When body and mind grow weak, the Self gathers in all the powers of life and descends with them into the heart. As prana leaves the eye, it ceases to see. He is becoming one, say the wise; he does not see. He is becoming one; he no longer hears. He is becoming one; he no longer speaks, or tastes, or smells, or thinks, or knows. By the light of the heart the Self leaves the body by one of its gates; and when he leaves, prana follows, and with it all the vital powers of the body.
Brihadaranyaka Upanishad
