The non duality has to be grasped in the midst of the experience of duality. That is, being aware of the truth and untruth, reality and unreality. And able establish himself in the truth and able to view and judge, the three states, on the base of the soul, which is ultimate truth/Brahman.
It is unfortunate people are stuck with their accumulated ideas based on the false self, and imagine about non duality and say nothing exist other then the source. But a Gnani says everything exists [illusion] but everything is created out of the soul/spirit. Everything rises from the soul/sprit, and subsides as soul/sprit. Thus, the whole diversity has no relevance, on the standpoint of the soul as self.
Everything [illusion] is nothingness [spirit] realizing the three states are created out nothingness [Soul/Spirit]. Nothingness is the nature of the soul/spirit, because there is no second thing exist other then itself.
