Fundamentally, the Upanishads are an extended commentary on the Vedas, the most ancient Hindu scripture, but they go far beyond mere explication.

"Upanishad” is a Sanskrit term meaning “sitting beside” or “sitting with.” It refers to students learning ancient and esoteric religious lore at the feet of teachers. The Hindu scripture known as the Upanishads is a collection of such teachings.

Like the Bible, the Upanishads consist of many different texts by many authors, taught over the course of many centuries. There are 13 principal Upanishads, the oldest and most important of the collection. Ninety-five shorter texts were added over the years, bringing the total to 108 Upanishads in the canonical collection.

The authorship of the principal Upanishads is unknown, or at best conjectural. They were composed over more than 500 years, from the seventh through the first centuries B.C., with possible later interpolations. They were transmitted orally for several centuries before being finally written down. Although the Buddha is never mentioned explicitly, some of the later Upanishads may have been composed in part as responses to Buddhist teachings.

Following the destruction of Jerusalem’s temple by the Romans in A.D. 70, Christians and Jews increasingly developed allegorical and esoteric interpretations of Jewish temple ritual and sacrifices (see the Epistle to the Hebrews). In similar fashion, the Upanishads represent a movement towards allegorization of the intricate Vedic sacrifices. Indeed, the Upanishads assume their readers are familiar with the Vedic sacrificial system — these were originally priestly teachings. Fundamentally, the Upanishads are an extended commentary on the Vedas, the most ancient Hindu scripture, but they go far beyond mere explication. The Vedas, and later commentaries called the Brahmanas, focus on sacrifices, hymns and other activities Hindu priests should perform to honor their gods. The Upanishads explain the esoteric spiritual realities symbolized by Vedic sacrifices and hymns and why we should honor the gods.

The Upanishads can also be viewed as philosophical texts, exploring the nature of wisdom and how to acquire it. Like Plato’s works, many of the Upanishads are dialogues between teachers and students, including allegories, parables and enigmatic teachings. There are also moral texts, explaining the principles of righteousness — how to live a good life and attain salvation through release from rebirth. Several fundamental doctrines of later Hinduism make their first appearances in the Upanishads. Ahimsa (nonviolence) — the belief that one must not harm any living thing — is first discussed in the Chandogya Upanishad (8.15.1). The doctrine of karma (deeds) posits that one’s deeds have spiritual reality and power and cosmic significance; they are tethered to the soul through the cycles of rebirth, until ultimate salvation or damnation (Brihadaranyaka Upanishad 4.4.5-6). A third doctrine is that material reality is maya (illusion), and the true nature of the universe can be understood only through esoteric knowledge. (In this it’s broadly similar to the allegory of Plato’s cave, in “Republic” 514a–520a.) Whatever changes is transitory and unreal; only the eternal is ultimately real.

But fundamentally the Upanishads are mystical texts describing the nature of infinite ultimate divine reality (Brahman) and its relationship to the inner self, or soul (Atman). The fundamental claim of the Upanishads is that Atman is Brahman, or in other words, that the soul is part of, or equivalent with, ultimate reality. Thus the human soul is, at its essence, divine. The most famous explanation of this concept is the Sanskrit phrase “tat tvam asi” (“that thou art”), found in the Chandogya Upanishad, Book 6. There the priest Uddalaka teaches his son Shvetaketu the esoteric doctrine of their priestly clan. The human soul (Atman) is to ultimate reality (Brahman) as a drop of water is to the ocean. It may seem distinct, but when the water drop is returned to the ocean it becomes an indivisible and indistinguishable part of the ultimate (6.9). Uddalaka provides several other similar analogies, concluding each one with the famous dictum, “tat tvam asi” (“that thou art,” or “you are that”), that is, Shevtaketu’s soul/Atman is in essence divine.

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On the other hand, the Upanishads don’t necessarily represent a single, internally consistent world view. Just as Judaism in Jesus’ day was divided into several different denominations, the Upanishads represent different, even competing, worldviews, or even fledgling sects. The attempt to unify the sometimes seemingly disparate teachings of the Upanishads has been a major issue in Hindu theology, which has ultimately contributed to the theological intricacies of Hindu monistic (“advaita,” “non-dualistic”) and dualistic (“dvaita”) schools of thought. But that, as they say, is another story.

Further reading: “Upanishads” by Patrick Olivelle (Oxford, 2008).

Daniel Peterson founded the Middle Eastern Texts Initiative, chairs The Interpreter Foundation and blogs on Patheos. William Hamblin is the author of several books on premodern history. They speak only for themselves.