Message of Nisargadatta Maharaj 
The essence of Maharaj's teaching

Nisargadatta's teachings are rooted in the Advaita Vedanta interpretation of the Vedic idea "Tat Tvam Asi" (That Thou Art), meaning 'You are one with Divinity'.

It is hard to summarize the techings of Sri Nisargadatta Maharaj. He recommended the practice of looking within, and focusing the feeling 'I am', that had led to his own realization in less than three years. Maharaj says:

Just keep in mind the feeling "I am," merge in it, till your mind and feeling become one. By repeated attempts you will stumble on the right balance of attention and affection and your mind will be firmly established in the thought-feeling "I am."

According to Nisargadatta Maharaj, our true nature is perpetually free peaceful awareness, in Hinduism referred to as Brahman. Awareness is the source of, but different from, the personal, individual consciousness, which is related to the body. The mind and memory are responsible for association with a particular body;  awareness exists prior to both mind and memory. It is only the idea that we are the body that keeps us from living what he calls our "original essence", the True Self, in Hinduism referred to as Atman.

Maharaj describes this essence of self as pure, free, and unaffected by anything that occurs. He compares it to a silent witness that watches through the body's senses, yet is not moved, either to happiness or sadness, based on what it sees.

For Nisargadatta, the Self is not one super-entity which knows independently, regardless of things; there is no such super-entity, no Creator with infinite intellect. God does not exist independently from creation. What does exist is the "total acting" (or functioning) of the Ultimate or Absolute Reality along the infinite varying forms in manifestation. This Absolute Reality is identical to The Self.

In his famous work 'I am That', Nisargadatta quotes:

The seeker is he who is in search of himself.

Give up all questions except one: ‘Who am I?’ After all, the only fact you are sure of is that you are. The ‘I am’ is certain. The ‘I am this’ is not. Struggle to find out what you are in reality.

To know what you are, you must first investigate and know what you are not.

Discover all that you are not -- body, feelings thoughts, time, space, this or that -- nothing, concrete or abstract, which you perceive can be you. The very act of perceiving shows that you are not what you perceive.

The clearer you understand on the level of mind you can be described in negative terms only, the quicker will you come to the end of your search and realize that you are the limitless being.

Nisargadatta also predicates the radical idea that there is no such thing as a "doer". According to him and other teachers of Vedanta, since our true nature or identity is not the mind, is not the body, but the witness of the mind and body, we, as pure awareness, do nothing. The mind and body act of their own accord, and we are the witness of them, though the mind often thinks it acts. This false idea (that the mind is the self) is what keeps us from recognizing our Self. Nisargadatta cautions:

'"The life force [prana] and the mind are operating [of their own accord], but the mind will tempt you to believe that it is "you". Therefore understand always that you are the timeless spaceless witness. And even if the mind tells you that you are the one who is acting, don't believe the mind. [...] The apparatus [mind, body] which is functioning has come upon your original essense, but you are not that apparatus." 

When asked about his biographical details, Maharaj used to say that "I was never born". For him, the essence of his being is eternal, pure and ever free awareness which is not confined to a specific body-mind.