The duality (mind or universe) disappears as non-duality (deep sleep).

Dualists could not distinguish between ‘I’ and ‘formless witness.’ They take ‘I’ as the self and witness. Their highest was the Jiva.

People are so much attached to the 'I' that they do not want to think that 'I' itself is mere illusion. Thus it is necessary to know the fact that “I’   is mind. And mind is universe. And universe is waking experience. And waking experience is illusion. Thus they are unable to detach the ‘I’ from the Real witness.

And dualists and intellectuals keep on objecting: - If everything else is false, your statement that self is Brahman is itself false.  Their

But when one says consciousness is false, there must be the awareness, consciousness, behind the very statement. The duality (mind or universe)   disappears as non-duality (deep sleep).  One has to rely upon that which is permanent. Formless witness or consciousness alone is permanent. Anything that one say is a he is saying within the illusion, but there is consciousness (formless witness)   before any statement could be made.

Again as a step leading to formless witness, the arguments, the three states avails. But if one knows the existence of the formless witness of the three states, nothing else remains to be known.