The essence of Mundaka Upanishad: Not be satisfied with rituals, yoga but inquire into the world around you, which you can see.+

From the Advaitic perspective, yoga is dualistic. The dualistic experience is nothing but ignorance.  

Yoga helps the yogi by giving him the feeling that the world is not worth bothering about, it detaches him from the world; it makes him treat the world as a dream, i.e. an idea. It does the same to his ego to some extent because he becomes indifferent to what happens to him. But the great secret is that this is only feeling, he feels these things only but does not know that the world is an illusion. Such knowledge can come only after Soulcentric inquiry and in no other way. That is why yogi cannot be Gnani. It is the difference between feeling and knowledge. 

The feeling of the yogi that the world is unreal may change tomorrow because all emotions are liable to change; and the fact is that yogis do change, as when going after women they lose their sense of world unreality though previously they felt it.

A permanent view of the world as unreal can come only after intellectual inquiry; such knowledge cannot change. Were the yogi of sufficiently sharp intellect he could discover the ideality of the world by reasoning alone and then it would not be necessary for him to have gone through yoga practice at all; that is why we say yoga is for dull or middling intellects.

The essence of Mundaka Upanishad: Not be satisfied with rituals, yoga, etc. which are good in their own way, but inquire. Into what? Brahman and Atman are things you can never see. So do not inquire into them. Inquire into the world around you, which you can see. Science tells you it is passing away every second. Everything is dying repeatedly. Where is it going? Thus you follow up your inquiry into what you can lay hands on. How can you inquire into Atma which you cannot see? So first we deal with the known and seen, this inquiry leads up to the unknown in the end.

Yoga cannot remove ignorance. It is only a step. It removes obstructions. It is not possible by mental control alone, by yoga, to achieve Brahman, but at best one falls asleep. It is like eating fire or leading an elephant by a thread or draining an ocean drop by drop, to try the yogic way. When the yogi shuts his eyes and does not see the world he is like the cat in the Indian proverb who shuts its eyes when drinking forbidden milk although other people are there, and it imagines it is unobserved because it cannot see them.  Yogi does not examine the phenomenal world and hence cannot realize Brahman for he takes that world as real but runs away from it.

 Gnana cannot come if anything is left out. The whole universe must be included. Only when all is known can all be known to be but ideation. Hence yogis blotting all out in samadhi cannot lead to Gnana. The ‘I-thought', the ego, belongs to the Seen as does the universe thought. The yogi may get the knowledge that the Seer is separate from Seen, but he will never know Brahman without inquiring into the world, because he is giving up the world, and hence cannot discover his unity with the world. The Gnani regards everything in the world as Brahman; the yogi rejects the world. Thus there is a fundamental difference.

That is why Sage Sankara said: ~ VC-63- “Without causing the objective universe to vanish and without knowing the truth of the ‘Self’ how is one to achieve Liberation.  

The universe in which we exist will not remain as reality when wisdom dawns. The universe is a mere illusion created out of consciousness and there is a conscious awareness of unity in diversity because there is no second thing that exists other than consciousness.

Realization is of the Soul as the ‘Self’ is -Perfection.  The non-dualistic truth cannot be comprehended by the dualistic perspective.

The universe is a reality only from the dualistic perspective. The universe is an illusion from the nondualistic perspective.  From the non-dualistic perspective the Soul, which is present in the form of consciousness is the sole reality. There is no second thing that exists other than the Soul in non-dualistic reality.

The Soul is the unknown reality on the dualistic perspective and the universe is the known reality on the dualistic perspective. The universe hides the Soul, which is the cause of the universe and it itself is uncaused.

Self-knowledge or Brahma Gnana or Atma Gnana helps to mentally transcend the mind into the Soul, which is the ultimate truth or Brahman or God in truth.

As the conviction of the Soul, the 'Self' becomes firm then gradually the ‘Self’ is free from experiencing the dualistic illusion (universe) as a reality.

When the Soul, the ‘Self’ becomes freed from all the obstruction by fully eliminating the ignorance it remains in its own awareness.  When the Soul, remains in its own awareness in the midst of the waking experience then the universe is merely an illusion created out of the soul, which is present in the form of consciousness.

When the Soul, the innermost 'Self' gives up identification with the waking entity then and recognizes its true nature is as undifferentiated awareness, unaffected by form, time, and space, and enter into its own awareness of its formless, timeless, and spaceless true nature.

The universe 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.

People's approach is more practical, and they stuck with the reality of the universe, they take it as real. That is why all the confusion.

The look of an object will depend upon the medium through which the observer views it. In fact, our mental and intellectual conditions determine the world, observed and experienced. The commoner viewing the world will see differently from a Gnani viewing the same world. Each one interprets the world that they see in terms of their existing knowledge. The commoner sees everything based on the ego, therefore, experiences the birth, life, death, and the world as a reality, whereas a Gnani sees everything as the consciousness and he is fully aware of the fact that, there is no second thing that exists other than the Soul or the consciousness. Thus, all the egocentric knowledge has to be bifurcated to realize the ultimate truth, which is beyond form, time, and space.

Sage Sankara says: ~ V.63 ~ "Without knowing and examining the external world, one can’t know Truth, as the idea that the external world exists, won't go. It can go only by an inquiry into the nature of the external world.

To say the universe is an illusion without first examining it and inquiring into its nature thoroughly is to delude oneself. This world is common to every one of us; therefore the seeker of truth must begin his inquiry with it. 

It is only after he has inquired into the nature of the objective world, he realizes the universe which through ignorance appears as of diverse forms, is nothing else but the Soul which is present in the form of consciousness is absolutely free from all the limitations of human thought.

Remember:~

This universe in which you exist is merely an illusion created out of single stuff. That single stuff is the Soul which is present in the form of consciousness.

The experience of birth, life, death, which happens within the universe is merely an illusion.

The form, time, and space are within the universe. Without form, time, and space there is no universe.

The universe is present only when the mind is present. The mind is present when ‘I’ is present.

From the standpoint of the Soul of the  Self, the universe is merely an illusion. 

If the universe is merely an illusion, then the experience of birth, life, and death happens within the universe is merely an illusion. 

If the universe is merely an illusion, then form, time, and space are merely an illusion. 

If form, time, and space are merely an illusion, then the mind is merely an illusion. 

If the mind is merely an illusion, then the ‘I’ is merely an illusion. 

If the ‘I’ is merely an illusion then the substance and the witness of the ‘I’ which is the formless Soul alone is real and eternal.

The mind is one.  The mind is present in the form of the ‘I’. The ‘I’ is present in the form of the universe. The universe appears as waking or dream (duality) and disappears as deep sleep (nonduality). : ~ Santthosh Kumaar