The Upanisads, literally meaning “to sit near someone,” constitute the concluding portion of the Vedas, the first original Hindu scripture, which has four sections: Samhitās or collections—hymns, prayers, and formulas of sacrifice; Brāhmanas—prose treatises discussing the significance of sacrificial rites and ceremonies; Āranyakas, or forest texts; and the Upanisads, or later Vedas. The Upanisads are the main basis for the Vedānta school of philosophy. The doctrines of the Upanisads were imparted orally. Groups of students sat near the teacher to learn from him the truths by which ignorance could be destroyed. The authors of the Upanisads, of which there are more than two hundred are not known. The principal Upanisads are the Ísa, Kena, Katha, Praśna, Mundaka, Māndūkya, Taittirīya, Aitareya, Chāndogya, and Brhadāranyaka Upanisads. Śankara, the Vedānta philosopher, wrote commentaries on the above ten and on the Śvetāśvatara Upanisads. In addition, the Kausitaki, Mahānārāyana and the Maitri are also considered principal Upanisads. These Upanisads were written partly in prose and partly in verse.
The Upanisads are concerned with the meaning of the sacrificial rites, and in the process of discussing them, they introduce some profound metaphysical and religious ideas. With the Upanisads began the period of speculative research into human nature and the individual’s position in the universe. The practical result of the Upanisads was to depersonalize the universe and to minimize the importance of earlier Vedic gods. The Upanisads were not philosophical treatises, but they contained certain fundamental ideas that form the basis of a philosophical system out of which the orthodox schools of Indian philosophy—Sāmkhya, Yoga, Nyāya, Vaísesika, Mīmāmsā, and Vedānta—developed their systems.
The Upanisads have for their ideal the realization of Brahman, becoming one with God. The world is not an end in itself. It comes from God, through his mysterious power, and it ends in God. Everything in the phenomenal world, including the individual, must realize the infinite, must strive to reclaim the highest. The Absolute is the highest and most desirable ideal. The performance of duty is necessary if one is to achieve the highest perfection. Morality is valuable because it leads one toward this highest perfection. Inner purity is more important than outer conformity. The ethics of the Upanisads insist on the transformation of the whole person. In the process of this transformation, one knows that one’s liberation from the phenomenal appearance depends on oneself and not upon the grace of transcendent deity. The idea of rebirth, the idea that the individual who has not gained the ultimate reality will be subject to the cycle of birth and death, is also presented for the first time in the Upanisads.