Upanishads (Samskrit : उपनिषद्) are the concluding segments, available as a part of Aranyakas. Since they expound the various spiritual and dharmika siddhantas and tattvas that leads a sadhaka to the highest purpose of Moksha and because they are present at the end of the Vedas, they are also referred to as the Vedanta (वेदान्तः) . They do not forbid the rituals or rites prescribed in the Karmakanda but expound that only through Jnana one can attain moksha.
वेदान्तो नामोपनिषत्प्रमाणं तदनुसारीणि। शारीरकसूत्राणि च । vedānto nāmopaniṣatpramāṇaṁ tadanusārīṇi। śārīrakasūtrāṇi ca ।
Sadananda Yogindra, in his Vedantasara says that"Vedanta has the Upanishads for its evidence and includes the Sharira Sutras (Vedanta Sutras or Brahma Sutras) and other works which corroborate it". 
- 1 परिचयः ॥ Introduction
- 2 व्युत्पत्तिः ॥ Etymology
- 3 Classification of Upanishads
- 4 Authorship
- 5 Interpretations
- 6 विषयविभागम् ॥ Contents
- 6.1 मौलिकसिद्धान्ताः ॥ Core Siddhantas
- 6.2 उपनिषद्वैशिष्ट्यम् ॥ Vaishishtya
- 6.3 Mahavakyas
- 6.4 Prasthana Trayi
- 7 References
परिचयः ॥ Introduction
The Vedas have been divided into four styles of texts – the Samhitas, the Aranyakas, the Brahmanas and the Upanishads. The subject matter of the whole Veda is divided into Karma-Kanda, Upasana-Kanda and Jnana-Kanda. The Karma-Kanda or Ritualistic section deals with various sacrifices and rituals. The Upasana-Kanda or Worship section deals with various kinds of worship or meditation. The Jnana-Kanda or Knowledge-Section deals with the highest knowledge of Nirguna Brahman. The Samhitas and the Brahmanas constitute Karma-Kanda; the Aranyakas constitute Upasana-Kanda; and the Upanishads constitute Jnana-KandaThe Upanishads along with the Bhagavadgita and Brahmasutras constitute the Prasthana Trayi (प्रस्थानत्रयी) and both the Gita and Brahmasutras are based again on the Upanishads. They are also the foundational sources for all Darshana shastras and including the Jain and Buddhist philosophies.
According to Dr. K. S. Narayanacharya, these are four different modes of expressing the same truths, each as a cross check against others so that misrepresentations are avoided, a method used and valid even today.
Most of the Upanishads are in forms of dialogues between a master and a disciple. In Upanishads, a seeker raises a topic and the enlightened guru satisfies the query aptly and convincingly. Chronology and dating of Upanishads is not attempted in this article.
व्युत्पत्तिः ॥ Etymology
There are different versions about the meaning of Upanishad as given by many scholars. The term Upaniṣad term consists of उप (upa) and नि (ni) उपसर्ग-s (Upasargas or Prefixes) and सद् धातुः (Sad dhatu) followed by किव्प् प्रत्ययः (Kvip pratyaya as Suffix) used in the sense of विशरणगत्यवसादनेषु । viśaraṇagatyavasādaneṣu Shri Adi Shankaracharya explains in his commentary on Taittiriyopanishad about the meanings of Sad (सद्) dhatu thus 
- विशरणम् (नाशनम्) to destroy : They destroy the seeds of Avidya causing samsara in a Mumukshu (a sadhaka who wants to attain Moksha), hence this Vidya is called Upanishads.
अविद्यादेः संसारबीजस्य विशरणाद् विनाशनादित्यनेन अर्थयोगेन विद्या उपनिषदुच्यते । avidyādeḥ saṁsārabījasya viśaraṇād vināśanādityanena arthayogena vidyā upaniṣaducyate ।
- गतिः (प्रपणम् वा विद्यर्थकम्) to obtain or to know : That vidya which leads to or make the sadhaka obtain Brahma, is called Upanishad.
परं ब्रह्म वा गमयतोति ब्रह्म गमयितृत्वेन योगाद् विद्योपनिषद् । paraṁ brahma vā gamayatoti brahma gamayitr̥tvena yogād vidyopaniṣad ।
- अवसादनम् (शिथिलर्थकम्) to loosen or to dissolve : Through which cycles of birth, aging etc painful process are loosened or dissolved (that is bondages of samsara are dissolved allowing the sadhaka to attain the Brahma).
His also defines the primary meaning of Upanishad as Brahmavidya (ब्रह्मविद्या । Knowledge of Brahma) and secondary meaning as ब्रह्मविद्याप्रतिपादकग्रन्थः (Brahmavidya pratipadaka granth । texts which teach Brahmavidya). Shankaracharya's commentaries of the Kaṭha and Brhadaranyaka Upanishad also support this explanation.
गर्भवासजन्मजराद्युपद्रववृन्दस्य लोकान्तरेपौनपुन्येन प्रवृत्तस्य अवसादपितृत्वेन उपनिषदित्युच्यते । garbhavāsajanmajarādyupadravavr̥ndasya lokāntarepaunapunyena pravr̥ttasya avasādapitr̥tvena upaniṣadityucyate ।
- नि (ni) उपसर्ग (Upasarga or Prefix) in front of सद् धातुः (Sad dhatu) also means 'to sit'.
- उप (upa) Upasarga is used to mean 'nearness or close to'.
- उपनिषद् term thus means "to sit near".
