The apparent duality between the transcendent Divine and the manifested universe comes down to the spiritual experiences that embody these two (apparently opposite) things. The classic experience of a timeless, unmoving, Self-Existence that is both beyond all manifested forms and existences, and independent of them, is the basis for the seeking of the traditional Yoga of knowledge. This experience is overwhelming in its intensity when it takes up the consciousness of the individual seeker and cannot be questioned or quibbled about.
Sri Aurobindo describes the experience: “For on one side he is aware of this Self everywhere, this ever-lasting Spirit-Substance–Brahman, the Eternal–the same self-existence here in time behind each appearance he sees or senses and timeless beyond the universe. He has this strong overpowering experience of a Self that is neither our limited ego nor our mind, life or body, world-wide but not outwardly phenomenal, yet to some spirit-sense in him more concrete than any form or phenomenon, universal yet not dependent for its being on anything in the universe or on the whole totality of the universe; if all this were to disappear, its extinction would make no difference to this Eternal of his constant intimate experience.”
He continues: “A changeless imperishable infinity, a timeless eternity, a self-awareness which is not this receptive and reactive or tentacular mental consciousness, but is behind and above it and pressent too below it, even in what we call Inconscience, a oneness in which there is no possibility of any other existence, are the fourfold character of this settled experience.”
At the same time it must be noted that there is no real “duality” here. There is no inherent opposition to the manifestation; rather, this Self-Existent takes on all the forms, and all the actions in the manifestation as the Time-Spirit. “Yet this eternal Self-Existence is seen by him also as a conscious Time-Spirit bearing the stream of happenings, a self-extended spiritual Space containing all things and beings, a Spirit-Substance which is the very form and material of all that seems non-spiritual, temporary and finite. For all that is transitory, temporal, spatial, bounded, is yet felt by him to be in its substance and energy and power no other than the One, the Eternal, the Infinite.”
This is the reconciliation between the two great Upanishadic statements “One without a second” and “All this is the Brahman.”
Sri Aurobindo, The Synthesis of Yoga, Part One: The Yoga of Divine Works, Chapter 4, The Sacrifice, The Triune Path and the Lord of the Sacrifice, pg. 111