The principal Upanishads are accepted to be those which Shankaracharya (AD 788-820) chose to comment upon. They are 10 in number and are enumerated in the Indian tradition as follows: Isha, Kena, Katha, Prashna, Mundaka, Mandukya, Taittiriya, Aitareya, Chhandogya, and Brihadaranyaka. According to some scholars, Shankara also commented upon an 11th Upanishad, the Shvetashvatara.
The Isha Upanishad embodies in its very opening verse the central theme of the Upanishads, namely, the spiritual unity and solidarity of all existence. The Kena illumines the nature of knowledge by pointing out the eternal knower behind all acts of knowing, and purifies man’s concept of ultimate reality of all touch of finitude and relativity by revealing its character as the eternal Self of man and the Self of the universe. The Katha holds a special fascination for all students of the Upanishads for its happy blend of charming poetry, deep mysticism, and profound philosophy; it contains a more unified exposition of Vedanta than any other single Upanishad; its charm is heightened by the two characters of its dialogue, namely, old Yama, the teacher, and young Nachiketa, the student.
The Prashna as the name implies, is an Upanishad of questions; each of its six chapters comprises a question asked by each of a group of six inquiring students on various aspects of Vedanta, and the answers given by their teacher, the sage Pippalada. The Mundaka, after classifying all knowledge into Para, higher, and Apara, lower, and describing all sciences, art, literature, politics and economics - in fact, all positive knowledge, the knowledge of the changeful many - as Apara, and boldly including even the holy Vedas and all sacred books in this category, proclaims that one knowledge as Para, by which the imperishable changeless reality (of the One behind the many) is realized. And the Upanishad sings in ecstasy the glorious vision of the One in many.
In the brief compass of its twelve verses of condensed thought, the Mandukya surveys the whole of experience through a study of the three states of waking, dream and dream-less sleep, and reveals the Atman, the true Self of man, the Turiya or the Fourth, as it puts it, as pure consciousness, eternal and non-dual. It proclaims one of the four Maha-Vakyas: “This Atman (Self of man) is the Brahman.”
The Taittiriya, after majestically proclaiming that “the knower of the Brahman attains Supreme” describes the five Koshas or sheaths that enclose and hide Brahman, and demonstrates the technique of piercing these sheaths of relativity and finitude with a view to reaching the infinite and the eternal at the core of experience. It also provides a scientific definition of the Brahman as, “That from which all beings are born, by which, after being born, they live, and into which they merge when they cease to be.” The Aitareya establishes the spiritual character of the Self of man, and proclaims this truth in another of the four Maha-Vakyas: “Brahman is pure Consciousness.”
The Chhandogya introduces us to charming truth seekers like Satyakama, Shvetaketu, and Narada, and outstanding spiritual teachers like Aruni, Sanatkumara, and Prajapati. Through several illuminating teacher-student dialogues, the Upanishad helps us to discriminate the reality of being from the appearance of becoming. In a brief utterance of deep spiritual and philosophical import, treated as another of the four Mahavakyas, it sings in refrain the divinity of man: “That thou art.” It is an impressive account of man’s spiritual education, his growth from worldliness to spirituality.
The Brihadaranyaka, the longest of the Upanishads, is, as the name implies, a big (brihat) forest (aranya) of philosophical thought and spiritual inspiration. Four outstanding personalities illumine its pages - two men and two women - Janak, the philosopher-king, Yajnavalkya, the philosopher-sage, Maitreyi, the deeply spiritual wife of Yajnavalkya, and Gargi, the gifted woman speaker and philosopher, who is foremost among questioners of Yajnavalkya in philosophical debate. The Upanishad majestically expounds, through its fascinating dialogues, the central theme of all the Upanishads, namely, the divinity of man and the spiritual solidarity of the whole universe in Brahman. It contains another of the four Mahavakyas: ”I am Brahman” It goes to the eternal credit of Shankara that, through his masterly commentaries on the principal Upanishads, he brought out of obscurity this immortal literature, as also the great Bhagavad Gita and made them accessible and intelligible to a wider audience.
This is an excerpt from “The Message of The Upanishads” by Swami Ranganathananda