Brihadaranyaka UpanishadThe Brihadaranyaka Upanishad, which is generally recognized to be the most important of the Upanishads, consists of three sections ('Kandas'), the Madhu Kanda which expounds the teachings of the basic identity of the individual and the Universal Self, the Muni Kanda which provides the philosophical justification of the teaching and the Khila Kanda, which deals with certain modes of worship and meditation, ('upasana'), hearing the 'upadesha' or the teaching ('sravana'), logical reflection ('manana'), and contemplative meditation ('nididhyasana').
TS Eliot's landmark work The Waste Land ends with the reiteration of the three cardinal virtues from this Upanishad: 'Damyata' (restraint), 'Datta' (charity) and 'Dayadhvam' (compassion) followed by the blessing 'Shantih shantih shantih', that Eliot himself translated as "the peace that passeth understanding."
Svetasvatara UpanishadThe Svetasvatara Upanishad derives its name from the sage who taught it. It is theistic in character and identifies the Supreme Brahman with Rudra (Shiva) who is conceived as the author of the world, its protector and guide. The emphasis is not on Brahman the Absolute, whose complete perfection does not admit of any change or evolution, but on the personal 'Isvara', omniscient and omnipotent who is the manifested Brahma. This Upanishad teaches the unity of the souls and world in the one Supreme Reality. It is an attempt to reconcile the different philosophical and religious views, which prevailed at the time of its composition.
Isavasya UpanishadThe Isavasya Upanishad derives its name from the opening word of the text 'Isavasya' or 'Isa', meaning 'Lord' that encloses all that moves in the world. Greatly revered, this short Upanishad is often put at the beginning of the Upanishads, and marks the trend toward monotheism in the Upanishads. Its main purpose is to teach the essential unity of God and the world, being and becoming. It is interested not so much in the Absolute in itself ('Parabrahman') as in the Absolute in relation to the world ('Paramesvara'). It says that renouncing the world and not coveting the possessions of others can bring joy. The Isha Upanishad concludes with a prayer to Surya (sun) and Agni (fire).
Prasna UpanishadThe Prashna Upanishad belongs to the Atharva Veda and has six sections dealing with six questions or 'Prashna' put to a sage by his disciples. The questions are: From where are all the creatures born? How many angels support and illumine a creature and which is supreme? What is the relationship between the life-breath and the soul? What are sleep, waking, and dreams? What is the result of meditating on the word Aum? What are the sixteen parts of the Spirit? This Upanishad answers all these six vital questions.
Mandukya UpanishadThe Mandukya Upanishad belongs to the Atharva Veda and is an exposition of the principle of Aum as consisting of three elements, a, u, m, which may be used to experience the soul itself. It contains twelve verses that delineate four levels of consciousness: waking, dreaming, deep sleep, and a fourth mystical state of being one with the soul. This Upanishad by itself, it is said, is enough to lead one to liberation.
Maitri UpanishadThe Maitri Upanishad is the last of what are known as the principal Upanishads. It recommends meditation upon the soul ('atman') and life ('prana'). It says that the body is like a chariot without intelligence but it is driven by an intelligent being, who is pure, tranquil, breathless, selfless, undying, unborn, steadfast, independent and endless. The charioteer is the mind, the reins are the five organs of perception, the horses are the organs of action, and the soul is unmanifest, imperceptible, incomprehensible, selfless, steadfast, stainless and self-abiding. It also tells the story of a king, Brihadratha, who realized that his body is not eternal, and went into the forest to practice austerity, and sought liberation from reincarnating existence.