Chandogya Upanishad · Anchored at ChU 8.7.1.
A god and a demon study with the Creator for the same Self. One takes the first answer home content; the other keeps coming back, uneasy.
The Self that is free of evil, ageless and deathless, sorrowless — that, said Prajapati, is to be sought and known. The word spread through the worlds, and two students came to learn it: Indra on behalf of the gods, and Virochana on behalf of the demons. They came separately, as rivals, and studied with the Creator for thirty-two years before he gave them his first answer.
"The person seen in the eye — that is the Self," he told them, "the immortal, the fearless: Brahman." And to test them: "Look at yourselves in a pan of water, and tell me what you do not understand." They saw their own reflections, well-dressed and groomed. "We see the Self entire," they said, "down to the hair and nails." Both went away satisfied. Virochana carried that answer home to the demons as the highest truth: the Self is the body; so adorn the body, serve the body, and you serve the Self.
But on his way back Indra saw the flaw: if the reflection is the Self, then when the body is blind, lame, or dies, the Self would be blind, lame, and dead. That cannot be the deathless thing he was promised. So he returned — and returned again, and again, across a hundred years and four ever-deeper answers — until Prajapati taught him the Self that is neither the body, nor the dreamer, nor even the sleeper, but the awareness present through them all.
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