Thus Upanishad came to mean as ' to sit near the Guru (preceptor) to obtain the 'secret knowledge' or Brahmavidya (as per Shabdakalpadhruma : उपनिषद्यते प्राप्यते ब्रह्म-विद्या अनया इति)
Generally, Upanishads are synonymous with Rahasya (रहस्यम्) or secrecy. Upanishads themselves mention statements such as
मोक्षलक्षणमित्येतत्परं रहस्यम् इत्येवं । mokṣalakṣaṇamityetatparaṁ rahasyam ityevaṁ । (Mait. Upan. 6.20)
सैषा शांभवी विद्या कादि-विद्येति वा हादिविद्येति वा सादिविद्येति वा रहस्यम् । saiṣā śāṁbhavī vidyā kādi-vidyeti vā hādividyeti vā sādividyeti vā rahasyam । (Bahvrchopanishad)
when discussing some important siddhantas. Probably such usages are given to prevent and caution against giving this knowledge to the undeserving.
In the mukhya upanishads, there are many instances of रहस्यम् meaning secret or hidden knowledge especially in Atharvaveda upanishads. Kaushitaki Upanishad for example, contains detailed siddhantas of मनोज्ञानम् and तत्वज्ञानम् (Psychology and metaphysics). Apart from them they also contain मृतकज्ञानम् (siddhantas around death, travel of Atman etc), बालमृत्यु निवारणम् (preventing untimely childhood deaths) शत्रुविनाशार्थ रहस्यम् (secrets about the destruction of enemies) etc. Chandogya Upanishads gives the secrets about the origin of worlds, Jiva, Jagat, Om and their hidden meanings.
Classification of Upanishads
More than 200 Upanishads are known, of which, the first dozen or so are the oldest (प्राचीनम्) most important and are referred to as the principal or main (mukhya) Upanishads. The rest of them aid in explaining bhakti or jnana concepts and many are without bhashyas. Some scholars accept 12 Upanishads and some even consider 13 to be the principal Upanishads and some others accept 108 Upanishads given by Muktikopanishad.
There is no fixed list of the Upanishads as newer ones, beyond the Muktika Upanishad list of 108 Upanishads, have continued to be composed and discovered. A collection of Upanishads, namely Upanishad Samgrahah by Pt. J. K. Shastri contains 188 upanishads. Pracheena Upanishads have long been revered in Sanatana Dharma traditions, and many sampradayas have interpreted the concepts of Upanishads to evolve their sampradaya. These "new Upanishads" number in the hundreds, cover diverse range of topics from physiology to renunciation.
Basis for Classification
Many modern and western indology thinkers have put forth their contemplations on the classification of Upanishads and it is based on the following factors
- the presence or absence of Shankaracharya's bhasyas (Ten for which bhashyas are available are Dasopanishads and the rest describing devatas. Vaishnava, Shaiva, Shakta, Saurya etc)
- the ancientness of Upanishad based on association with Aranyakas and Brahmanas
- the ancientness and modernness of the Upanishads based on description of deities and other aspects ( Given by Shri Chintamani Vinayak on Page 256 of Reference )
- the shanti patha given in each of the Upanishads
- the ancientness and modernness of the Upanishads having prose or metrical compositions (mostly given by Western Indologists like Dr. Daison)
दशोपनिषदः ॥ Dasopanishads
Muktikopanishad lists the following ten as principal Upanishads which have received attention from Shri Adi Shankaracharya in form of his bhasyas and are considered ancient (प्राचीनम्).
ईश-केन-कठ-प्रश्न-मुण्ड-माण्डूक्य-तित्तिरः । ऐतरेयं च छान्दोग्यं बृहदारण्यकं तथा ॥ īśa-kēna-kaṭha-praśna-muṇḍa-māṇḍūkya-tittiraḥ । ēētarēyaṁ ca chāndōgyaṁ br̥hadāraṇyakaṁ tathā ॥
The 10 Mukhya Upanishad on which Adi Shankaracharya commented are:
- ईशावाश्योपनिषद् ॥ Ishavasya Upanishad (Shukla Yajur Veda)
- केनोपनिषद् ॥ Kena Upanishad (Sama Veda)
- कठोपनिषद् ॥ Katha Upanishad (Yajur Veda)
- प्रश्नोपनिषद् ॥ Prashna Upanishad (Atharva Veda)
- मुण्डकोपनिषद् ॥ Mundaka Upanishad (Atharva Veda)
- माण्डूक्योपनिषद् ॥ Mandukya Upanishad (Atharva Veda)
- तैत्तियोपनिषद् ॥ Taittiriya Upanishad (Yajur Veda)
- ऐतरेयोपनिषद् ॥ Aitareya Upanishad (Rig Veda)
- छान्दोग्योपनिषद्॥ Chhandogya Upanishad (Sama Veda)
- बृहदारण्यकोपनिषद् Brhadaranyaka Upanishad (Yajur Veda)
Apart from these ten, कौषीतकि (Kaushitaki) श्वेताश्वतर (Shvetasvatara) and मैत्रायणीय (Maitrayaniya) Upanishads are regarded as ancient since the first two of these three found a mention by Shankaracharya in his Brahmasutra bhashyas along with dashopanishad bhashyas. However, there are no available commentaries on these given by him.
