VIII: 20

Chapter 8, Verse 20

"Beyond the manifest
And the unmanifest,
There is another Existence
Which is changeless and eternal.
When all things pass away,
It remains forever."

Swami Shivananda:

Krishna is referring to the Eternal Para Brahm, distinct from the unmanifested, primordial Nature.  It has a different quality.  It changelessly remains after all beings from Brahmaa the Creator down to an ant or a blade of grass have lived out their existences.

Paramahansa Yogananda:

Krishna is explaining that the unmanifested state of cosmic dissolution is only a temporary resting place for those who will again emerge as players in the cosmic drama.  Beyond the periodically unmanifested state of subtle cosmic vibration, there remains an ever-existent vibrationless essence.

Even the pure, undifferentiated aspect of Spirit in creation has an impermanence, which is punctuated by comings and goings.  This heart of the Universe, at the time of cosmic dissolution, for want of a form in which to pulse, is resolved again into Spirit.  This is symbolized as Vishnu, the Preserver of the worlds, asleep on Shesha, the thousand-headed serpent, awaiting the dawn of the next cycle of manifestation when he will send forth again those worlds and beings that he has preserved in an unmanifested state during the long night of cosmic dissolution.

Eckhart Tolle:

Most feel that their identity, their sense of self, is something incredibly precious that they don't want to lose.  That is why that there is such a strong fear of death.  It is frightening that "I" could cease to exist.  However, the mistake is restricting the "I" to a particular name and form and the stories that go along with them.  That "I" is no more than a temporary formation in the field of consciousness.  As long as that form identity is all that they know, they remain unaware of the priceless quality of their own essence, their innermost sense of "I Am," which is pure consciousness itself.  It is the eternal in them and in you.  It is the only thing that cannot be lost.