The Upanishads represent the loftiest heights of ancient Indo-Aryan thought and culture. They form the wisdom portion or Gnana-Kanda of the Vedas, as contrasted with the Karma-Kanda or sacrificial portion. In each of the four great Vedas--known as Rik, Yajur, Sama and Atharva--there is a large portion which deals predominantly with rituals and ceremonials, and which has  for its aim to show man how by the path of right action he may prepare himself for higher attainment. Following this in each Veda is another portion called the Upanishad, which deals wholly with the essentials of philosophic discrimination and ultimate spiritual vision.
      For this reason the Upanishads are known as the Vedanta, that is, the end or final goal of wisdom (Veda, wisdom; anta, end). The name Upanishad has been variously interpreted. Many claim that it is a compound Sanskrit word Upa-ni-shad, signifying "sitting at the feet or in the presence of a teacher"; while according to other authorities it means "to shatter" or "to  destroy" the fetters of ignorance. Whatever may have been the technical reason for selecting this name, it was chosen  undoubtedly to give a picture of aspiring seekers "approaching" some wise Seer in the seclusion of an Himalayan forest, in order  to learn of him the profoundest truths regarding the cosmic universe and God. 

    Because these teachings were usually given in the stillness of some distant retreat, where the noises of the world could not disturb the tranquillity of the contemplative life, they are known also as Aranyakas, Forest Books. Another reason for this name may be found in the fact that they were intended especially for the Vanaprasthas (those who, having fulfilled all their duties in the world, had retired to the forest to devote themselves to spiritual study). The form which the teaching naturally assumed was that of dialogue, a form later adopted by Plato and other Greek philosophers. As nothing was written and all instruction was
upanishads transmitted orally, the Upanishads are called Srutis, "what is heard." The term was also used in the sense of revealed, the Upanishads being regarded as direct revelations of God; while the Smritis, minor Scriptures "recorded through memory," were traditional works of purely human origin.

     It is a significant fact that nowhere in the Upanishads is mention made of any author or recorder. No date for the origin of the Upanishads can be fixed, because the written text does not limit their antiquity. The word Sruti makes that clear to us. The teaching probably existed ages before it was set down in any written form. The text itself bears evidence of this, because not infrequently in a dialogue between teacher and disciple the teacher quotes from earlier Scriptures now unknown to us. 

      The ennobling lessons of these Scriptures are as practical for the modern world as they were for the Indo-Aryans of the earliest Vedic age. Their teachings are summed up in two Maha-Vakyam or "great sayings":--Tat twam asi (That thou art) and Aham Brahmasmi (I am Brahman). This oneness of Soul and God lies at the very root of all Vedic thought, and it is this dominant ideal of the unity of all life and the oneness of Truth which makes the study of the Upanishads especially beneficial at the present moment.