Upanishads as Part of Aranyakas
Many Upanishads are the final or exclusive portions of the Aranyakas or the Brahmanas. But these refer mainly to the dasha upanishads. It may be seen from the table below that some Upanishads not classified in dashopanishads are from Aranyakas. (Ex: Mahanarayaneeya Upanishad, Maitrayaniya Upanishad) while the Atharvana veda associated Upanishads do not have corresponding Brahmanas or Aranyakas as they are unavailable.
|Veda||What part of Brahmana or Aranyaka constitutes Upanishad||Name of Upanishad||Name Comes from||Contents|
|RigVeda||4th to 6th Adhyayas of 2nd Prapathaka of 2nd Aranyaka of Aitareya Aranyaka (Page 250 of Reference )||ऐतरेयोपनिषद् ॥ Aitareya Upanishad||Mahidasa Aitareya composed this Upanishad||Consists of 3 Adhyayas|
|3rd to 6th Adhyayas of Shankhyayana Aranyaka (Page 251 of Reference )||कौषीतकि उपनिषद् ॥ Kaushitaki Upanishad||Given by Kaushitaki Rshi||Consists of 4 Adhyayas|
|Yajurveda||Krishna||7th to 9th Prapathakas of Taittiriya Aranyaka (Page 251 of Reference )||तैत्तियोपनिषद् ॥ Taittiriya Upanishad||From source Taittiriya Aranyaka||Consists of 3 Vallis or chapters : Shikshavalli, Brahmavalli (Anandavalli) and Bhruguvalli|
|10th Prapathaka of Taittiriya Aranyaka (also considered as Khila khanda) (Page 251 of Reference )||नारायणीयोपनिषद् ॥ Narayanopanishad
महानारायणीय उपनिषद् ॥ Mahanarayaniya Upanishad
|From the description of Narayana as Supreme Brahman.||Consists of collection of passages including both prose and mantras (totaling to 150 divided into two adhyayas).|
|Kathasamhita or Kathavalli (Page 54 of Reference )||कठोपनिषद् ॥ Kathopanishad or काठकोपनिषद् ॥ Kathakopanishad||Comes from the source Katha samhita||2 Adhyayas with 3 vallis each (total 6 vallis) contains 119 mantras.|
|Maitrayaniya Aranyaka (Page 251 of Reference )||मैत्रायणीय Maitrayaniya Upanishad||Comes from the source Maitrayaniya Aranyaka||Contains 7 Prapathakas|
|Shukla||Last 6 Adhyayas of Shatapatha Brahmana (Page 56 of Reference )||Brhadaranyakopanishad||Contains 6 Adhyayas|
|40th Adhyaya of माध्यन्दिन-शतपथ-ब्राह्मणम् ॥ Madhyandina Shatapatabrahmana||ईशावाश्योपनिषद् ॥ Ishavasya Upanishad||The first word of the first mantra ॐ ईशा वास्यमिदँ सर्वं यत्किञ्च जगत्यां जगत् ।||Contains 18 mantras|
|Samaveda||10th Anuvaka of 4th Adhyaya of जैमिनीय ॥ Jaiminiya (Talavakara तलवकार) Brahmana (Page 253 of Reference )||केनोपनिषद् ॥ Kena Upanishad or Talavakaropanishad||The first word of the first mantra ॐ केनेषितं पतति प्रेषितं मनः||Contains 4 Khandas with 32 mantras in all.|
|Last 10 Adhyayas of Chandogyabrahmana of Kauthuma Shakha (Page 55 of Reference )||छान्दोग्योपनिषद्॥ Chhandogya Upanishad||Contains 8 Prapathakas or Adhyayas each having varying number of Kandas and mantras (totaling to 154 Khandas).|
|Adharvaveda||Associated with Pippalada brahmana (Page 54 of Reference )||प्रश्नोपनिषद् ॥ Prashna Upanishad||Comes from the Prashna or question and answer format seen in this upanishad.||Mostly in prose it has six Prashnas (sections)with 16, 13, 12, 11, 7 and 8 passages respectively.|
|Associated with Shaunaka samhita (Page 54 of Reference )||मुण्डकोपनिषद् ॥ Mundaka Upanishad||3 Mundakas (chapters) each containing 2 Khandas with 64 mantras in all.|
|Associated with Atharvaveda (Page 55 of Reference )||माण्डूक्योपनिषद् ॥ Mandukya Upanishad||Composed by the son of Mandukya maharshi.||Contains 12 mantras only|
Deity and Samkhya Based Classification
Pt. Shri. Chintamani Vinayak Vaidya has assigned the ancientness (प्राचीनता) or modernness (अर्वाचीनता) of Upanishads using the two factors
- Siddhanta of Anatmarupa Brahma (a supreme power beyond and above deities)
- Vishnu or Shiva deities are accepted as paradevata (supreme deity) and are praised
- Principles of Samkhya Siddhanta (Prakriti, Purusha, Gunas-Satva, Rajas and Tamas)
It is proved beyond doubt that the ancient upanishads have described a Supreme Anatmarupa Brahma, above the vaidika deities, who has created regulated and maintained order of the creation. They are thus very ancient and include Aitareya, Isha, Taittiriya, Brhadaranyaka, Chandogya, Prashna, Mundaka and Mandukya Upanishads.
Only in the more recent upanishads one can see the older ones in praise of Vishnu as the Supreme Being followed by the more recent one's in praise of Shiva. In this group is classified Kathopanishad where Vishnu is the Supreme Being. Krishna Yajurveda Upanishads are famous for their Shiva and Rudra stutis (Rudra Prashna is a famous stuti) and in this way Shevetashvatara Upanishad which accepts Shiva as paradevata is more recent compared to Kathopanishad. In this series, Maitrayaniya Upanishad which accepts all the Trimurti's (Brahma Vishnu and Shiva) is more recent than the above two mentioned Upanishads.
