The sages of the Upanishads show a unique preoccupation with states of consciousness. They observed dreams and the state of dreamless sleep and asked what is “known” in each, and what faculty could be said to be the knower. What exactly is the difference between a dream and waking experience? What happens to the sense of “I” in dreamless sleep? And they sought invariants: in the constantly changing flow of human experience, is there anything that remains the same? In the constantly changing flow of thought, is there an observer who remains the same? Is there any thread of continuity, some
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