Sri Aurobindo translates Shwetashwatara Upanishad, Chapter Six, Verse 19: “Who hath neither parts nor works, for He is utterly tranquil, faultless, stainless, therefore He is the one great bridge that carrieth us over to Immortality, even as when a fire hath burnt up all its fuel.”
When we reflect on the fire that has burnt up all its fuel, we recognize that the active manifestation of fire ends, while the principle of fire remains in a potential form. Should new fuel, new opportunity for action arise, the fire can be reignited. Similarly, when the seeker systematically quiets the mind, withdraws the senses from their objects, and focuses on the unmoving, unlimited, timeless Eternal, he can attain a shift of awareness from the ever-active focus on the external world and the action of the three Gunas, to that of the Timeless Eternal. This shift of awareness is exactly what is described in the texts on Raja Yoga as the seeker achieves various stages of the state of consciousness called Samadhi.
A number of Upanishads describe the path to Immortality as the shift of consciousness from the focus on the outer events, actions and objects in the world of forms, all of which are transient in nature, to the transcendent, unlimited Eternal, achieved through knowledge by identity. The Transcendent is not definable using any terms conceivable by the mind. Any attempt to limit falls short of the reality. The sages say “not this, not that” in order to make it clear that trying to identify the Eternal through individual forms is an approach that cannot possibly be successful. It is by systematically withdrawing the mind’s attention to each of the objects presented to the senses, and from each of the directions for outer action proposed by the mind’s activity, that the “mind stuff” (chitta) can be brought to a state of absolute stillness, where the only awareness at that point is the existential awareness of Sat-Chit-Ananda, unconditioned by form, process, action, event or time. This timeless state is one of immortal existence.
The Eternal is not limited even by its absence of forms or movement. Therefore, just as the fire becomes quiescent when it has consumed all its fuel, the potential for fire remains. So also, the Eternal possesses always the potential for action. The ancient scriptures speak of the “waking” and the “sleep” of Brahma. In one status, the manifested universe is active, in motion, the three Gunas are continually acting upon one another and trying to achieve a state of equilibrium. In the other status, everything is withdrawn and there is simply the unmoving stillness of the vast conscious awareness that is the Absolute, the Eternal, the Transcendent.
Sri Aurobindo, The Upanishads, Shwetashwatara Upanishad, pp.369-384