@XXSpadeMasterXX,
Thanks for asking the pertinent questions Spade.
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This is the problem IGM, that is faced when someone tries to mount an argument against dualism
for the opposite of it. [A rather dualist framing, by the way.]
Or, in other words, to try to argue for non-dualism, to claim that dualism is "just a fiction".
Dualism is not just a "fiction". In Mahayana (your practice I think?) terms dualism is a
Upaya, it is needed in order to reach enlightenment at which point you will see the truth of it in different terms.
Non-dualism is actually arguing for the
less enlightened view of pre-dualism. The view that we all had prior to gaining awareness of the self.
If you follow down this "non-dualist" intellectual path, I think where you will eventually arrive is a place indistinguishable from nihilism. This will not make an ethical practice very easy or natural.
I suggest, if you haven't already, look into framing a discussion in terms of dualism as opposed to trans-dualism.
To maybe explore the notion that there is a way to reconcile such things as 'self' and 'not self'.
Maybe a good jumping off point is thinking about the extensibility of self.
Which, by the way, is often very much related to some meditative exercises that you might already be familiar with.