The Upanishads are a collection of texts in the Vedic Sanskrit languagewhich contain the earliest emergence of some of the central religious concepts of Hinduism, some of which are shared with Buddhism and Jainism. The Upanishads are considered by Hindus to contain revealed truths (Sruti) concerning the nature of ultimate reality (brahman) and describing the character and form of human salvation (moksha).
The Upanishadic age was characterized by a pluralism of worldviews. While some Upanishads have been deemed 'monistic', others, including the Katha Upanishad, are dualistic. The Maitri is one of the Upanishads that inclines more toward dualism, thus grounding classical Samkhya and Yoga, in contrast to the non-dualistic Upanishads eventuating in Vedanta. They contain a plurality of ideas, which only by the later Vedanta-schools were tried to reconcile.
The Upanishads are treatises on Brahman-knowledge, that is knowledge of Ultimate Hidden Reality, and their presentation of philosophy presumes, it is by a strictly personal effort that one can reach the truth. In the Upanishads, states Radhakrishnan, knowledge is a means to freedom, and philosophy is the pursuit of wisdom by a way of life.
The Upanishads include sections on philosophical theories that have been at the foundation of Indian traditions. For example, the Chandogya Upanishad includes one of the earliest known declaration of Ahimsa (non-violence) as an ethical precept. Discussion of other ethical premises such as Damah (temperance, self-restraint), Satya (truthfulness), Dana (charity), Arjava (non-hypocrisy), Daya (compassion) and others are found in the oldest Upanishads and many later Upanishads. Similarly, the Karma doctrine is presented in the Brihadaranyaka Upanishad, which is the oldest Upanishad.
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