This question has been asked by devotees of Vadanta and told by the Gurus from ancient times. And this subject is described in Upnishadas.
As Gurus has been described, my Guru St. Nikka Singh ji used to describe us, when I was learning the meaning of Bhagwad Gita from him.
When we used to read the 13th chapter of Bhagwad Gita he often said,” It is very important to understand it “and as he told us by his own experience and by knowledge of this 13th chapter chapter I will describe it something here.
Knowledge Described by Guru and Bhagwad Gita 13th chapter; This knowledge is like a knowledge of theory. When a devotee understands it, then traditional Gurus told him, now you do meditation and Nam Japa which are the same, and by your effort and faith, devotion one day you will find your own experience in your heart. This knowledge is like a road map, when you have a road map in your pocket it will easy for you to find your destination.
In the 13th Chapter of Bhagwad Gita it is like this;
This body O Kuntiputra (son of Kunti) is called Field and who knows. This body is called its Knower (Observer),it has been cleared by ancient Saints (Rishes).-1solaka chapter 13; This is an ancient traditional knowledge Vadanta. Here given an example of Field and master of the field. Our body is like the farm and Our Self is like a Farmer. As a farmer knows his farm like this our Self knows to our body. I know my body, my mind, my pleasure and pain, my body changes as childhood, youth and old age but I never change. My mind changes happiness, sadness, but I still remains the same. Every day three stages come to my body as awakening, dream and deep sleep. These stats comes and goes, but I will be the same. That is why ‘I am” is a real master of the field.
My guru to explain it as very important to know that I am a Knower (Observer) not observed as body mind. It has been explained in every Upnishids very briefly. Now it is like a road map in the pocket and when you will do your Nam Japa, meditation with keen interest with more faith and devotion, then one day you will find in your heart this Bliss which cannot be described by language.that is why in every Upnishidas in the last they said; Nati-Nati. Meaning that we tried our best, but we cannot describe it, it is not a matter of words.