Nonduality is the English equivalent of the Sanskrit term, Advaita. Dvaita is a Sanskrit word that means duality, dualism, and dual. “A” is a prefix that makes it negative. So Advaita means not-dual or nondual. The philosophy becomes nondualism and when we speak of it as a subject it is nonduality.
For me, it basically means the appearance of two or even a multitude of separate things are not really separate at all. There is no “other.” The appearance of separateness (otherness) is an illusion caused by ignorance.
For example, there is an aspen forest in Utah called the Pando. If we were there and I asked you, how many aspen trees do you see…you might say hundreds. However, it is only the appearance of many, many aspen trees. In reality, it is a clonal colony of a single male quaking aspen that has been scientifically determined to be a single living organism (by identical genetic markers). It is a contender for one of the world’s largest organisms.
We are fooled by what we see and the assumptions we make based on what we see. We see many aspen trees and therefore think there are many separate trees. In reality, it is one tree…think of it as a tree on its side underground with only the branches sticking up out of the ground.
Another example, which I have given in public talks, would be my loop example. Imagine I held in my hand what looks like to be a loop of flat ribbon. I would ask, how many sides does this loop have? I would hear in reply, two. I would ask, is there an inside and outside to this loop? I would hear, yes.
What people do not see is that within the palm of my hand (where I am holding the loop) is a twist. That makes this a Mobius strip. If you are unfamiliar with a Mobius strip, it is a surface with one continuous side formed by joining the ends of a rectangular strip after twisting one end 180°. As there is only one continuous side, there really is no inside or outside. The ignorance of the twist makes all the difference in the world and makes the illusion of a loop believable.
Nonduality means “you” the reader and “I” the writer are not really two separate things. I am not other than you. This usually throws people for a loop and they say, “how can this be?”
Here is another analogy in answer. When you dream at night, where does the dream come from? It comes from our minds…the brain. While you are dreaming, however, you see a whole world and you (the dream character) move around in that world and do whatever you do. If you accidentally kick a rock, it hurts and you say, “ouch, that rock hurt me.” You think of that rock as separate and distinct from yourself. You bump into a dream character and have a discussion…or maybe it is the kind of dream that someone is trying to kill you. All the while, you believe this other dream character is separate and distinct from yourself. I am myself…and and they are themselves. When you wake up it all vanishes.
The question then is was the dream character you were playing and the dream character that was trying to kill you or the dream character you had a conversation with…were they really other than you? I would say no…they were all you. Even the rock was you…you created it via the brain. This is not to say that everyone in the dream was not real except you…because even that dream character you played in your dream was not real. Perhaps that dream character you played could fly, but that is not something you can do when you are awake.
Whether in a dream or in the real world, we tend to believe the world we are presented with. It is an illusion. Not saying that it does not exist, but rather it is not as it appears to be. Like a mirage appears to be water, but there isn’t any water there. The appearance exists…the water we think it is doesn’t exist.
Sometimes people think that nonduality means being in some state of constant “oneness” or “nothingness” (two sides of the same coin). Such a spiritual experience can happen, but that is not really what nonduality is about. Nonduality is not a belief system or a constant spiritual experience. So, it needs direct insight…it needs the deep realization and understanding that “otherness” is an illusion. It would be more along the lines of realizing you are dreaming while you are dreaming (kind of like a lucid dreamer). You know it is a dream, you know it is all you and you created it, and yet you still only have this one dream character to experience and play with within the dream. It is not that the experience of the dream (i.e. duality/otherness) ends.
Knowing it is a dream, however, makes all the difference in the world. When the other dream character comes to kill you, would you still be afraid…knowing it is just a dream and you will only wake up unharmed?
How you interact with and feel about the world is very different when you realize it is just a dream. Then as a lucid dreamer (lucid liver perhaps), you can never be fooled into believing the world you see again. As second-hand knowledge that may become a belief, it just doesn’t work. You have to see it for yourself.