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@Fallaneze
Well first, monism isn't necessarily true.
(IFF) two substances can interact (natural and super-natural or mental and physical) (THEN) they must be fundamentally the same.
(IFF) two substances can NOT interact (natural and super-natural or mental and physical) (THEN) they must be UNDETECTABLE to each other.
(THEREFORE) de facto monism is necessarily true.
Second, we have evidence of a mental reality and no evidence of a non-mental reality. So why is physicalism a better explanation than idealism when idealism is evidenced by a mental reality whereas physicalism, which posits a non-mental reality, has no evidence whatsoever?
We have evidence that our "mental reality" is shockingly incomplete.
We can deductively reason that noumenon is a logical necessity.
We can also logically deduce that we can't say much about noumenon except that it is non-infinite and is likely composed of some portion of unknown and some portion of unknowable features.