Upanishad upa (near) and shad (to sit), meaning “sitting down near”; inspired by the action of sitting at the feet of an illuminated teacher to engage in a session. There is an attempt in these texts to shift the focus of religious life from external rites and sacrifices to internal spiritual quests in the search for answers. Probably between c. 800 BCE and c. 500 BCE, some people during this time decided to engage in the pursuit of spiritual progress, living as ascetic hermits, rejecting ordinary material concerns and giving up family life. The Upanishads are commonly referred to as Vedanta. Vedanta has been interpreted as the "last chapters, parts of the Veda" and alternatively as "object, the highest purpose of the Veda".
All Upanishads are associated with one of the four Vedas; various philosophical theories in the early Upanishads have been attributed to famous sages such as Yajnavalkya, Uddalaka Aruni, Shvetaketu, Shandilya, Aitareya, Balaki, Pippalada, and Sanatkumara. Women, such as Maitreyi and Gargi participate in the dialogues and are also credited in the early Upanishads. There are some exceptions to the anonymous tradition of the Upanishads. The Shvetashvatara Upanishad, for example, includes closing credits to sage Shvetashvatara, and he is considered the author of the Upanishad.
More than 200 Upanishads are known, but 14 are the most important: Isa, Kena, Katha, Prasna, Mundaka, Mandukya, Taittiriya, Aitareya, Chandogya, Brhadaranyaka, Svetasvatara, Kausitaki, Mahanarayana and the Maitri. These texts are found mostly in the concluding part of the Brahmanas and Aranyakas and were, for centuries, memorized by each generation and passed down orally.
The Upanishads tell us that the core of our own self is not the body, or the mind, but atman or “Self”. Atman is the core of all creatures, their innermost essence. It can only be perceived by direct experience through meditation. It is when we are at the deepest level of our existence. Brahman is the one underlying substance of the universe, the unchanging “Absolute Being”, the intangible essence of the entire existence. It is the undying and unchanging seed that creates and sustains everything. It is beyond all description and intellectual understanding.
Two different types of the non-dual Brahman-Atman are presented in the Upanishads. The one in which the non-dual Brahman-Atman is the all-inclusive ground of the universe and another in which empirical, changing reality is an appearance (Maya). The Upanishads describe the universe, and the human experience, as an interplay of Purusha (the eternal, unchanging principles, consciousness) and Prakṛti (the temporary, changing material world, nature). The Purusha manifests itself as Ātman (soul, self), and the Prakrti as Māyā. The Upanishads refer to the knowledge of Atman as "true knowledge" (Vidya), and the knowledge of Maya as "not true knowledge" (Avidya, Nescience, lack of awareness, lack of true knowledge). In the Upanishads, Māyā is the perceived changing reality and it co-exists with Brahman which is the hidden true reality. Maya, or "illusion", is an important idea in the Upanishads, because the texts assert that in the human pursuit of blissful and liberating self-knowledge, it is Maya which obscures, confuses and distracts an individual.
One of the great insights of the Upanishads is that atman and Brahman are made of the same substance. When a person achieves moksha or liberation, atman returns to Brahman, to the source, like a drop of water returning to the ocean. The Upanishads claim that it is an illusion that we are all separate: with this realization we can be freed from ego, from reincarnation and from the suffering we experience during our existence. Moksha, in a sense, means to be reabsorbed into Brahman, into the great World Soul.
Along with the Bhagavad Gita and the Brahmasutra, the mukhya Upanishads (known collectively as the Prasthanatrayi – three main sources for all schools of Vedanta) provide a foundation for the several later schools of Vedanta, including two influential monistic schools; Buddhism and Jainism.