The Upanishads ( Sanskrit: Upaniṣad ; IPA: [ʊpən̪ɪʂəd̪] ) are a collection of texts that contain some of the central philosophical concepts of Hinduism, some of which are shared with Buddhism and Jainism. The Upanishads are considered by Hindus to contain utterances ( śruti ) concerning the nature of ultimate reality ( brahman ) and describing the character of and path to human salvation ( mokṣa or mukti ).
The Upanishads are commonly referred to as Vedānta , variously interpreted to mean either the "last chapters, parts of the Veda " or "the object, the highest purpose of the Veda".  The concepts of Brahman (Ultimate Reality) and Ātman (Soul, Self) are central ideas in all the Upanishads, and "Know your Ātman" their thematic focus.  The Upanishads are the foundation of Hindu philosophical thought and its diverse traditions. Of the Vedic corpus, they alone are widely known, and the central ideas of the Upanishads are at the spiritual core of Hindus.
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. The mukhya Upanishads are found mostly in the concluding part of the Brahmanas and Aranyakas and were, for centuries, memorized by each generation and passed down orally. The early Upanishads all predate the Common Era, some in all likelihood pre-Buddhist (6th century BCE), down to the Maurya period. Of the remainder, some 95 Upanishads are part of the Muktika canon, composed from about the last centuries of 1st-millennium BCE through about 15th-century CE. New Upanishads, beyond the 108 in the Muktika canon, continued to be composed through the early modern and modern era, though often dealing with subjects which are unconnected to the Vedas.
Along with the Bhagavad Gita and the Brahmasutra, the mukhya Upanishads (known collectively as the Prasthanatrayi ) provide a foundation for the several later schools of Vedanta, among them, two influential monistic schools of Hinduism.
With the translation of the Upanishads in the early 19th century they also started to attract attention from a western audience. Arthur Schopenhauer was deeply impressed by the Upanishads and called it "the production of the highest human wisdom". The 19th-century transcendentalists noted the influence of the Upanishads in western philosophy. 
The Sanskrit term Upaniṣad (u = at, pa = foot, nishat =sitting down) translates to "sitting at the foot/feet of", referring to the student sitting down near the teacher while receiving esoteric knowledge. Monier-Williams' Sanskrit Dictionary adds that, "According to native authorities Upanishad means 'setting to rest ignorance by revealing the knowledge of the supreme spirit.'"
Shri Adi Shankara explains in his commentary on the Kaṭha and Brihadaranyaka Upanishad that the word means Ātmavidyā , that is, "knowledge of the Self ", or Brahmavidyā "knowledge of Brahma". Other dictionary meanings include "esoteric doctrine" and "secret doctrine". The word appears in the verses of many Upanishads, such as the fourth verse of the 13th volume in first chapter of the Chandogya Upanishad. Max Muller as well as Paul Deussen translate the word Upanishad in these verses as "secret doctrine",  Robert Hume translates it as "mystic meaning",  while Patrick Olivelle translates it as "hidden connections".
The authorship of most Upanishads is uncertain and unknown. Radhakrishnan states, "almost all the early literature of India was anonymous, we do not know the names of the authors of the Upanishads".  The various philosophical theories in the early Upanishads have been attributed to famous sages such as Yajnavalkya, Uddalaka Aruni, Shvetaketu, Shandilya, Aitareya, Balaki, Pippalada and Sanatkumara.  Women, such as Maitreyi and Gargi participate in the dialogues and are also credited in the early Upanishads. 
There are exceptions to the anonymous tradition of the Upanishads and other Vedic literature. The Shvetashvatara Upanishad, for example, includes closing credits to sage Shvetashvatara , and he is considered the author of the Upanishad. Scholars believe that early Upanishads, were interpolated  and expanded over time, because of the differences within manuscripts of the same Upanishad discovered in different parts of South Asia, differences in non-Sanskrit version of the texts that have survived, and differences within each text in terms of the meter,  the style, the grammar and the structure. The texts as they exist now is believed to be the work of many authors.
