The philosophy of nonduality comes from the East, where it is called Advaita Vedanta. It says that everything is an integral part the One Spirit, and it is only illusion, which makes us think that we are separate from one another and from the spirit.
Nonduality is not just a theory, but one can realize it experimentally and live a life in which nonduality is a practical reality. You do not need to live in Ashram or on the Himalayas in order to taste it.
This philosophy has always existed, but during the last century more people have come to know about it. Maybe the world has become more ready to accept it, and so several spiritual teachers appeared to teach it. Some are known and some are not so much known. Among the great and known masters of nonduality are Sri Ramana Maharshi, H.W.L. Poonja, also known as Papaji, Sri Nisargadatta Maharaj and Ramesh balsekar.
According to this philosophy there is one spirit in the whole Universe, which is eternal and unchangeable. Everything is in it, and was and is manufactured by it.
Look at your physical body. It is made from the same kind of materials like any other body. Your feelings and thoughts are also composed from the same stuff like anyone else's. It is the spirit that infuses life into the body, feelings and thoughts making them look as a separate unit.
In order to explain the phenomena of the world the philosophy of nonduality says that some kind illusion called Maya arises in the omnipotent, omniscient and omnipresent spirit. This illusion - Maya, causes the erroneous notion of many separate units, and that everyone is a separate entity.
The Mind and the Ego
Let me now share with you a story. A big piece of sponge was divided into many small pieces, which were thrown into a large container of water. All those pieces of sponge got soaked in the water, which was one homogenous body of water. After a little while, the water in each piece of sponge started identifying itself with the sponge, and began feeling and believing that it was a separate independent entity.
The water stayed as one body of water. Each quantity of water present in each piece of sponge stayed connected with all the water in the container, but strangely, the water present in each piece of sponge started believing and acting as if it was part of the sponge, and was a definite entity separate from all the other pieces of sponge and the water in the container.
The water is the spirit, and the pieces of sponge are the materials composing the physical body and the feelings and thoughts. There is one spirit expressing itself through various forms, but each form thinks and feels that it is separate and independent.
This erroneous belief in a separate individuality is the ego. From this ego come disagreements, selfishness, anger, resentment and violence. Each expression of the spirit, through each body, erroneously believes in its separateness as a unit, and fights for its existence. These units, when united with other similar units possessing similar characteristics are called a family, tribe, nation or race.
It is not easy to see that everything in the Universe is the expression of the one Spirit. The illusion of life is so perfect that it is hard to look through this illusion.
When the mind is active, everything looks real. The mind is the projection machine, which constantly projects pictures on the screen we call reality. In sleep or while there is no consciousness of the world such as in a swoon, this projection machine is temporarily switched off and everything disappears. The world reappears after the mind is switched on again.
It is the mind and ego, which are responsible for the appearance of the world. When you are able to switch off the mind and the ego you realize in a direct manner the Spirit that is always here, beyond the illusion of the outside world.
The Way to Realizing Nonduality
Even if you only accept the philosophy of nonduality in theory, you stop being hurt by people and outside events. You stop giving importance to unimportant matters, and get a glimpse of the taste of real freedom. When you free yourself from the erroneous idea that you are a separate ego, happiness, joy and inner strength step into your life.
By developing the power of concentration, and through meditation and detachment the mind becomes quiet, and nonduality becomes a fact for us. Then life is lived differently and in a better way.
A practical method that Sri Ramana Maharshi has taught and advocated, and which I can vouch for from personal experience is called the "Vichara". It is a form of meditation in which you incessantly ask yourself in your mind the question, "Who am I?"
Do not try to find the answer through the mind. Just ask the question and let the answer come from the inside. Keep looking inside yourself and asking the question, and your awareness of this "I" will increase. You will find out that as your mind becomes concentrated and silent it becomes easier to experience this inner awareness.
Do not pay attention to the ideas and answers that come from your mind. The answer comes through the intuition, as an inner certainty and knowledge. It is not the mind that KNOWS. It is the PURE CONSCIOUSNESS, your real I that knows.
If the answer comes as a thought, ask yourself whose thought it is, and where did it come from. Look inside to find its source. This source is what we are seeking in this meditation.
This kind of meditation may be practiced often, and everywhere, not just while sitting and meditating at definite times. When the awareness of this "I" increases you will find that at times you experience a kind of a silent, free consciousness, and yet be able to function normally. You will know what Oneness means.
Though on the material level you continue to function as a separate unit, you come to understand the illusion of the world, and experience through your awakened consciousness the reality of nonduality and the Oneness of the Spirit. This is spiritual awakening and enlightenment.