The duality exists until the mind exists.

Advaita only means negation of duality. The soul, which is present in the form of consciousness, is ultimate truth or Brahman. The consciousness is the cause of the origin, maintenance, and withdrawal of the universe is Advaita (i.e. non-dual), it means that the consciousness transcends all conceptions, positive and negative. Nothing positive can ever be imagined or said about it.

The consciousness is existence absolute, awareness absolute.  Existence absolute means that consciousness is not unreal or non-existent. And it is not unconsciousness.  Nothing positive can be stated about consciousness.

In one of the Upanishads the disciple says to the Master: "Lord, tell me the nature of Atman". The Teacher remains silent. Again the question is asked, and again the answer is silence. By his silence the Teacher indicated that the nature of Atman is in inexplicable, indescribable and unimaginable.

The soul, the innermost self is the One without a second. There is not the least shadow of multiplicity in the soul, which is present in the form of consciousness." 

We are talking about non-duality within the duality. The duality exists until the mind exists. Absent of mind is absent of duality. Therefore,   by understanding “What is mind?” and “What is the substance of the mind?” one becomes aware what is reality, and what is illusion.  Only by perfect understanding and realization of the non-dual truth, one will be able to overcome the duality. 

Since we are conditioned to view, judge and conclude the truth on the base of ‘I’, which is the false self, within the false experience, we are unaware what is truth, and what is illusion.  Man and his world exist within the illusion.  Therefore, it is erroneous to view and judge the truth on the base of illusory entity.

  
Truth cannot be got in world. The truth is hidden within the three states as their formless substance and witness.  The formless substance is the soul. And the soul is the true self.

The soul, the innermost self  is not an entity, or identity within the waking  or dream, but it is the source of the three states, from where the three states erupt and subsides. 

When one is able to view and judge the worldview on the standpoint soul as self, then he will realize there is neither the creation, nor the creator, but only non-dual soul  or spirit will prevail as ultimate reality.  Therefore, it is erroneous to conclude the truth on the base of religion and god. 

All religious and yogic practices are based on the `ego' or waking entity  as self, which is false self  within the false experience (waking or duality).  The Waking experience is mere illusion, same way as the dream. Hence it is very much necessary to verify truth of the ‘I’, to realize the fact that ‘I’ itself is mere illusion. ‘I’ is not the self, because it comes and goes as waking or dream. Absent of ‘I’ absent of waking or dream or mind. Absent of waking or dream is absent of illusion or duality.   The knower of the ‘I’ is formless soul. When one who realizes the soul as the true self, and then he becomes free from experiencing the duality as reality. 

This formless path is very difficult for those who stick to their inherited or accepted truth, but not for the serious seekers who are in search of only non dual truth nothing but non dual truth.