Scholars are uncertain about the exact centuries in which the Upanishads were composed. The chronology of the early Upanishads is difficult to resolve, states philosopher and Sanskritist Stephen Phillips, because all opinions rest on scanty evidence and analysis of archaism, style and repetitions across texts, and are driven by assumptions about likely evolution of ideas, and presumptions about which philosophy might have influenced which other Indian philosophies. Indologist Patrick Olivelle says that "in spite of claims made by some, in reality, any dating of these documents [early Upanishads] that attempts a precision closer than a few centuries is as stable as a house of cards". Some scholars have sought to analyse similarities between Hindu Upanishads and Buddhist literature to establish chronology for the Upanishads.
Patrick Olivelle gives the following chronology for the early Upanishads, also called the Principal Upanishads:
- The Brhadaranyaka and the Chandogya are the two earliest Upanishads. They are edited texts, some of whose sources are much older than others. The two texts are pre-Buddhist; they may be placed in the 7th to 6th centuries BCE, give or take a century or so.
- The three other early prose Upanisads—Taittiriya, Aitareya, and Kausitaki come next; all are probably pre-Buddhist and can be assigned to the 6th to 5th centuries BCE.
- The Kena is the oldest of the verse Upanisads followed by probably the Katha, Isa, Svetasvatara, and Mundaka. All these Upanisads were composed probably in the last few centuries BCE. 
- The two late prose Upanisads, the Prasna and the Mandukya, cannot be much older than the beginning of the common era.
Stephen Phillips places the early Upanishads in the 800 to 300 BCE range. He summarizes the current Indological opinion to be that the Brhadaranyaka, Chandogya, Isha, Taittiriya, Aitareya, Kena, Katha, Mundaka, and Prasna Upanishads are all pre-Buddhist and pre-Jain, while Svetasvatara and Mandukya overlap with the earliest Buddhist and Jain literature.
The later Upanishads numbering about 95, also called minor Upanishads, are dated from the late 1st-millennium BCE to mid 2nd-millennium CE. Gavin Flood dates many of the twenty Yoga Upanishads to be probably from the 100 BCE to 300 CE period. Patrick Olivelle and other scholars date seven of the twenty Sannyasa Upanishads to likely have been complete sometime between the last centuries of the 1st-millennium BCE to 300 CE. About half of the Sannyasa Upanishads were likely composed in 14th- to 15th-century CE.
The general area of the composition of the early Upanishads was northern India, the region bounded on the west by the upper Indus valley, on the east by lower Ganges region, 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. This region covers modern Bihar, Nepal, Uttar Pradesh, Uttarakhand, Himachal Pradesh, Haryana, eastern Rajasthan and northern Madhya Pradesh.
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.
The Chandogya Upanishad was probably composed in a more Western than an Eastern location in Indian subcontinent, possibly somewhere in the western region of the Kuru-Panchala country. 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. In fourth chapter of the Kaushitaki Upanishad, a location named Kashi (modern Varanasi) is mentioned.
Muktika canon: major and minor Upanishads
There are more than 200 known Upanishads , one of which, Muktikā Upanishad, predates 1656 CE and contains a list of 108 canonical Upanishads, including itself as the last. The earliest ones such as the Brihadaranyaka and Chandogya Upanishads date to the early centuries of the 1st millennium BCE, and the latest to around the mid 2nd-millennium CE during a period of Islamic invasions and political instability.  Various scholars include the earliest 10, 11, 12 or 13 Upanishads as Mukhya (major) or Principal Upanishads, all composed in the 1st-milliennium BCE. The remainder 95 to 98 are called "minor Upanishads", and were likely composed between the last centuries of the 1st-millennium BCE and about mid 2nd-millennium CE. These are further divided into Upanishads associated with Shaktism (goddess Shakti), Sannyasa (renunciation, monastic life), Shaivism (god Shiva), Vaishnavism (god Vishnu), Yoga, and Sāmānya (general, sometimes referred to as Samanya-Vedanta). 