Katha Upanishad (which has no principles of Samkhya) is ancient as against Shvetashvatara (which expounds principles of Samkhya and its preceptor Kapila Maharshi), further more recent is Maitrayaniya Upanishad where samkhya philosophy along with the description of the gunas is given in detail.
Shanti Patha Based Classification
Some upanishads are not related to any veda, while some are definitely associated with one or the other veda. Based on the Shanti patha that is given in the beginning of the Upanishads the following classification is proposed.(Page 288-289 of Reference )
|Rigveda||वाङ् मे मनसि प्रतिष्ठिता मनो मे वाचि प्रतिष्ठितमाविरावीर्म एधि ॥ vāṅ mē manasi pratiṣṭhitā manō mē vāci pratiṣṭhitamāvirāvīrma ēdhi ॥||Aitareya, Kaushitaki, Atmaprabodha, Nadabindu, Nirvana, Mudgala, Akshamalika, Tripura, Saubhagya, Bahvrucha Upanishads (10)|
|Krishna Yajurveda||ॐ सह नाववतु । सह नौ भुनक्तु । सहवीर्यं करवावहै । oṁ saha nāvavatu । saha nau bhunaktu । sahavīryaṁ karavāvahai ।||Kathavalli (commonly referred to as Kathopanishad), Taittiriya, Narayana, Brahma, Kaivalya, Shvetasvatara, Garbha, Amrtabindu, Amrtanada, Kalagnirudra, Kshurika, Sarvasara, Shukarahasya, Tejobindu, Dhyanabindu, Brahmavidya, Yogatattva, Dakshinamurty, Katha (different from Kathavalli), Skanda, Shareeraka, Varaha, Yogashikha, Ekakshara, Akshi, Avadhuta, Rudrahrdaya, Yogakundalini, Panchabrahma, Kalisantarana, Pranagnihotra, Sarasvatirahasya Upanishads (32)|
|Shukla Yajurveda||ॐ पूर्णमदः पूर्णमिदं पूर्णात् पूर्णमुदच्यते । oṁ pūrṇamadaḥ pūrṇamidaṁ pūrṇāt pūrṇamudacyatē ।||Ishavasya, Brhadaranya, Jabala, Subaala, Hamsa, Paramahamsa, Mantrikaniralamba, Tarasara, Trishikhi, Brahmanamandala, Brahmana, Advayataraka, Paingala, Bhikshuka, Turiyateeta, Adhyatma, Muktika, Shatyayani, Yajnavalkya Upanishads (19)|
|Samaveda||ॐ आप्यायन्तु ममाङ्गानि वाक्प्राणश्चक्षुः
श्रोत्रमथो बलमिन्द्रियाणि च सर्वाणि । oṁ āpyāyantu mamāṅgāni vākprāṇaścakṣuḥ
śrōtramathō balamindriyāṇi ca sarvāṇi ।...
|Kena, Chandogya, Aaruni, Maitrayani, Maitreyi, Vajrasuchi, Yoga, Chudamani, Vasudeva, Sanyasa, Avyakta, Savitri, Rudrakshajabala, Darshanajabali, Kundika, Mahopanishad Upanishads (16)|
|Atharvaveda||ॐ भद्रं कर्णेभिः शृणुयाम देवाः भद्रं पष्येमाक्षभिर्यजत्राः । oṁ bhadraṁ karṇēbhiḥ śr̥ṇuyāma dēvāḥ bhadraṁ paṣyēmākṣabhiryajatrāḥ ।||Prashna, Mundaka, Mandukya, Brhadjabala, Nrsimhatapini, Narada parivrajaka, Sitaa, Sharabha, Mahanarayana, Ramarahasya, Ramatapini, Shandilya, Paramahamsa parivrajaka, Annapurna, Surya, Atma, Parambrahma, Pashupata, Tripuratapini, Devi, Bhavana, Bhasmajabala, Ganapati, Mahavakya, Gopalatapini, Krishna, Hayagriva, Garuda, Dattatreya, Atharvashikha, Atharvashiro Upanishads (31)|
Content Based Classification
Based on their content Upanishads can be divided into six categories.
- Vedanta siddhanta
- Yoga siddhanta
- Samkhya siddhanta
- Vaishnava siddhanta
- Shaiva siddhanta
- Shakta siddhanta
The authorship of most Upanishads is uncertain and unknown. The various philosophical theories in the early Upanishads have been attributed to famous seers such as Yajnavalkya, Uddalaka Aruni, Shvetaketu, Shandilya, Aitareya, Balaki, Pippalada and Sanatkumara. Women, such as Maitreyi and Gargi participated in the dialogues and are also credited in the early Upanishads. Prasnopanishad is based on Prashna (questions) and Uttara (answer) format between gurus and shishyas, as such a number of rshis are mentioned in this Upanishad.
There are exceptions to the anonymous tradition of the Upanishads and other Vedic literature. The Shvetashvatara Upanishad, for example, includes credits to the seer Shvetashvatara in 6.21, and he is considered the author of the Upanishad.
Upanishads not only speak about evolution and manifestation of the world, as 'srshti', but also about its dissolution which make them a welcome support towards a better understanding of ancient discoveries. Inquiries into origin of the worldly things have been widely discussed. However, in such matters as above, one finds that Upanishads abound in statements that are apparently contradictory in their nature.
