You had inquired about “non-dualism.” I think it can be helpful, in the first place, to designate a number of referents for “dualism” so then we can attempt to grasp what is being rejected. Three meanings of dualism that come to mind are
(1) epistemological (i.e. subject-object bifurcation),
(2) philosophical-metaphysical (e.g. Cartesian),
and (3) moral-metaphysical (i.e. good versus evil)
(3) It has been observed that evil is often just too much or too little of something which in another context or proportion would not be evil. I think that does a good job capturing the third kind of dualism according to the enumeration above but a number of points may be necessary to form a more comprehensive picture of moral dualism. First, I would also add the temporal dimension to the notion above and say “the right thing at the wrong time is not the right thing.” Further, I would note that it may be useful to differentiate good and evil as principles from good things and evil things in which those principles appear or inhere. The second is obviously dependent on context while the first is not. In fact, it is just the order and relation amongst the first thing that defines any particular context and differentiates it from any other one. Plato offers a very useful discussion of this issue in the Phaedo, which I will excerpt below. Only substitute “good” and “evil” for “heat” and “cold” and I think the connection should be clear.
(2) Philosophical dualism is usually thought of according to the opposition of body and soul or matter and spirit. Cartesius is perhaps the most prominent exponent of this sort of dualism. I think all sophisticated forms of philosophical dualism conceive of the distinction as a polarity rather than a binary. I am thinking of Aristotelian hylomorphism, which is essentially a codification of ideas that Plato floated in a number of dialogues, including the Phaedo.
(1) Epistemological dualism, for lack of a better term, has to do with the subject-object structure of perception. Many schools of yoga and hermeticism try to overcome this mode of perception, which is ultimately accomplished by dissolving one of the terms into the other. Because each term only makes sense in correlation to its apparent opposite, a successful dissolution of one term into the other is really a disassemblage of the whole structure of perception. Calling it “monism” however risks propagating the same subject-object mode of cognition that it is attempting to overcome because it is posited in just the same mode as any other abstract philosophical system. For this reason, “non-dualism,” which is a translation of the Sanskrit term advaita, has been used as a way to avoid positing anything at all and thereby become a positing subject set against a world of posited objects all over again:
When I get to the bottom, I go back to the top of the slide
Where I stop, and I turn, and I go for a ride
‘Til I get to the bottom, and I see you again!
Yes, I have quoted Lennon-McCartney in the manner that Plato and Aristotle quoted Homer.
I hope something I said was interesting or helpful. Perhaps others have something to add.
Plato in the Phaedo, on the exclusivity of contrary principles:
Hereupon one of the company, though I do not exactly remember which of them, said: In heaven’s name, is not this the direct contrary of what was admitted before—that out of the greater came the less and out of the less the greater, and that opposites were simply generated from opposites; but now this principle seems to be utterly denied.
Socrates inclined his head to the speaker and listened. I like your courage, he said, in reminding us of this. But you do not observe that there is a difference in the two cases. For then we were speaking of opposites in the concrete, and now of the essential opposite which, as is affirmed, neither in us nor in nature can ever be at variance with itself: then, my friend, we were speaking of things in which opposites are inherent and which are called after them, but now about the opposites which are inherent in them and which give their name to them; and these essential opposites will never, as we maintain, admit of generation into or out of one another. At the same time, turning to Cebes, he said: Are you at all disconcerted, Cebes, at our friend’s objection?
No, I do not feel so, said Cebes; and yet I cannot deny that I am often disturbed by objections.
Then we are agreed after all, said Socrates, that the opposite will never in any case be opposed to itself?
To that we are quite agreed, he replied.
Yet once more let me ask you to consider the question from another point of view, and see whether you agree with me:—There is a thing which you term heat, and another thing which you term cold?
But are they the same as fire and snow?
Most assuredly not.
Heat is a thing different from fire, and cold is not the same with snow?
And yet you will surely admit, that when snow, as was before said, is under the influence of heat, they will not remain snow and heat; but at the advance of the heat, the snow will either retire or perish?
Very true, he replied.
And the fire too at the advance of the cold will either retire or perish; and when the fire is under the influence of the cold, they will not remain as before, fire and cold.
That is true, he said.
And in some cases the name of the idea is not only attached to the idea in an eternal connection, but anything else which, not being the idea, exists only in the form of the idea, may also lay claim to it. I will try to make this clearer by an example:—The odd number is always called by the name of odd?