Some of the Upanishads are categorized as "sectarian" since they present their ideas through a particular god or goddess of a specific Hindu tradition such as Vishnu, Shiva, Shakti, or a combination of these such as the Skanda Upanishad. These traditions sought to link their texts as Vedic, by asserting their texts to be an Upanishad, thereby a Śruti . Most of these sectarian Upanishads, for example the Rudrahridaya Upanishad and the Mahanarayana Upanishad, assert that all the Hindu gods and goddesses are the same, all an aspect and manifestation of Brahman, the Vedic concept for metaphysical ultimate reality before and after the creation of the Universe. 
The Mukhya Upanishads can be grouped into periods. Of the early periods are the Brihadaranyaka and the Chandogya , the oldest.
The Aitareya, Kauṣītaki and Taittirīya Upanishads may date to as early as the mid 1st millennium BCE, while the remnant date from between roughly the 4th to 1st centuries BCE, roughly contemporary with the earliest portions of the Sanskrit epics. 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 , but other scholars date these earlier. 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|
There is no fixed list of the Upanishads as newer ones, beyond the Muktika anthology of 108 Upanishads, have continued to be discovered and composed. In 1908, for example, four previously unknown Upanishads were discovered in newly found manuscripts, and these were 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 , were incomplete and inconsistent, likely poorly maintained or corrupted.
Ancient Upanishads have long enjoyed a revered position in Hindu traditions, and authors of numerous sectarian texts have tried to benefit from this reputation by naming their texts as Upanishads. These "new Upanishads" number in the hundreds, cover diverse range of topics from physiology to renunciation to sectarian theories. They were composed between the last centuries of the 1st millennium BCE through the early modern era (~1600 CE). While over two dozen of the minor Upanishads are dated to pre-3rd century CE, many of these new texts under the title of "Upanishads" originated in the first half of the 2nd millennium CE, they are not Vedic texts, and some do not deal with themes found in the Vedic Upanishads.
The main Shakta Upanishads, for example, 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. Sectarian texts such as these do not enjoy status as shruti and thus the authority of the new Upanishads as scripture is not accepted in Hinduism.
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. During the modern era, the ancient Upanishads that were embedded texts in the Vedas, were detached from the Brahmana and Aranyaka layers of Vedic text, compiled into separate texts and these were then gathered into anthologies of Upanishads. These lists associated each Upanishad with one of the four Vedas, many such lists exist, and these lists are inconsistent across India in terms of which Upanishads are included and how the newer Upanishads are assigned to the ancient Vedas. In south India, the collected list based on Muktika Upanishad, and published in Telugu language, became the most common by the 19th-century and this is a list of 108 Upanishads. In north India, a list of 52 Upanishads has been most common.
The Muktikā Upanishad's list of 108 Upanishads groups the first 13 as mukhya , 21 as Sāmānya Vedānta, 20 as Sannyāsa, 14 as Vaishnava, 12 as Shaiva, 8 as Shakta, and 20 as Yoga.  The 108 Upanishads as recorded in the Muktikā are shown in the table below. The mukhya Upanishads are the most important and highlighted.
The Upanishadic age was 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 schools of Hinduism, in contrast to the non-dualistic Upanishads at the foundation of its Vedanta school. They contain a plurality of ideas.
Sarvepalli Radhakrishnan states that the Upanishads have dominated Indian philosophy, religion and life ever since their appearance.  The Upanishads are respected not because they are considered revealed ( Shruti ), but because they present spiritual ideas that are inspiring.  The Upanishads are treatises on Brahman-knowledge, that is knowledge of Ultimate Hidden Reality, and their presentation of philosophy presumes, "it is by a strictly personal effort that one can reach the truth".  In the Upanishads, states Radhakrishnan, knowledge is a means to freedom, and philosophy is the pursuit of wisdom by a way of life. 
The Upanishads include sections on philosophical theories that have been at the foundation of Indian traditions. For example, the Chandogya Upanishad includes one of the earliest known declaration of Ahimsa (non-violence) as an ethical precept.  Discussion of other ethical premises such as Damah (temperance, self-restraint), Satya (truthfulness), Dāna (charity), Ārjava (non-hypocrisy), Daya (compassion) and others are found in the oldest Upanishads and many later Upanishads.  Similarly, the Karma doctrine is presented in the Brihadaranyaka Upanishad, which is the oldest Upanishad.
