There are a number of 108 Upanishads recognized in the Indian Philosophy, available from the ancient time and the famous saint; Samkara has made his comments on about ten of them that are considered as the chief Upanishads. Though, no exact date can be assigned to the Upanishads, these are undoubtedly the oldest and the most authoritative in the Indian philosophy. The earliest of the Upanishads date back to the pre-Buddhistic era and there a few Upanishads that were written after Buddha. The most possible time for the composition of those Upanishads is in between the completion of the Vedic hymns and the rise of Buddhism, in the sixth century B.C. There are also some dates available for the earliest Upanishads and the accepted dates for these Upanishads are 1000 B.C to 300 B.C. The popular religious teacher, Samkara did make his comments on about ten of the Upanishads that were composed at a later period, during the pre-Buddhistic period of about 400 or 300 B.C.
The Upanishads that are in prose are considered as the oldest ones and all of them are non-sectarian. To name some of the early Upanishads, the likes of the Aiterya, the Kausitaki, the Taittiriya, the Chandogya, the Brhadaranyaka, and parts of the Kena, can be mentioned. The verses 1-13 of the Kena, and iv, 8-21 of the Brhadaranyaka are considered to form the transition to the metrical Upanishads and they also can be put down as later additions. However, the Kathopanishad is considered as the later one and the elements of the Samkhya and the Yoga systems can be found here. The Kathopanishad also have frequent quotes from the other Upanishads and also from the Bhagavad-Gita. The Mandukya Upanishad is the latest one among the pre-sectarian Upanishads and it contains the elements of both the Samkhya and the Yoga systems. Apart from this, the Atharva-Veda Upanishads are also considered to grow at a later period. Among the other Upanishads, the Svetasvatara is known to be composed at such a period, when a number of philosophical theories were fermenting in India.