Some describe the world to be real while others call it a illusion. One calls the Atman as essentially different from Brahman, while other texts describe the essential identity of the two. Some call Brahman the goal and the Atman the seeker, another draws an eternal verity of both. In between these extreme positions, there are varieties of other views. Yet all the divergent conceptions are based on the Upanishads. One must bear in mind that such views and perspectives have been traditionally existing from time immemorial in Bharatavarsha and the founders of these schools of thought are outstanding spokesmen of those systems. So is the case of the rshis and maharshis associated with the shad darshanas; they simply were their best expounders or codifiers.
Although everyone of these six systems of thought claims to derive its authority from the Upanishads, it is the Vedanta that bases itself wholly on them. In the Upanishads, the highest truths are given out as and when they were glimpsed by the rshis, hence may lack the systematic arrangement which can be expected of leisurely deliberation.
The task of introducing order into Upanishad thoughts taken up by Badarayana, in the sutra format (Brahmasutras), failed to convey the exact meanings as intended by him. As a consequence the Brahmasutras also suffered the same fate as Upanishads with commentators interpreting them as per their predilections and training.
विषयविभागम् ॥ Contents
The main subject of Upanishads is the discussion about Paramatmatattva. There are two kinds of Vidyas : Para (पराविद्या) and Apara (अपराविद्या). Of these Paravidya is supreme and is called Brahmavidya (ब्रह्मविद्या). Upanishads present the detailed discussion about Paravidya. Aparavidya is primarily related to Karmas, hence called Karmavidya (कर्मविद्या). The fruits of Karmavidya are perishable while the results of Brahmavidya are indestructable. Aparavidya may not lead one to Moksha (may lead to svarga and other worlds) but Paravidya always gives Moksha.
मौलिकसिद्धान्ताः ॥ Core Siddhantas
The central concepts found in the Upanishads involve the following aspects that are the fundamental and unique values of Sanatana Dharma which have been guiding the Chitta (Manas) of people of Bharatavarsha from ages. None of these concepts have ever been mentioned or used in any kind of ancient literatures in any other part of the world.
- ब्रह्मन् ॥ Brahman, Paramatma (परमात्मा), That (तत्), Purusha (पुरुषः) Nirguna Brahman (निर्गुणब्रह्मन्) (Supreme Being, Ultimate Reality)
- आत्मा ॥ Atman, जीवात्मा ॥ Jivatma, Ishvara (ईश्वरः), Sat (सत्), Saguna Brahman (सर्गुणब्रह्मन्), the Subject (Self)
- प्रकृतिः ॥ Prkriti, Asat (असत्), Not Self, The Object (Material Cause)
- मनः ॥ Manas (Prajna, Chitta, Samkalpa)
- कर्म ॥ Karma (Action) of Past, Present and Future
- माया ॥ Maya (Illusion), Shakti, Power, the Will, of Ishvara.
- जीवः ॥ Jiva (Embodiment of Atman in an Upadhi), The Many, arising from Mulaprkriti.
- सर्गः ॥ Sarga (Origin) of Srshti (सृष्टिः)
- ज्ञानम् ॥ Jnana (Knowledge)
- अविद्या ॥ Avidya (Ignorance)
- मोक्षम् ॥ Moksha (The Paramapurushartha)
The Upanishads speak about the identity of the Supreme Being, the Brahman, the individual Atman, their mutual relationship, the Universe (jagat) and man’s place in it. In short they deal with Jiva, Jagat, Jnana and Jagadishwara and ultimately the path to Brahman called as mokṣa or mukti.
Brahman and Atman
Brahman and Atman are two concepts that are unique to Bharatiya Jnana siddhantas which are highly developed in the Upanishads. From the root cause Prkriti the world came into being. Paramatma is Nitya, Puratana, Shasvata (Eternal) devoid of the cycles of birth and death. The Sharira or body is subjected to death and birth but Atma residing in it continues to exist. Just like butter is evenly distributed in milk so also Paramatma is all-pervading in the world. Just like the sparks arise out of fire so also the beings take shape from the Paramatma. Such aspects which are explained in Upanishads have been widely discussed and elucidated in the Darshana shastras.
ब्रह्मन् ॥ Brahman
The word Brahman is used to represent the all pervading, ultimate reality without a second. While this is a principle of universal acceptance for all sects of Vedantins, there is a variation in these schools as to relationship between Brahman and Jivatma.
The Unity which never appears but which IS, is implied in the very existence of universes and systems, worlds and individuals. IT (tat) is not only recognised now in all religions, but also in all philosophy and in all science as a fundamental necessity. Endless disputes and controversies have surrounded IT, many names describe IT and many have left it unnamed, but none have denied IT (except the Charvakas and other atheists). 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 process of the mankind. 
Nirguna Representation of Brahman
One which is described as without a second, is Infinite, Absolute, eternal is called as तत् । THAT, without attributes, gunas, beyond name and form, and cannot be explained by any similies or worldly descriptions is the Nirguna Brahman.
Chandogya Upanishad expounds the Nirguna Brahmatattva through the Mahavakyas such as
एकमेवाद्वितीयम्। ekamevādvitīyam। One only, without a second. (Chand. Upan. 6.2.1)
सर्वं खल्विदं ब्रह्म । sarvaṁ khalvidaṁ brahma । All this is verily Brahman. (Chand. Upan. 3.14.1)
Shevatasvatara Upanishad says
यदाऽतमस्तन्न दिवा न रात्रिर्न सन्नचासच्छिव एव केवलः ।... ॥ १८ ॥ yadā'tamastanna divā na rātrirna sannacāsacchiva eva kevalaḥ ।... ॥ 18 ॥ (Shvet. Upan. 4.18)
When there was mere tamas, and neither day nor the night, neither the universe (having a form) or without a form, there existed that pure auspicious principle alone signifying the One Principle.
