Khandogya-Upanishad: Sixth Prapathaka, Tenth Khanda

1. “These rivers, my son, run, the eastern (like the Ganga) toward the east, the western (like the Sindhu) toward the west. They go from sea to sea (i.e. the clouds lift up the water from the sea to the sky, and send it back as rain to the sea). They become indeed sea. And as those rivers, when they are in the sea, do not know, I am this or that river,

2. “In the same manner, my son, all these creatures, when they have come back from the True, know not that they have come back from the True. Whatever these creatures are here, whether a lion, or a wolf, or a boar, or a worm, or a midge, or a gnat, or a mosquito, that they become again and again.

3. “That which is that subtile essence, in it all that exists has its self. It is the True. It is the Self, and thou, O Svetaketu, art it.”

“Please, Sir, inform me still more,” said the son.

“Be it so, my child,” the father replied.