nārāyaṇa (Mahānārāyaṇa), Kali-Saṇṭāraṇa (Kali)||Śvetāśvatara, Kaivalya, Kālāgnirudra, Dakṣiṇāmūrti, Rudrahṛdaya, Pañcabrahma||Amṛtabindu, Amṛtanāda, Kṣurika, Dhyānabindu, Brahmavidyā, Yogatattva, Yogaśikhā, Yogakuṇḍalini|
|Shukla Yajurveda||Bṛhadāraṇyaka, Īśa||Subāla, Mantrikā, Nirālamba, Paiṅgala, Adhyātmā, Muktikā||Jābāla, Paramahaṃsa, Advayatāraka, Bhikṣu, Turīyātīta, Yājñavalkya, Śāṭyāyani||-||Tārasāra||-||Haṃsa, Triśikhi, Maṇḍalabrāhmaṇa|
|Atharvaveda||Muṇḍaka, Māṇḍūkya, Praśna||Sūrya, Ātmā||Parivrāt (Nāradaparivrājaka), Paramahaṃsaparivrājaka, Parabrahma||Sītā, Annapūrṇa, Devī, Tripurātapani, Bhāvana||Nṛsiṃhatāpanī, Mahānārāyaṇa (Tripādvibhuti), Rāmarahasya, Rāmatāpaṇi, Gopālatāpani, Kṛṣṇa, Hayagrīva, Dattātreya, Gāruḍa||Śira, Atharvaśikha, Bṛhajjābāla, Śarabha, Bhasma, Gaṇapati||Śāṇḍilya, Pāśupata, Mahāvākya|
See also 
- Advaita Vedanta, summarized by Shankara (788–820), advances a non-dualistic (a-dvaita) interpretation of the Upanishads."
- "These Upanishadic ideas are developed into Advaita monism. Brahman's unity comes to be taken to mean that appearances of individualities.
- "The doctrine of advaita (non dualism) has is origin in the Upanishads."
- These are believed to pre-date Gautam Buddha (c. 500 BCE)
- The breakdown of the Vedic cults is more obscured by retrospective ideology than any other period in Indian history. It is commonly assumed that the dominant philosophy now became an idealist monism, the identification of atman (self) and Brahman (Spirit), and that this mysticism was believed to provide a way to transcend rebirths on the wheel of karma. This is far from an accurate picture of what we read in the Upanishads. It has become traditional to view the Upanishads through the lens of Shankara's Advaita interpretation. This imposes the philosophical revolution of about 700 C.E. upon a very different situation 1,000 to 1,500 years earlier. Shankara picked out monist and idealist themes from a much wider philosophical lineup.
- In this Introduction I have avoided speaking of 'the philosophy of the upanishads', a common feature of most introductions to their translations. These documents were composed over several centuries and in various regions, and it is futile to try to discover a single doctrine or philosophy in them.
- The Upanishadic age was also characterized by a pluralism of worldviews. While some Upanishads have been deemed 'monistic', others, including the Katha Upanishad, are dualistic.
- The Maitri is one of the Upanishads that inclines more toward dualism, thus grounding classical Samkhya and Yoga, in contrast to the non-dualistic Upanishads eventuating in Vedanta.
- For instances of Platonic pluralism in the early Upanishads see Randall.
- The date of the Buddha's birth and death are uncertain: most early 20th-century historians dated his lifetime as c. 563 BCE to 483 BCE, but more recent opinion dates his death to between to between 486 and 483 BCE or, according to some, between 411 and 400 BCE.
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Further reading 
- Edmonds, I.G (1979), Hinduism, New York: Franklin Watts, ISBN 0-531-02943-3
- Embree, Ainslie T (1966), The Hindu Tradition, New York: Random House, ISBN 0-394-71702-3
- Frances Merrett, ed. (1985), The Hindu World, London: MacDonald and Co
- Pandit, Bansi; Glen, Ellyn (1998), The Hindu Mind, B&V Enterprises, ISBN 81-7822-007-5
- Radhakrishnan, Sarvapalli (1994) , The Principal Upanishads, New Delhi: HarperCollins Publishers India, ISBN 81-7223-124-5
- Wangu, Madhu Bazaz (1991), Hinduism: World Religions, New York: Facts on File, ISBN 0-8160-4400-7
- Max Müller, translator, The Upaniṣads, Part I, New York: Dover Publications, Inc., 1962, ISBN 0-486-20992-X.
- Max Müller, translator, The Upaniṣads, Part II, New York: Dover Publications, Inc., 1962, ISBN 0-486-20993-8.
- Three Upanisads of The Vedanta by J.L. Bansal
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