These common and well known examples amply illustrate the concept of Nirguna or formless Brahman.
Brahman Represented by Pranava (Omkara)
That this Nirguna Brahman is also referred by Omkara or Pranavanada is also well illustrated in the Upanishads. Kathopanishad states that
सर्वे वेदा यत्पदमामनन्ति तपाँसि सर्वाणि च यद्वदन्ति । यदिच्छन्तो ब्रह्मचर्यं चरन्ति तत्ते पदँ संग्रहेण ब्रवीम्योमित्येतत् ॥ १५ ॥ (Kath. Upan. 1.2.15)
sarvē vēdā yatpadamāmananti tapām̐si sarvāṇi ca yadvadanti . yadicchantō brahmacaryaṁ caranti tattē padam̐ saṁgrahēṇa bravīmyōmityētat .. 15 ..
एतद्ध्येवाक्षरं ब्रह्म एतद्ध्येवाक्षरं परम् । एतद्ध्येवाक्षरं ज्ञात्वा यो यदिच्छति तस्य तत् ॥ १६ ॥ (Kath. Upan. 1.2.16)
ētaddhyēvākṣaraṁ brahma ētaddhyēvākṣaraṁ param . ētaddhyēvākṣaraṁ jñātvā yō yadicchati tasya tat .. 16 ..
Meaning : That which all the Vedas declare, that which all austerities utter, That desiring which they lead the life of Brahmacharya, That Word I tell thee briefly : it is Aum. That word is even Brahman; that Word is even the Supreme.
आत्मन् ॥ Atman, The Saguna Representation of Brahman
The next important concept is that of Saguna Brahman, also Supreme like the Nirguna Brahman, except that here there are a few limiting adjuncts (name, form etc), called variously as Atman, Jivatma, Inner Self, Self, Consciousness etc. The Individual Self, Atman, is the internal ruler, the that immortal part of a visible entity, which includes all living beings including humans, animals and trees. The discussion about Brahman being gross and subtle is raised by Satyakama as explained in the Prashnopanishad.
एतद्वै सत्यकाम परं चापरं च ब्रह्म यदोङ्कारः । etadvai satyakāma paraṁ cāparaṁ ca brahma yadoṅkāraḥ । (Pras. Upan. 5.2)
Meaning : Verily, O Satyakama, this Omkara is the Supreme and the lower Brahman. Brhdaranyaka Upanishad also says the following about the existence of two forms of Brahman - the Sat and Asat forms.
द्वे वाव ब्रह्मणो रूपे मूर्तं चैवामूर्तं च मर्त्यं चामृतं च स्थितं च यच्च सच्च त्यच्च ॥ १ ॥ dve vāva brahmaṇo rūpe mūrtaṁ caivāmūrtaṁ ca martyaṁ cāmr̥taṁ ca sthitaṁ ca yacca sacca tyacca ॥ 1 ॥ (Brhd. Upan. 2.3.1)
Meaning : There are two states of Brahman, मूर्तं । gross (with form, body and organs) and अमूर्तं । subtle (formless), mortal (मर्त्यं) and immortal (अमृतं), finite and infinite, existent and beyond (existence). This second, lower, gross, mortal, finite, existent Brahman is not "another" but is Brahman conditioned - therefore limited, manifesting and is thus Saguna with Attributes. The subtle formless Brahman has already been described as Nirguna Brahman.
यो दिवि तिष्ठन्दिवोऽन्तरो यं द्यौर्न वेद यस्य द्यौः शरीरं यो दिवमन्तरो यमयत्य् एष त आत्माऽन्तर्याम्यमृतः ॥ ८ ॥ yo divi tiṣṭhandivo'ntaro yaṁ dyaurna veda yasya dyauḥ śarīraṁ yo divamantaro yamayaty eṣa ta ātmā'ntaryāmyamr̥taḥ ॥ 8 ॥ (Brhd. Upan. 3.7.8)
The Vedanta Darshana extensively debates the concept of Plurality (the Manyness, Bahupurushas) based on the various explanations of Saguna Brahman according to that particular school of thought.
Unity of Atman and Brahman
Atman is the predominantly discussed topic in the Upanishads, but one finds two distinct versions. Some state that Brahman (Highest Reality, Universal Principle, Being-Consciousness-Bliss) is identical with Atman (Advaita siddhanta), while others state Atman is part of Brahman but not identical (Visishtadvaita and Dvaita siddhantas of Vedanta). This ancient debate flowered into various dual, non-dual theories in Hinduism. More about these aspects are discussed under the heading Brahman.
That Brahman and Atman are one and the same was proposed in Chandogya Upanishads mahavakyas. One of them being the following
स य एषोऽणिमैतदात्म्यमिदँ सर्वं तत्सत्यँ स आत्मा तत्त्वमसि श्वेतकेतो | sa ya eṣo'ṇimaitadātmyamidam̐ sarvaṁ tatsatyam̐ sa ātmā tattvamasi śvetaketo | (Chand. Upan. 6.8.7)
That which is this subtle essense, all this has got That as the Self, That is Truth, That is Self. You are That O! Svetaketu. In the Mandukya Upanishad another mahavakya stresses this point
सर्वं ह्येतद् ब्रह्मायमात्मा ब्रह्म सोऽयमात्मा चतुष्पात् ॥ २ ॥ sarvaṁ hyetad brahmāyamātmā brahma so'yamātmā catuṣpāt ॥ 2 ॥ (Mand. Upan. 2)
All this is surely Brahman. This Self is Brahman. The Self, such as It is, is possessed of four quarters.