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 Kaushitaki Upanishad asserts that "external rituals such as Agnihotram offered in the morning and in the evening, must be replaced with inner Agnihotram, the ritual of introspection", and that "not rituals, but knowledge should be one's pursuit". The Mundaka Upanishad declares how man has been called upon, promised benefits for, scared unto and misled into performing sacrifices, oblations and pious works. Mundaka thereafter asserts this is foolish and frail, by those who encourage it and those who follow it, because it makes no difference to man's current life and after-life, it is like blind men leading the blind, it is a mark of conceit and vain knowledge, ignorant inertia like that of children, a futile useless practice.  The Maitri Upanishad states,
The performance of all the sacrifices, described in the Maitrayana-Brahmana, is to lead up in the end to a knowledge of Brahman, to prepare a man for meditation. Therefore, let such man, after he has laid those fires,  meditate on the Self, to become complete and perfect.
The opposition to the ritual is not explicit in the oldest Upanishads. On 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, Vedic gods such as the Agni , Aditya , Indra , Rudra , Visnu , Brahma and others become equated in the Upanishads to the supreme, immortal and incorporeal Brahman-Atman of the Upanishads, god becomes synonymous with Self, and is declared to be everywhere, inmost being of each human being and within every living creature.   The one reality or ekam sat of the Vedas becomes the ekam eva advitiyam or "the one and only and sans a second" in the Upanishads. Brahman-Atman and Self-realization develops, in the Upanishad, as the means to moksha (liberation, freedom in this life or after-life).  
According to Jayatilleke, the thinkers of Upanishadic texts can be grouped into two categories. One group, which includes Early Upanishads along with some Middle and Late Upanishads, were composed by metaphysicians who used rational arguments and empirical experience to formulate their speculations and philosophical premises. The second group includes many middle and later Upanishads, where their authors professed theories based on yoga and personal experiences. Yoga philosophy and practice, adds Jayatilleke, is "not entirely absent in the Early Upanishads". The development of thought in these Upanishadic theories contrasted with Buddhism, since the Upanishadic inquiry assumed there is a soul ( Atman), while Buddhism assumed there is no soul ( Anatta ), states Jayatilleke.
Brahman and Atman
Two concepts that are of paramount importance in the Upanishads are Brahman and Atman . The Brahman is the ultimate reality and the Atman is individual self (soul). Brahman is the material, efficient, formal and final cause of all that exists. It is the pervasive, genderless, infinite, eternal truth and bliss which does not change, yet is the cause of all changes. Brahman is "the infinite source, fabric, core and destiny of all existence, both manifested and unmanifested, the formless infinite substratum and from which the universe has grown". Brahman in Hinduism, states Paul Deussen, as the "creative principle which lies realized in the whole world".
The word Atman means the inner self, the soul, the immortal spirit in an individual, and all living beings including animals and trees.  Ātman is a central idea in all the Upanishads, and "Know your Ātman" their thematic focus. These texts state that the inmost core of every person is not the body, nor the mind, nor the ego, but Atman – "Soul" or "Self". Atman is the spiritual essence in all creatures, their real innermost essential being. It is eternal, it is ageless. Atman is that which one is at the deepest level of one's existence.
Atman is the predominantly discussed topic in the Upanishads, but they express two distinct, somewhat divergent themes. Some state that Brahman (Highest Reality, Universal Principle, Being-Consciousness-Bliss) is identical with Atman , while others state Atman is part of Brahman but not identical.  This ancient debate flowered into various dual, non-dual theories in Hinduism. The Brahmasutra by Badarayana (~ 100 BCE) synthesized and unified these somewhat conflicting theories, stating that Atman and Brahman are different in some respects particularly during the state of ignorance, but at the deepest level and in the state of Self-realization, Atman and Brahman are identical, non-different.
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.
Two different types of the non-dual Brahman-Atman are presented in the Upanishads, according to Mahadevan. The one in which the non-dual Brahman-Atman is the all inclusive ground of the universe and another in which empirical, changing universe is a form of Maya , often translated as "illusion".
