The 2 energetic monisms do not conflict so much as cover different scopes. Idealism and neumonism are congruent kissing cousins.
Subjective idealism has the mind producing each individual’s world and goes no further. Meanwhile, neumonism postulates Nature as the product of something noumenal, such as energy, which is insubstantially conceptual.
Philosophically, idealism is an inadequate account for the nature of existence, in failing to identify how different minds may share the same sense of reality. Neumonism goes beyond idealism in positing an independent source of minds (e.g., energy), but does not identify an origin for that energy. Thus, though on the right track, these monisms fall short of a comprehensive explanation.
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The seeming but chimerical connection between the mind and body provides the best empirical evidence to infer the nature of monistic reality. Saliently, the choice of matterism or energyism centers on a single issue in physics: whether space and time are real, or only relations.
If space and time are absolute, we live in a physical world. If they are only relational, an amazingly convincing simulation is constantly before us.
For spacetime to be real requires inherent integrity. Instead, cosmological observations confirming Einstein’s relativity theories show spacetime as anything but incorruptible. Spacetime predictably distorts under the sway of gravity and is observer dependent. The only possible conclusion is that the spacetime coordinate system is merely a set of relations, and so cannot buttress matterism. In short, relativity alone proves energyism, with wave/particle duality as icing on the cake.
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Mediocre minds usually dismiss anything which reaches beyond their own understanding. ~ French author François de La Rochefoucauld
Matterism is literal minded in taking what is perceived as being reality (naïve realism). In contrast, energyism posits manifestation as a persistent illusion of objects which act as a puppet show of energetic interactions. The conclusion of energyism coincides with modern physics.
Materiality is to energyism analogous to what classical physics and thermodynamics are to modern physics: an approximation which works at ambient scale but is not fundamental. Matter is not to be taken literally as elemental. Actuality is an artifice of a deeper, immaterial reality.
Probable impossibilities are to be preferred to improbable possibilities. ~ Aristotle