Is assuming this world as illusion or unreal enough? Does it not contradict day-2-day experience?
In the process of neti-neti or as taught by bhagavAn in gItA chapter 13 that kshetra (field of observation, which is the world we live in or experience) is different from kshetraGYA (witness, observer known as sAkshI). There are various reasons why we must superimpose the unreality of this world firmly in our mind even though we experience this world and are affected by different situations we face in our life. Of-course, certain level of purity is necessary in order to be a witness. Just like water when mixed with milk becomes inseparable from milk, our consciousness is mixed i.e. strongly embedded, attached) to our mind, which in turn is strongly attached to 5 senses, which in turn rely on physical gross body. We must by any valid means separate this consciousness from mind, body and senses.
One way of doing it is to make our mind understand and agree that this world is unreal. This is not an assumption, but an affirmation. Affirmations are like auto suggestions which are used to marathon runners, psychiatrists, hypnotists and all those who work with subconscious mind. Suppose you have got fracture in hand. By repeatedly reminding or auto-suggesting your mind that 'I am healthy', 'my hand is healed', 'There is no more fracture in my hand', 'My hand is strong and healthy', 'I am completely healthy', etc. By daily repeating these instructions with calm mind, relaxed body, without any thoughts in background and without having any doubt on affirmation whether they will work or not, one recovers very fast. This happens when the prime thought in your mind becomes 'My hand is healed' and there is no contradictory thought of failure to recover. There is no fear at the back of mind while repeating affirmations. Affirmations also must be accompanied with bhAva or emotions. Just before you go to sleep, get into sleepy state. Then with relaxed body and mind repeat affirmation 3-4 times and go to sleep. It so happens that they may continue throughout night in subconscious mind. When there are no stray thoughts, mind is calm, and thoughts are said with strong faith, they enter into subconscious mind. Once they enter subconscious mind, subconscious mind will make sure that whatever has entered inside it gets executed. Subconscious mind does not do rational thinking, yet it has 90 % power as compared to the conscious mind which has just 10 % power. Hence subconscious mind over powers conscious mind and executes what was feeded to it without discriminating.
When any thought is repeated or any action is repeated, it becomes habit. Similarly, Self Enquiry or neti neti or OM chanting becomes habit and goes on spontaneously. In this way, one can stay connected with Brahman even if one is engrossed in work. Chanting goes on in subconscious mind. As soon as one becomes free, mantra re-enters conscious mind and continues by itself without any effort on our part. Similarly, the process of removing wrong belief that 'this world is real' or 'I am body' or I am mind or I am jIva (incarnated soul) can be removed by putting right belief in subconscious mind. Hence though we experience exactly opposite, we keep affirming that this world is unreal, I am not body, mind, intellect, ego or soul, I am Brahman, unchanging, immortal, unborn supreme reality.
Subconscious mind is the key to our spiritual progress. All questions like
Why and how to prayers work?
Why prayers does not work all time?
Why we get opposite to what we pray?
How can one heal oneself very fast in half of expected time?
etc, can be answered to taking subconscious mind into account
There are other practical reason for affirming that this world is unreal. Once you mentally accept that this world is unreal and temporary, you will make every effort to remain conscious, not to get pulled into it. Mind which is supposed to control 5 senses has become their slave. Senses are directed extrovert. One sees anything when light projects on retina, is projected to brain as an inverted image. However, according to shAstra-s mind travels along with senses or via senses to the object, feels it and then infers a meaningful suggestion to it. Remember that mind and brain are different. So we must make efforts to stop mind running behind senses and its hunting for worldly object. Pull it backwards and make it stable. Then make this introvert mind merge into it's source which is Brahman. Mind itself does not have any power. It is jada (gross). Like moon it reflects the power of Atman or Brahman. When mind moves towards it's source, it merges in it. Hence in order to create dispassion we must cultivate in our mind very firmly that this world is unreal. But developing just dispassion is not enough. We need to surrender ourselves to Ishvara or Brahman and pray to take us beyond mAyA. Since one disassociates oneself from sense objects, body, mind, ego and intellect by practising dispassion, surrender to Ishvara and by being witness, this path is called as 'path of negation' by it 'neti-neti' or 'Self Enquiry'. Even by chanting OM, one is technically doing neti-neti, as one is separating or dis-associating 'I' or 'Self' (kshetraGYA) from all that is non-self (kshetra). Mind focusses on OM and later on, it searches for the origin of OM and merge in it. Origin of OM, which is turiyA avasthA or peace is nothing but Brahman.
When one becomes a witness, one gets separated from the objection of observation. When one gets attracted or associates oneself with any object, one becomes fused with it, mixes with it and considers oneself to be non-different from the object. We all unknowingly accept ourselves as 'body'. Accepting unreality and temporary nature of world helps us becomes witness to it and not get attached to it. Staying as witness increases our awareness as a separate entity then object of observation. Finally one separates from mind-body-intellect-ego-soul complex and becomes aware of non-dual Brahman. But to be a witness, mind has to stop longing for sense objects. Hence establishing unreality of this world is necessary.
We understand that there is no assumption of being 'Brahman'. Just assuming does not make one 'Brahman'. One has to realise one's true nature. Hence 'I am Brahman' or 'Brahman satya, jagat mithyA' is not just assumption, but a process of 'neti-neti' or 'the path of negation'.