Verse 23

23. For me there is neither earth,  nor water,  nor fire,  nor air,  nor ether.  Thus realizing the observer,  who lies in the cavity of the heart,  who is without parts,  and without a second,  the Witness of all,  beyond both existence and non-existence one attains the Pure Consciousness Itself.

"For me there is neither earth,  nor water,  nor fire,  nor air,  nor ether".

For the Sage at rest meditating in the last ashram of life there is neither home nor away neither becoming nor having become although the knowledge of all these exists.
Likewise with the Truth Consciousness and Bliss of the Absolute there is nothing relevant but all things are known  (are knowledge).

"Thus realizing the observer,  who lies in the cavity of the heart,  who is without parts,  and without a second,  the Witness of all"

"The cavity of the heart",  it should be understood,  literally means cavity or void or ether.  The meaning is all that Exists is Absolute Consciousness.  Therefore,  Consciousness or Absolute knowledge is Existence itself and relates to no thing.  Whatever or Wherever the organ of perception  (here the heart),  may appear to be,  it is manifest within,  as it were,  Absolute Knowledge.  So,  here the  "empty"  cavity of the heart is the absolute and so can be known there.
The Absolute Self is known within the heart of the intellect of the Sage as the subtle "self". This  "self"  provides the light of knowledge that guides the meditating Sage to Realisation of the Absolute Self the  "Witness of all".

"beyond both existence and non-existence one attains the Pure Consciousness Itself"

The Absolute is the Existence principle itself therefore it is the Existence of existence,  likewise with non-existence the Absolute is beyond all such labels.
Realising the Absolute Self thus the Sage Realises Absolute Consciousness and finds his way to unity as that Consciousness Alone.

Through divine discrimination the truth is now heard in the words of the Sages and the scriptures. 
The enlightened seeker,  the one to whom has been revealed the truth of Existence will no longer see Truth in duality.  A sincere attempt,  a decision to attain higher Truth through Self Realisation requires surrendering,  relinquishing dualistic personal beliefs and support.  Although now accepted to be false,  giving up ambition and desire is as if sawing through the very branch that is the sole support between the seekers reason for being and oblivion below.  The courageous,  desiring that Truth felt through the love of the Absolute,  suffer the heat of this fear and surrender themselves to the reality of the Absolute.
That heat caused by the action of the irrepressible working for Truth is real.  It may cause actual distress.  It has an emotional content.  It is a drama.
The Experience is of two worlds,  or as Dogen Zenji states in his Genjokoan  "When one side is lit,  the other side is dark".  The child clinging fearfully to the branch of the tree,  the world he knows,  is afraid to give up the little security he has even though his Father is close with arms outstretched urging the child to let go.  Once the move is made,  and the child is safe in the world of the Fathers arms the world of the tree distorted by fear is forgotten.
Before Self Realisation the concept of "I Alone Exist" is difficult.  After Self Realisation the understanding that assumes duality is profoundly alien.
The Absolute,  the primal Consciousness,  the eternal Observer is unmoved throughout this play but the material body and emotions however,  being a prop within the play registers all consequences.