Upanishad's says:- "He who thinks he knows, does not know." This means that to know anything implies a second, an object of knowledge, hence duality, i.e. no Gnana.
Gnani and Illusion
A King who was greatly incensed at the wisdom taught by Gnani that everything here below is an illusion. He wanted to teach the exponent of this doctrine a lesson. So he invited the then Gnani to his palace. That Gnani went there and stoutly maintained that everything in this world was illusion. The king had arranged to let loose a hungry lion against Gnani. The beast rushed at Gnani who took to a precipitate flight to save himself.
'Oh, Venerable Sir,' shouted the king, 'why do you run so fast seeing that the lion is only an illusion?'
'Oh, king,' said Gnani in his course of his flight, 'my running too is an illusion. Everything in this world is an illusion.’
Similarly, we all are searching truth within the illusion not being aware of the fact that, the illusion is created out of single stuff which is: Consciousness or Brahman.
Thus searching the truth in illusion with the illusory self, within the illusory experience, has to be illusion. The illusion is created, and sustained, and finally dissolves as consciousness, which is Soul, the innermost self. Since, there is no second thing other than the consciousness; the consciousness itself is God or Brahman or Christ. This conviction derives only through from deeper inquiry, analysis and reasoning.