The Upanishads describe the universe, and the human experience, as an interplay of Purusha (the eternal, unchanging principles, consciousness) and Prakṛti (the temporary, changing material world, nature).  The former manifests itself as Ātman (Soul, Self), and the latter as Māyā. The Upanishads refer to the knowledge of Atman as "true knowledge" ( Vidya ), and the knowledge of Maya as "not true knowledge" ( Avidya , Nescience, lack of awareness, lack of true knowledge).
Hendrick Vroom explains, "the term Maya [in the Upanishads] has been translated as 'illusion,' but then it does not concern normal illusion. Here 'illusion' does not mean that the world is not real and simply a figment of the human imagination. Maya means that the world is not as it seems; the world that one experiences is misleading as far as its true nature is concerned." According to Wendy Doniger, "to say that the universe is an illusion (māyā) is not to say that it is unreal; it is to say, instead, that it is not what it seems to be, that it is something constantly being made. Māyā not only deceives people about the things they think they know; more basically, it limits their knowledge."
In the Upanishads, Māyā is the perceived changing reality and it co-exists with Brahman which is the hidden true reality. Maya , or "illusion", is an important idea in the Upanishads, because the texts assert that in the human pursuit of blissful and liberating Self-knowledge, it is Maya which obscures, confuses and distracts an individual.  
Schools of Vedanta
The Upanishads form one of the three main sources for all schools of Vedanta, together with the Bhagavad Gita and the Brahmasutras. Due to the wide variety of philosophical teachings contained in the Upanishads, various interpretations could be grounded on the Upanishads. The schools of Vedānta seek to answer questions about the relation between atman and Brahman, and the relation between Brahman and the world. The schools of Vedanta are named after the relation they see between atman and Brahman:
- According to Advaita Vedanta, there is no difference.
- According to Vishishtadvaita the jīvātman is a part of Brahman, and hence is similar, but not identical.
- According to Dvaita, all individual souls (jīvātmans) and matter as eternal and mutually separate entities.
Advaita literally means non-duality, and it is a monistic system of thought. It deals with the non-dual nature of Brahman and Atman. Advaita is considered the most influential sub-school of the Vedanta school of Hindu philosophy. Gaudapada was the first person to expound the basic principles of the Advaita philosophy in a commentary on the conflicting statements of the Upanishads. Gaudapada's Advaita ideas were further developed by Shankara. King states that Gaudapada's main work, Māṇḍukya Kārikā, is infused with philosophical terminology of Buddhism, and uses Buddhist arguments and analogies. King also suggests that there are clear differences between Shankara's writings and the Brahmasutra , and many ideas of Shankara are at odds with those in the Upanishads. Radhakrishnan, on the other hand, suggests that Shankara's views of Advaita were straightforward developments of the Upanishads and the Brahmasutra , and many ideas of Shankara derive from the Upanishads.
Shankara in his discussions of the Advaita Vedanta philosophy referred to the early Upanishads to explain the key difference between Hinduism and Buddhism, stating that Hinduism asserts "Atman (Soul, Self) exists", while Buddhism asserts that there is "no Soul, no Self".  
The Upanishads contain four sentences, the Mahāvākyas (Great Sayings), which were used by Shankara to establish the identity of Atman and Brahman as scriptural truth:
- "Prajñānam brahma" - "Consciousness is Brahman" ( Aitareya Upanishad)
- "Aham brahmāsmi" - "I am Brahman" ( Brihadaranyaka Upanishad)
- "Tat tvam asi" - "That Thou art" ( Chandogya Upanishad)
- "Ayamātmā brahma" - "This Atman is Brahman" ( Mandukya Upanishad)
The Dvaita school was founded by Madhvacharya. Dvaita is regarded as the best philosophic exposition of theism. Madhva, much like Adi Shankara claims for Advaita, states that his theistic Dvaita Vedanta is grounded in the Upanishads. 
