The inquiry and reasoning reveals:-
Where does the experience of duality /mind subside?
It is in formless witness, which is the source from where the mind rises and subsides. The mind subsides as soul, which in the form of consciousness. That is dual [mind] becoming non-dual [deep sleep]. When the mind raises the sense of duality is felt, because the soul becomes the waking or dream experience. When the Soul/Self remains in its formless non-dual true nature then the experience of duality is non-existent. The whole experience of waking or dream is Soul/consciousness, because it is the formless substance and witness of the three states. There is no second substance in the three states other than Soul/consciousness.
Deep sleep is the illustration that proves that the experience of duality [waking or dream] subsides as soul/spirit. When Soul/Self wakes up consciously in its own formless non-dual true nature the experience of duality becomes mere mirage.
Even the dream world is not different form the soul/consciousness, because the formless substance and witness of the dream itself is soul/consciousness. Similarly, the formless substance and witness of the Waking experience is also is the soul/conciousness. Therefore waking or dream both are illusion witnessed by the soul/self. In reality that is as soul, the experience of diversity is nonexistent there is nothing exist other than the soul, the whole diversity is mere mirage created out of single stuff ,that is the soul or spirit. Therefore, the diversity is mere division within the soul, which is in the form of consciousness.