Upanisad the fourth and final tier of Vedic literature. The 'Upanisad' literally means esoteric knowledge, which is read with the preceptor in seclusion. But as the connotation suggests, the word refers to the special treatises written towards the close of the Vedic era in literature. Vedic literature can be split into four kinds of treatises - Sanghita, Brahmana, Aranyaka and Upanisad. According to the Vedic tradition, the Aranyakas are included in Brahmanas and the Upanisads in the Aranyakas. As each of the Brahmanas and Aranyakas are associated with a Samhita, the Upanisads are considered in the same fashion.

Although there are an indefinite number of Upanisads, thirteen of them are reputed to be of great import. These are Aitareya, Kausitaki (of Rg Veda), Chhandogya, Kena (of Sama Veda), Taittiriya, Katha, Shvetashvatara, Maitrayaniya (of Krishva Yajur Veda), Brhadaranyaka, Isha (of Shukla Yajur Veda), Mundaka, Prashna and Mandukya (of Atharva Veda). While some of them are written in verse, others are written in both verse and prose. The Upanisads date back to the pre-Buddhist period (6th century BC).

Indian philosophy was first laid out in the Upanisads. They reveal ways of getting rid of ritual complexities and contain subtle decisions on worldly rites of the early days and instructions on getting into a novel world of thought. Some Upanisads depict different aspects of contemporary social life. A few scholars believe that the knowledge of science was first manifested in the Upanisads.

The Upanisads do not deal with gods or goddesses; rather Brahma is central to the deliberation of the books. They hold a brief for the one Brahma as the centre of the universe: He is the Truth, the Conscious and the One worth Knowing; the rest is untruth, the ignorant and the uninformed. The animate and Brahma are one, with no difference between them. Salvation of the animate comes through the encounter with Brahma. And this, according to the Upanisads, is the sole objective of human life.

These treatises indicate that India had been resplendent with knowledge even when the rest of the world was sunken in the darkness of the pre-civilisation days. Like the vedas, gita, ramayana and the mahabharata, the Upanisads are also considered as part of the scriptures of the Hindus. Many among the Hindu community recite from the Upanisads just as they also read from the Gita. Brahmaism, the doctrine propagated by Raja rammohun roy and Maharshi debendranath tagore, was based on the principles of the Upanisads. The Upanisads are the epitome of Indian philosophy. [Mrinal Kanti Gangopadhyay]