The fourth layer of Vedic literature according to the traditional division, the Upaniṣads are also referred to as the Vedānta—the conclusion, essence, or culmination of the Veda. A distinction is normally made between an effectively closed canon of principal, or major, early Upaniṣads, composed or compiled between approximately the 7th century bce and the early centuries of the Common Era, and a still open-ended category of ‘minor’ Upaniṣads. Collections of the major early Upaniṣads usually include the following (listed in a tentative chronological order): Bṛhadāraṇyaka, Chāndogya, Taittirīya, Aitareya, Kauṣītaki, Kena, Kaṭha, Ῑśā, Śvetāśvatara, Muṇḍaka, Praśna, Māṇḍūkya, and Maitrāyaṇī. (Medieval anthologies, however, often contain much longer lists of what are regarded as ‘major’ Upaniṣads.) The minor Upaniṣads consist of texts numbered in their hundreds which employ the cachet of the name for their own purposes—e.g. those collected as Saṃnyāsa Upaniṣads.
Each extant early Upaniṣad is attributed to, and was originally orally transmitted by, a particular Vedic school or śākhā. (So, for example, the Taittirīya Upaniṣad, belongs to the Taittirīya Saṃhitā of the Black Yajur Veda.) The division between Āraṇyaka and Upaniṣad is not always clearly demarcated, either in terms of content or classification: the last book of the Śatapatha Brāhmaṇa, for instance, is referred to as an Āraṇyaka, but so is the Upaniṣad that completes it, the Bṛhadāraṇyaka.
The earliest Upaniṣads, such as the Bṛhadāraṇyaka and the Chāndogya, remain closely linked to Vedic ritualism; they continue the speculation on the underlying meaning of ritual found in the texts that chronologically precede them, the Brāhmaṇas and Āraṇyakas, particularly in relation to the notion of bandhu, ‘homology’ or ‘equivalence’ (which is also one of the early meanings of ‘upaniṣad’). One important result of this is the formulation of an equivalence between the essence of the individual, the ātman, and the power which underlies the universe, brahman (neut.). Knowledge (jñāna)—i.e. the active realization of such an equation—liberates the knower from the cycle of karma and rebirth—a complex of ideas which itself first comes to textual light in the Bṛhadāraṇyaka and Chāndogya. There it is presented as a secret knowledge (upaniṣad) of hidden equivalences, privately transmitted from teacher to pupil, outside the setting of the village (grāma). The Upaniṣads are therefore the texts which contain this secret teaching, much of it put into the mouths of kṣatriyas (and sometimes of women). (Some recent scholarship has suggested that these new teachings may be associated with the rapid growth of cities in this period, and represent the views of ‘city’ brahmins.) Variations on these themes, and other cosmological and soteriological speculations (many of them related to early yogic and Sāṃkhya ideas) were added in the later Upaniṣads, with a notable theistic strand emerging in texts such as the Kaṭha and the Śvetāśvatara. The major Upaniṣads are thus the repositories of many of the theological and philosophical ideas which come to dominate later Vedāntic thought. Unsystematic themselves (but like the rest of the Veda regarded as revelation), they were systematized, partly through the synthesis made in the Brahmasūtra and the work of its commentators, and partly through numerous commentaries on particular Upaniṣads, such as those made by Śaṅkara, which shaped their thought for the theological purposes of particular Vedāntic schools. Indeed, as two of the prasthāna traya, the Brahmasūtra and the principal Upaniṣads required commentaries from anyone wishing to found a new Vedānta subschool. In more recent times, many of those involved in the Hindu reform movements of the 19th century (such as Rāmmohun Roy) have eulogized the Upaniṣads as containing the basis of a pure, essential (not to say universal) Hinduism. Similarly, the early Upaniṣads, cut free of their Vedic moorings, were among the first texts to attract the attention of Western philosophers and Orientalists. For summaries of specific content, see the entries on individual Upaniṣads.