The Upanishads are the first Indian works in which philosophical speculation is presented in a broad and developed form. Furthermore, in them we find the seeds of many doctrines that would be developed several centuries later by the great philosophical systems of India, especially those of the various schools of Vedanta, the most prevalent and influential orthodox philosophical school throughout Indian culture.
It can be said that Upanishadic thought marked the course of subsequent philosophical speculation in India, focusing on the problem of the Absolute and the means of reaching it and achieving it, and making metaphysical inspiration its principal, and indeed, almost exclusive, inspiration.