Radhakrishnan

 “Though in some sense the Upanishads are the continuation of the Vedic religion, they are in another sense a strong philosophical protest against the religion of the Brahmanas. 

It is in the Upanishads that the tendency to spiritual monism, which, in one form or another, characterizes much of Indian philosophy, was first established whose intuition rather than reason was first recognized as the true guide of ultimate truth.”