The Inquiry and Reasoning reveals that:
The dream or Waking experiences are absent, in deep sleep. The witness can remain with or without the dream or waking experiences.
The deep sleep experience proves that the witness can remain without the experience of duality. Therefore, one can come to firm conclusion to get the conviction that the witness is formless and apart.
One has to hold on to the formless witness, which is the soul, as Self and learn to view and judge the worldview on the base of soul as self to realize the fact that waking experience also unreal similar to the dream.