The Upanishads give us several concise descriptions of the Reality which are at once startlingly clear and simple, and yet, have been interpreted in ways that are contradictory. “One without a Second” and “All This is the Brahman” both try to express the essential truth of existence that there is ONE consciousness, ONE being, and that all that we see in the manifested universe, all these changing, phenomenal forms, are THAT.
The statement “one without a second” has been interpreted to mean that only the Absolute, the Eternal is real, and nothing else exists; thus, the world is an illusion! Taken out of context of the second statement that “all this is the Brahman”, it has led to renunciation of the world and attempts to reach the Absolute through denying the reality of the manifested universe.
Sri Aurobindo rejects this kind of one-sided analysis in favor of an integrated view: “the fundamental reality of the Absolute is to our spiritual perception a Divine Existence, Consciousness and Delight of Being which is a supracosmic Reality, self-existent, but also the secret truth underlying the whole manifestation; for the fundamental truth of Being must necessarily be the fundamental truth of Becoming. All is a manifestation of That; for it dwells even in all that seem to be its opposites and its hidden compulsion on them to disclose it is the cause of evolution, on Inconscience to develop from itself its secret consciousness, on the apparent Non-Being to reveal in itself the occult spiritual existence, on the insensible neutrality of Matter to develop a various delight of being which must grow, setting itself free from its minor terms, its contrary dualities of pain and pleasure, into the essential delight of existence, the spiritual Ananda.”
Sri Aurobindo, The Life Divine, Book 2, Part 2, Chapter 16, The Integral Knowledge and the Aim of Life; Four Theories of Existence