The nature of the illusion is impermanent and constant flux whereas the nature of the soul or consciousness is permanent and still.

The nature of the mind, which is present in the form of universe, is constant flux.  The universe appears as waking or dream. Without being aware of the existence of the formless witness  of the three states,  people think everything is in flux in the universe.

 The permanent thing is the substance of the mind or universe.  The formless substance is the witness of the mind (I) or universe.  The formless substance and witness are one in essence. That essence is soul, which is present in the form of consciousness.  The soul or consciousness is real and permanent.  The universe is mere mirage created out of consciousness.

World is both real and unreal. It is real because it is a manifestation of consciousness, but is unreal, in the sense, that it is not absolute and eternal like consciousness itself.

Intellectuals   approach was more practical, and they stuck with the reality of the world, they take it as real.

On the standpoint the soul, the innermost self the universe is mere mirage.  The soul is the permanent all else mere illusion.  Man and exist in the unreal universe and by discussing on the standpoint of the ego (waking entity) which is the false self within the false experience will leads to perversity.    

In self-awareness the body is not considered as body, ego is not considered as ego and the world is not considered as world because body, ego and the world too are nothing but consciousness.   In self-awareness there is no flux. There is unity in diversity.  Only in ignorance the illusion is experienced as reality. The nature of the illusion is impermanent and constant flux whereas the nature of the soul or consciousness is permanent and still.