The Upanishads, a part of the Vedas, are ancient Sanskrit texts that contain some of the central philosophical concepts and ideas of Hinduism, some of which are shared with Buddhism, and Jainism. Among the most important pieces of literature in the history of Indian religions and culture, the Upanishads played an important role in the development of spiritual ideas in ancient India, marking a transition from Vedic ritualism to new ideas and institutions. Of all Vedic literature, the Upanishads alone are widely known, and their central ideas are at the spiritual core of Hindus.
Upanishads are the result of the quest for answers to problems of life, death, rebirth, and liberation. The enlightened master, who alone can answer these questions, reply to the questions put to them. These sets of questions are recorded in various Upanishads.
Eeshaavaasyopanishad
The Eeshaavaasyopanishad is one of the shortest Upanishads, embedded as the final chapter of the Shukla Yajurveda. It is a Mukhya Upanishad and is known in two recensions, called Kanva and Madhyandina. The Upanishad is a brief poem, consisting of 17 or 18 verses, depending on the recension.
It is named after the first verse – Eeshaavaasyam idam sarvam... This is the 40th chapter of Shukla Yajur Veda. It is a very small Upanishad with just eighteen verses. These mantras give out the details of the spiritual science in a language known for its brevity.
Kenopanishad
The Kena Upanishad was probably composed sometime around the middle of the 1st millennium BCE. It has an unusual structure where the first 13 are verses composed as a metric poem, followed by 15 prose paragraphs of main text plus 6 prose paragraphs of epilogue. Paul Deussen suggests that the latter prose section of the main text is far more ancient than the poetic first section, and Kena Upanishad bridged the more ancient prose Upanishad era with the metric poetic era of Upanishads that followed.
It is named after the first verse – Keneshitam Patati Presitam manah….. This forms a part of the nineth chapter of Brahmana of Saama Veda’s Jaiminiya recension (shakha) school of Veda. Hence it is also called Braahmanopanishad. Brahmatattva i.e. supremacy of Brahman is the point established in this Upanishad.
Kathopanishad
The Kathakali Upanishad is an important ancient Sanskrit corpus of the Vedanta sub-schools and an influential Sruti to the diverse schools of Hinduism. It asserts that "Atman exists", teaches the precept "seek Self-knowledge which is Highest Bliss", and expounds on this premise like the other primary Upanishads of Hinduism. The Upanishad presents ideas that contrast Hinduism with Buddhism's assertion that "Soul, Self does not exist", and Buddhism's precept that one should seek "Emptiness which is Highest Bliss". The detailed teachings of Katha Upanishad have been variously interpreted, as Dvaita and as Advaita.
This forms a part of the Katha recension of the Krishna Yajur Veda, hence named Kathopanishad. It has two chapters and six sub-sections. This has the famous dialogue between Nachiketa, a young boy, and Yama the Lord of Death regarding the status of the soul after death. It deals with the knowledge leading to the liberation of the soul.
Prashnopanishad
This Upanishad is a part of the Pippalaada Samhita of Atharva Veda. It is in a form of a question and answers session between Rishi Pippalaada and his six disciples regarding the knowledge of Self. The Prashna Upanishad contains six Prashna, and each is a chapter with a discussion of answers. The chapters end with the phrase, prasnaprativakanam, which literally means, "thus ends the answer to the question". In some manuscripts discovered in India, the Upanishad is divided into three Adhyayas with a total of six Kandikas.
The first three questions are profound metaphysical questions but, states Eduard Roer, do not contain any defined, philosophical answers, are mostly embellished mythology and symbolism. The fourth section, in contrast, contains substantial philosophy. The last two sections discuss the symbol Om and Moksha concept. Roer as well as Weber suggest that the last two Prashnas may be spurious, later age insertion into the original Upanishad.
Mundakopanishad
Mundakopanishad forms a part of Shaunaka Shaakhaa of Atharva Veda. It is named Mundaka (tonsured head) because it was meant to cater to the inquiry of the Sanyaasins (the people with tonsured heads). It deals with the creation of the World and the Supreme Self.
The Mundaka Upanishad contains three Mundakams (parts), each with two sections. The first Mundakam, states Roer, defines the science of "Higher Knowledge" and "Lower Knowledge", and then asserts that acts of oblations and pious gifts are foolish, and do nothing to reduce unhappiness in the current life or next, rather it is the knowledge that frees. The second Mundakam describes the nature of the Brahman, the Self, the relation between the empirical world and the Brahman, and the path to knowing Brahman. The third Mundakam expands the ideas in the second Mundakam and then asserts that the state of knowing Brahman is one of freedom, fearlessness, complete liberation, self-sufficiency, and bliss.
Maandukya Upanishad
It is one of the smallest Upanishads with twelve mantras, belonging to Atharva Veda. It contains an exposition about the greatness and nature of OM. It is the only Upanishad that has been commented upon by Gaudapaadaachaarya, the grand preceptor of Aadi Shankara thus highlighting its greatness.
Taitreeya Upanishad
This is the seventh, eighth and nineth chapter of Taitreeya Brahmana of the Krishna Yajur Veda. The three chapters are named Shikshaavalli, Brahmaanandavalli, and Brighuvalli. The first chapter speaks about the greatness of spiritual education, the second regarding the nature of Supreme, and the third deals with the methodology adopted by Varuna to teach his son Brighu, the science of Supreme Self.
Aitreya Upanishad
This forms part of Aitreya Braahmana’s second chapter, belonging to Rig Veda. It contains three chapters dealing with the nature of creation (world), the individual Self (Jiva), and Brahman (the supreme Self). Chaandogya Upanishad :
It is found in the last eight chapters of the Taandya, Sadvimsha, and Mantra Brahmanas belonging to Kautama recension of Saama Veda. These eight chapters put together are called Chaandogya Brahmana or Chaandogya Upanishad. It contains a detailed exposition on various topics such as Upasana (worship), Antaryaamitva (presence of Lord in all beings), etc. It also contains, the famous mahaavaakya ‘Tat Tvam Asi’ – “Thou art That”. Brihadaaranyaka Upanishad :
Commentaries on Upanishads
Aadi Shankara
Aadi Shankara, who is considered as the authority to comment on many philosophical scripts was the oldest commentator on the 10 principal Upanishads along with a commentary on Shvetaashvatara Upanishad. His commentary was based on his philosophy of Monotheism i.e. Advaita.
Madhvaachaarya and Ranga Raamaanuja have written commentaries on the 10 Upanishads from the Dvaita and Vishishtadvaita point of view respectively.
Upanishad Brahma Yogin of the 17th century has commented on all 108 Upanishads and has offered a synthesis of all the Upanishads.