Adyatma means the study of Atman or spirit or consciousness. Atman means the Soul, which is in the form of consciousness within constitutes the most basic and fundamental study of what constitutes the real self of every Human experience.
Ego is the physical self. The soul is the eternal self. The best scientists and thinkers have failed to understand this cosmic phenomenon since they see all from the eyes of their bloated ego. Result ... they all see one aspect of the issue never arriving at the right conclusion. One has to penetrate the veil of duality and farther into the domain of reality.
For a Gnani the truth is totally opposite of the duality. He knows the fact that the soul is the true self and the three states are mere myth on the standpoint of the soul, which is the true self.
That alone is called ultimate truth, which is a fact that can never be changed under any circumstances. One must make an effort to know the ultimate truth or Brahman. The soul, which is present in the form of consciousness, is there always. Consciousness pervades in everything and everywhere in all the three states. Consciousness is the permanent thing and all else is a mere illusion.
There is nothing new to be acquired; only they have to be sharp enough to grasp and understand and assimilate it, when told about it. But there is a difference between understanding and realization. Effort is required for this understanding only, whereas once understood no special effort is needed to remember his understanding: until then he only has an idea of the soul, he only partially understands it. But once he thoroughly grasps what it is and that all these things are soul, he will then constantly find it present everywhere without further effort, because he will perceive the ultimate truth by understanding, even in the midst of worldly existence.
When there is only one thing that is soul, then there is nothing to change, nothing to appear or disappear; when one speaks of remembering or forgetting soul that implies he believe in the existence of something else, i.e. a second thing, which is to be remembered. That would show he doesn't know that all is one. But knowing it, there is no second, consequently no intermittent perception of the soul, but a permanent effortless understanding and assimilation that it alone is.
When one gets a glimpse of truth he has to try to repeat it a number of times in order to establish it. The soul, the innermost self must raise itself by the soul, the innermost self.
The difference between a physical truth and a spiritual truth, the former can evolve or change; the latter is ultimate and final.
The seeker has to verify and accept the truth and reject the untruth. When Buddha verified Vedic religion, god and scriptures and found them they are not the means to acquire self-knowledge or Brahma Gnana or Atma Gnana and rejected them. Then he was on his own and found the truth through deeper reasoning and got enlightenment. Without being judgmental about your views, I am expressing my views derived from deeper self-search.
It is impossible to prove the existence of God by any reasoning: one can only believe or assume. Religious believers see visions of gods and goddesses and adepts according to their own impressions remaining unconsciously in the mind from the inherited conditioning. This entire concept based confusion and doubts are present only till the ignorance is there. Once one discovers the fact that, the self is not physical but it is the soul, the innermost self then there is no scope for duality, which he experienced as reality.
"It is this Supreme Oneness which alone is real, since there is nothing else but the Self. Verily, there remains no other independent entity in the state of realization of the highest Truth."
First seeker must inquire into the nature of the physical body i.e. matter. Second he must inquire into the nature of the mind.
Observe at everything in nature because in everything there is consciousness. One should not avoid them, do not shut himself to Nature; and he should not shut himself away from the world which is as much consciousness, as anything else because the consciousness is the cause of the three states and it itself is uncaused.
To say this world is illusion without first examining them and inquiring into them thoroughly is to delude himself. The world is reality within the waking experience. Same way as the dream world was reality within the dream.
The person, who stamped their foot on the ground to refute to show the world, is real, ignore that in dream he would do exactly the same--stamp his dream foot on the ground and assert it to be real.