The Brhadaranyaka Upanishad (6.2) and the Chandogya Upanishad (5.10) contain the first references to soul transmigration, which draw a qualitative difference between two separate routes for the soul.
The way to the realm of the gods ended at the sun, and the person who traveled it never returned; the path to the world of the fathers ended at the moon, and the person who traveled it was finally reincarnated on earth.
The desire for individual spiritual awareness runs through all of the Upanishads, and this is the key to embarking on the road to the realm of the gods.
Those who achieve such realization reach the ultimate goal, but those who only perform good actions will return to earth, although with karmic rewards from their good efforts.
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