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.”