Upanishads are philosophical texts that contain inner or mystic teaching. These teachings come from questions pondered upon by deep thinkers and conveyed to pupils, usually in the solitude of a quiet forest. The pupils would sit around the teacher to listen and learn of the secret doctrines.
The doctrines of the Upanishads are considered to be part of the early sources of the Hindu religion. The oldest of these texts are the most important, and referred to as the principal, main or old Upanishads, also known as mukhya. The Brihadaranyaka, Jaiminiya Upanisadbrahmana and the Chandogya Upanishads were written during the pre-Buddhist time in India, and are the oldest ones. The Taittiriya, Aitareya and Kausitaki, are thought to have been composed after this era sometime after the 5th century BC because they show Buddhist influence. Although the Upanishads are known to have existed during the pre-Buddhist era, the exact dates of their compositions are still debated upon by scholars.
Two major concepts in the Upanishads teachings are characterized by the Brahman, which is the universal spirit, and the Atman, which is the individual Self. According to the doctrines of the Upanishads, the Brahman is "the infinite Spirit Source and fabric and core and destiny of all existence, both manifested and unmanifested and the formless infinite substratum and from whom the universe has grown". Brahman is considered the ultimate transcendent and immanent Spirit that is infinite in existence, and the sum total of all that ever is, was, or shall be. Atman, on the hand, is the immortal perfect Spirit of all living things.