The third school of Vedanta is the Vishishtadvaita, which was founded by Ramanuja. Ramanuja strenuously refuted Shankara's works. Visistadvaita is a synthetic philosophy bridging the monistic Advaita and theistic Dvaita systems of Vedanta. Ramanuja, just as Madhva claims for Dvaita sub-school, states that Vishishtadvaita is grounded in the Upanishads. 
Similarities with Platonic thought
Several scholars have recognised parallels between the philosophy of Pythagoras and Plato and that of the Upanishads, including their ideas on sources of knowledge, concept of justice and path to salvation, and Plato's allegory of the cave. Platonic psychology with its divisions of reason, spirit and appetite, also bears resemblance to the three gunas in the Indian philosophy of Samkhya.
Based on these common features some scholars, most notably E.J. Urwick and M.L. West, have argued that the Ancient Greek philosophy was influenced by, and borrowed some core concepts from, the Upanishads. Various mechanisms for such a transmission of knowledge have been conjectured including Pythagoras traveling as far as India; Indian philosophers visiting Athens and meeting Socrates ; Plato encountering the ideas when in exile in Syracuse; or, intermediated through Persia.
However other scholars, such as Arthur Berriedale Keith, J. Burnet and A.R. Wadia, believe that the two systems developed independently. They note that there is no historical evidence of the philosophers of the two schools meeting, and point out significant differences in the stage of development, orientation and goals of the two philosophical systems. Wadia writes that Plato's metaphysics were rooted in this life and his primary aim was to develop an ideal state. In contrast, Upanishadic focus was the individual, the self (atman, soul), self-knowledge, and the means of an individual's moksha (freedom, liberation in this life or after-life). 
The Upanishads have been translated into various languages including Persian, Italian, Urdu, French, Latin, German, English, Dutch, Polish, Japanese, Spanish and Russian. The Moghul Emperor Akbar's reign (1556–1586) saw the first translations of the Upanishads into Persian. His great-grandson, Sultan Mohammed Dara Shikoh, produced a collection called Oupanekhat in 1656, wherein 50 Upanishads were translated from Sanskrit into Persian.
Anquetil Duperron, a French Orientalist received a manuscript of the Oupanekhat and translated the Persian version into French and Latin, publishing the Latin translation in two volumes in 1801–1802 as Oupneck'hat . The French translation was never published. The Latin version was the initial introduction of Upanishadic thought to Western scholars. However, according to Deussen, the Persian translators took great liberties in translating the text and at times changed the meaning.
The first Sanskrit to English translation of the Aitareya Upanishad was made by Colebrooke,  in 1805 and the first English translation of the Kena Upanishad was made by Rammohun Roy in 1816.    Colebrooke was aware of 170 Upanishads. Sadhale's catalog from 1985, the Upaniṣad-vākya-mahā-kośa lists 223 Upanishads.
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. Other major translations of the Upanishads have been by Robert Ernest Hume (13 Principal Upanishads),  Paul Deussen (60 Upanishads), Sarvepalli Radhakrishnan (18 Upanishads), and Patrick Olivelle (32 Upanishads in two books).
Reception in the West
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.
Juan Mascaró states that the Upanishads represents for the Hindu approximately what the New Testament represents for the Christian, and that the message of the Upanishads can be summarized in the words, "the kingdom of God is within you".
Paul Deussen in his review of the Upanishads, states that the texts emphasize Brahman-Atman as something that can experienced, but not defined.  This view of the soul and self are similar, states Deussen, to those found in the dialogues of Plato and elsewhere. The Upanishads insisted on oneness of soul, 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.  Max Muller, in his review of the Upanishads, summarizes the lack of systematic philosophy and the central theme in the Upanishads as follows,
There is not what could be called a philosophical system in these Upanishads. They are, in the true sense of the word, guesses at truth, frequently contradicting each other, yet all tending in one direction. The key-note of the old Upanishads is "know thyself," but with a much deeper meaning than that of the γνῶθι σεαυτόν of the Delphic Oracle. The "know thyself" of the Upanishads means, know thy true self, that which underlines thine Ego, and find it and know it in the highest, the eternal Self, the One without a second, which underlies the whole world.