मनस् ॥ Manas
Manas (not equivalent to Mind but used in that sense) is also called as Prajna, Chitta, Samkalpa which is engaged in a Vrtti (वृत्तिः) or states of existence (Yoga Darshana describes 6 such states). The thinking nature of man has been understood as the very essence of human beings since ancient times in India. Serious searches for unravelling the mystery of Manas and its impacts on life proved decisive in deepening the philosophical thoughts of human race making definite impacts upon the socio-cultural standards of life. Studies of Manas have contributed much in the fields of arts and science. It is a matter of fact that all philosophical thoughts and knowledge systems in India spring out explicitly or implicitly from the Vedas. The Upanishads being an integral part of Vedas represent the philosophical zenith of the Vedic thoughts and in depth discussions on Manas contribute to their uniqueness.
Aitareya Upanishad describes the origin of cosmic mind along with the origin of universe in a sequential manner.
हृदयं निरभिद्यत हृदयान्मनो मनसश्चन्द्रमा । hr̥dayaṁ nirabhidyata hr̥dayānmano manasaścandramā । (Aite. Upan. 1.1.4)
A heart parted open and from it came the mind. from the internal organ, mind, came the Moon.
Thought becomes the power that triggers the process of creation driven by the supposition of a cosmic mind or cosmic intelligent behind creation. While Brhadaranyaka says एतत्सर्वं मन एव | etatsarvaṁ mana eva |" (Brhd. Upan. 1.5.3) All this is the Mind itself", Ishavasya Upanishad refers to Manas in अनेजदेकं मनसो जवीयो । anejadekaṁ manaso javīyo । (Aite. Upan. 4)  the context of Atman being faster than the mind. Here the speed is described as a property of mind. Brhdaranyaka further says that सर्वेषा सङ्कल्पानां मन एकायनम् एव meaning Manas is the common ground for all imaginations and deliberations (Brhd. Upan. 4.5.12).
That Manas is not consciousness but is a subtle form of matter like the body is expounded in the Chandogya Upanishad. Further it says that Anna consumed is sorted in three ways post digestion. The grossest part becomes faeces; the middle component becomes flesh; the subtle ingredient becomes the mind. (Chan. Upan. 6.5.1)
The rituals of the Vedas, purify the Manas, discipline his Kama pravrtti and helps a Jiva to ascend in the path of attaining Brahmajnana.
माया ॥ Maya
Maya (not always meaning Illusion) is another most important concept touched upon in the Upanishads. The Supreme being or Paramatma, by his power of Maya projects the Universe and Jivatma (manifested Brahman) gets entangled in this Maya as long as He does not realize that His real nature is that of Paramatma. The siddhanta about Maya is mentioned as follows in the Upanishads.
Chandogya Upanishad explains the plurality feature as follows
तदैक्षत बहु स्यां प्रजायेयेति तत्तेजोऽसृजत । तत्तेज ऐक्षत बहु स्यां प्रजायेयेति तदपोऽसृजत । tadaikṣata bahu syāṁ prajāyeyeti tattejo'sr̥jata । tatteja aikṣata bahu syāṁ prajāyeyeti tadapo'sr̥jata । (Chan. Upan. 6.2.3)
That 'Sat' deliberated, 'may I become many; May I be born'. Then 'It' created Tejas (fire). The Fire deliberated 'may I become many; May I be born'. That created "Ap" or water. Shvetasvatara Upanishad says
क्षरं प्रधानममृताक्षरं हरः क्षरात्मानावीशते देव एकः । तस्याभिध्यानाद्योजनात्तत्त्वभावाद्भूयश्चान्ते विश्वमायानिवृत्तिः ॥ १० ॥ (Shvet. Upan. 1.10)
kṣaraṁ pradhānamamr̥tākṣaraṁ haraḥ kṣarātmānāvīśate deva ekaḥ । tasyābhidhyānādyojanāttattvabhāvādbhūyaścānte viśvamāyānivr̥ttiḥ ॥ 10 ॥
Matter (Pradhana) is the kshara or perishable. The jivatman is akshara or imperishable on account of being immortal. He, the only Supreme being, rules over both matter and Atman. By meditating on Him (अभिध्यानात्), being in "yoga" with Him (योजनात्), by the knowledge of identity with Him (तत्त्वभावाद्), one attains, in the end, freedom from the Maya of the world.
छन्दांसि यज्ञाः क्रतवो व्रतानि भूतं भव्यं यच्च वेदा वदन्ति । अस्मान्मायी सृजते विश्वमेतत्तस्मिंश्चान्यो मायया सन्निरुद्धः ॥ ९ ॥ (Shvet. Upan. 4.9)
chandāṁsi yajñāḥ kratavo vratāni bhūtaṁ bhavyaṁ yacca vedā vadanti । asmānmāyī sr̥jate viśvametattasmiṁścānyo māyayā sanniruddhaḥ ॥ 9 ॥
The shrutis (chandansi), the yajnas and kratus, the vratas (vows), the past, the future and all that the Vedas declare, have been produced from the imperishable Brahman. Brahman projects the universe through the power of Its maya. Again, in that universe Brahman as the jivatma gets entangled through the delusion of maya.
