As stated by Professor Cowell in Elphinstone's History of India -- 'they breathe a freedom of spirit unknown to any earlier work except the Rig-Veda . . . The great teachers of the higher knowledge and Brahmans are continually represented as going to Kshatriya Kings to become their pupils.' The 'Kshatriya Kings' were in the olden times, like the King-Hierophants of Egypt, the receptacles of the highest divine knowledge and wisdom, the Elect and the incarnations of the primordial divine Instructors -- the Dhyani Buddhas or Kumaras. There was a time, aeons before the Brahmans became a caste, or even the Upanishads were written, when there was on earth but one 'lip,' one religion and one science, namely, the speech of the gods, the Wisdom-Religion and Truth. This was before the fair fields of the latter, overrun by nations of many languages, became overgrown with the weeds of intentional deception, and national creeds invented by ambition, cruelty and selfishness, broke the one sacred Truth into thousands of fragments" (TG 354).
Thirteen of the principal Upanishads are: Aitareya, Kaushitaki, Kena, Taittiriya, Maitri, Katha, Brihadaranyaka, Prasna, Mundaka, Mandukya, Isa, Chhandogya, and Svetasvatara.

As stated by Professor Cowell in Elphinstone's History of India -- 'they breathe a freedom of spirit unknown to any earlier work except the Rig-Veda . . . The great teachers of the higher knowledge and Brahmans are continually represented as going to Kshatriya Kings to become their pupils.' The 'Kshatriya Kings' were in the olden times, like the King-Hierophants of Egypt, the receptacles of the highest divine knowledge and wisdom, the Elect and the incarnations of the primordial divine Instructors -- the Dhyani Buddhas or Kumaras. There was a time, aeons before the Brahmans became a caste, or even the Upanishads were written, when there was on earth but one 'lip,' one religion and one science, namely, the speech of the gods, the Wisdom-Religion and Truth. This was before the fair fields of the latter, overrun by nations of many languages, became overgrown with the weeds of intentional deception, and national creeds invented by ambition, cruelty and selfishness, broke the one sacred Truth into thousands of fragments" (TG 354).
Thirteen of the principal Upanishads are: Aitareya, Kaushitaki, Kena, Taittiriya, Maitri, Katha, Brihadaranyaka, Prasna, Mundaka, Mandukya, Isa, Chhandogya, and Svetasvatara.