The Upanishads are works of inspiration, not of reasoning; therefore we shall not find in them the development of thought or the logical connection of the sentences managed on the system of modern writers. The principle of our modern writing borrowed from the Greeks, who were the first nation to replace inspiration by intellect, resembles the progress of the serpent over a field, slow, winding, insinuating, covering perfectly every inch of the ground. The literary method of the ancients resembles the steps of a Titan striding from reef to reef over wide & unfathomable waters. The modern method instructs the intellect, the ancient illumines the soul. In the latter also there is a perfect logical sequence but this logic demands for our understanding&capacity to follow it something of the same illumination which presided at its construction. So profoundly characteristic is this difference that the Greek governs even his poetry by the law & style of the logical intellect, the Indian tends to subject even his prose to the law & style of the illuminated vision.
-Sri Aurobindo. The Isha Upanishad, CWSA vol. 17, p 370
Sri Aurobindo : The Upanishads came after the Vedas and they put in more plain language the same truth that was in the Veda. In the Veda the language is symbolic. But the Upanishads are equally great. Even in the Veda there are passages which clearly show that the Vedantic Truth is contained in the Veda. But it is surprising that the readers of the Veda miss those passages. For instance, the Veda says – “Riten ritam apihitam”, and then “it is That one” that is the source. It is clear that it refers to the Vedantic truth of the One. Similarly, the Upanishads speak of the Vedic symbols. The Ishopanishad speaks of the Vedic gods Sun – Surya and Agni, but you can see that the significance there is symbolic. Veda, Upanishad, Gita all are equally great.
(A.B.Purani. Evening Talks With Sri Aurobindo, 17Th January 1939)
- Our of the 108 Upanishads(उपनिषद्), there are there are 10 major ones : Isha, Kena, Katha, Prashna, Mundaka, Mandukya, Aitareya, Taittiriya, Brihadaranyaka, Chandogya.
- The Upanishads borrow many verses from the Rig Veda as described here.
- Summary of Upanishads at vedah.com
- wikipedia entry on Upanishads
This is a decent 15-minute video overview of the Upanishads produced by the Dr Sumit Kesarkar, although it does not cover Sri Aurobindo’s commentary.
Commentaries I found helpful
- Sri Aurobindo. Kena and other Upanishads (SABDA) (online PDF)
- Sri Aurobindo. Isha Upanishad (SABDA) (online PDF)
- Kapali Sastry. Lights on the Upanishads (SABDA) (amazon)
- M.P. Pandit. Upanishads: Gateways of Knowledge.
- Klaus Witz. Supreme Wisdom of the Upanishads (amazon) (google books)
- Swami Niranjanananda Saraswati. Dharana Darshan (amazon)
- Swami Brahmananda. The Supreme Knowledge: Revealed Through Vidyas in the Upanishads (amazon)
- N.S. Subrahmanian. Encyclopaedia of the Upanishads. New Delhi : Sterling, 1985. (amazon)
- Paul Deussen. Sixty Upanishads of the Veda (2 volumes); translated from German by V.M. Bedekar and G.B. Palsule. Delhi : Motilal Banarsidass, 1980, 2010. (amazon)
- Chitrita Devi. Upanishads for all. New Delhi: S. Chand, 1973. (Contains a verse-by-verse English translation of the ten principal Upanishads) (amazon) (google)
Click on Upanishads