Upanishads are based not upon theological reasoning, but on experience of spiritual life. And life is not dogmatic; in it opposing forces are reconciled – ideas of non-dualism and dualism, the infinite and the finite, do not exclude each other.
Moreover the Upanishads do not represent the spiritual experience of any one great individual, but of a great age of enlightenment which has a complex and collective manifestation, like that of the starry world. Different creeds may find their sustenance from them, but can never set sectarian boundaries round them; generations of men in our country, no mere students of philosophy, but seekers of life’s fulfillment, may make living use of the texts, but can never exhaust them of their freshness of meaning.
Rabindranath Tagore(Source: from the Forward by Rabindranath Tagore to The Philosophy of the Upanishads by S. Radhakrishnan)