मायां तु प्रकृतिं विद्यान्मायिनं च महेश्वरम् । तस्यावयवभूतैस्तु व्याप्तं सर्वमिदं जगत् ॥ १० ॥ (Shvet. Upan. 4.10)
māyāṁ tu prakr̥tiṁ vidyānmāyinaṁ ca maheśvaram । tasyāvayavabhūtaistu vyāptaṁ sarvamidaṁ jagat ॥ 10 ॥
Know that the prakriti or nature is maya and that Supreme Being (Mahesvara) is the mayin (the maker of Maya). The whole universe is filled with jivatmans which are parts of His being. Brhdaranyaka Upanishad says
इदं वै तन्मधु दध्यङ्ङाथर्वणोऽश्विभ्यामुवाच । तदेतदृषिः पश्यन्नवोचत् । रूपरूपं प्रतिरूपो बभूव तदस्य रूपं प्रतिचक्षणाय । इन्द्रो मायाभिः पुरुरूप ईयते युक्ता ह्यस्य हरयः शता दशेतिय् अयं वै हरयो ऽयं वै दश च सहस्रणि बहूनि चानन्तानि च । तदेतद्ब्रह्मापूर्वमनपरमनन्तरमबाह्यम् अयमात्मा ब्रह्म सर्वानुभूरित्यनुशासनम् ॥ १९ ॥ (Brhd. Upan. 2.5.19)
idaṁ vai tanmadhu dadhyaṅṅātharvaṇo'śvibhyāmuvāca । tadetadr̥ṣiḥ paśyannavocat । rūparūpaṁ pratirūpo babhūva tadasya rūpaṁ praticakṣaṇāya । indro māyābhiḥ pururūpa īyate yuktā hyasya harayaḥ śatā daśetiy ayaṁ vai harayo 'yaṁ vai daśa ca sahasraṇi bahūni cānantāni ca । tadetadbrahmāpūrvamanaparamanantaramabāhyam ayamātmā brahma sarvānubhūrityanuśāsanam ॥ 19 ॥
Darshanas particularly the Vedanta darshana of Sri Adi Shankaracharya highlights this Maya as the cause of bondage to samsara and that Brahman alone is real and all else is unreal.
सर्गः ॥ Sarga
Upanishads abound with the srsti siddhantas (theories of origin of universe) which have ramified and flowered when it came to the darshana shastras. The one classic concept of origin (synonymous with procession from or production of, the evolving of, the sending forth) of the Universe, the Srsti siddhanta proposes that Ishvara evolves all beings out of Himself. Mundakopanishad proclaims
यथोर्णनाभिः सृजते गृह्णते च यथा पृथिव्यामोषधयः संभवन्ति । यथा सतः पुरुषात् केशलोमानि तथाऽक्षरात् संभवतीह विश्वम् ॥ ७ ॥ (Mund. Upan. 1.1.7)
yathorṇanābhiḥ sr̥jate gr̥hṇate ca yathā pr̥thivyāmoṣadhayaḥ saṁbhavanti । yathā sataḥ puruṣāt keśalomāni tathā'kṣarāt saṁbhavatīha viśvam ॥ 7 ॥
उपनिषद्वैशिष्ट्यम् ॥ Vaishishtya
Although all Upanishads proclaim that the goal of human life, embroiled in the flow of samsara, is to attain Jnana that leads to Moksha, the ultimate paramapurushartha, each of the upanishads have their own special features about their siddhantas as follows 
- Aitareya upanishad establishes the characteristics of Brahma
- Brhadaranyaka gives the paths to higher worlds
- Katha discusses the doubts about post death path of a Jiva.
- Shvetasvatara says Jagat and Paramatma are Maya.
- Mundakopanishad stressed the fact that the whole Universe is nothing but Parabrahma
- Ishavasya defines that a Jnani is one who sees Self and Paramatma pervading the world.
- Taittiriyopanishad proclaims that Brahmajnana leads to Moksha.
- Chandogyopanishad gives the outline of how janmas (births) happen and paths to reach Brahmaloka.
- Prashnopanishad logically answers the questions regarding the nature of Atma.
- Mandukya upanishad proclaims Atman to be Brahman
The Upanishads include sections on certain siddhantas that have been the very foundation of Sanatana Dharma. 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 Brhadaranyaka Upanishad, which is the oldest Upanishad.
The Upanishads contain several mahā-vākyas or "Great Sayings" on the the most unique concept of Brahman which is one of the knowledge treasures belonging to Bharatavarsha.
|अहं ब्रह्म अस्मि ॥ aham brahmāsmi||Brhadaranyaka Upanishad 1.4.10||"I am Brahman"|
|अयम् आत्मा ब्रह्म ॥ ayam ātmā brahma||Mandukya Upanishad 1.2||The Atma is Brahman|
|सत्यं ज्ञानमनन्तं ब्रह्मा ॥|
|सर्वं खल्विदं ब्रह्म ॥ sarvam khalvidam brahma||Chandogya Upanishad 3.14.1||"All this is Brahman"|
|एकमेवाद्वितीयम् ॥ ekam evadvitiyam||Chandogya Upanishad 6.2.1||"That [Brahman] is one, without a second"|
|तत्त्वमसि ॥ tat tvam asi||Chandogya Upanishad 6.8.7 et seq.||"Thou art that" ("You are Brahman")|
|प्रज्ञानं ब्रह्म ॥ prajnānam brahma||Aitareya Upanishad 3.3.7||"Knowledge is Brahman"|
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