Nonduality is the view–but then it is more than just a view–that reality is just one substance. That is, all of reality is one seamless substance.
To write “just,” as I have in the opening definitional sentence, or “only” may be a bit misleading since it could be read as implying that there could be more than one substance (as when we say, “Put just enough creamer in my coffee”; here, we imply that you can or could have put in more than just enough). Yet “just” is meant to suggest exactly the opposite: there is and cannot be more than one substance. There is This.
This view, which is more than just a view, may strike you, upon first acquaintance, to be pretty intuitive. The truth is that it’s anything but.
For starters, this one seamless substance, as it is, must be beyond space and time and therefore must be infinite and eternal–to wit, utterly limitless. Limitless, unborn, and unconditioned.
Next, if all reality is one substance, then I am That, you are That, everyone is That, every sentient being is That, every plant is That, and everything is That. I am limitless and so is everything and everyone else. In the relative sense of form, we are but prisms or extrusions of Being. In the absolute sense, there is absolutely no difference because no thing, just the completely undifferentiated fabric of Pure Being.
Go further. If I am That, then I cannot be a separate self standing over and against a world of objects and others that are “not-me.” I think I’m a separate ego “in here” and that there’s an objective world of others and objects “out there,” but if non-duality is right, then I’m worse than mistaken. I’m living in ignorance. In fact, I’m leading my life in such a way, you might say, as to be false.
Go one step further for now. If I am That, then I am not this body (which I’ve, out of ignorance, taken to be mine) but nor am I this mind (which I’ve also taken to be mine or to be me). But why am I not this body, this mind, or this body-mind composite? Because I am That and That is unlimited whereas the body and mind are limited, bounded, finite. I cannot be the body, the mind, or both.
But then though the body and mind can perish, evidently Pure Being–that is, I–cannot perish. (These implications I’ve recently referred to as “the weirdness of nonduality” (here and here).)
All of the above, of course, is merely theoretical. The point of the mystical path, however, is to see intuitively and to experience fully all that, and indeed much more than, what is written above. For body and mind, as Zen master Dogen once said, to “drop off.”
To see that we are Pure Being and not an extruded separate doer: what a marvel!