If it doesn't matter what we call the primary substance, then why the debate for the past 1000 years? — Harry Hindu
The debate here is really whether substance is primary or emergent. I am saying monism doesn't work, and neither does dualism. The simplest possible workable metaphysics is triadic - the kind in which substantial being, or actuality, is emergent from a developmental process of becoming.
And this kind of self-organising systems approach is what we find as our best current scientific answer. Aristotle got it early on. Peirce picked up the threads in a modern way. And science makes sense of both mind and matter in terms of self-organising systems these days.
It seems to me that the debate stems from our preliminary assumption of dualism and is solved through the realization of monism. — Harry Hindu
You are trying hard to make monism work. And it kind of does work if the "one thing" is the idea of a developmental process in which a fundamental instability becomes emergently self-regulating so as to produce comparatively stable being.
So there is one basic thing. Nature. The cosmos. Physical existence. But it is a systemic process. A logic of development and emergent order. And it thus has an irreducible complexity that can be most simply described in terms of three moving parts. In other words, the form of a hierarchy.
Jumping ahead to the physics, this is exactly how the science has panned out. You can see it everywhere. The Planck scale is defined by just three constants - c, G and h - bound in reciprocal relations to breath measurable existence into space, time and energy. The cosmos is described in terms of its triadic hierarchical structure - a quantum microscale, a relativistic macroscale, and then the good old solid and substantial classical scale that emerges between these two systematic limits.
The very shape of physics expresses a triadic metaphysics.
So the big mistake is to look for a monism without internal complexity. The simplest possible metaphysics that makes sense in actual descriptions of the world is a monism - a presumption of a closed system with all its causality bound up within itself - in which there are enough internal parts in play to explain the emergence of complex stable structure. And hierarchical organisation is the simplest model of a complex system.
Once you can count to three in metaphysical terms, then dualism or twoness becomes a lot less psychologically threatening to your worldview. You get to have your monism - a monism of one world - but a monism with the necessary complexity to describe a world with internal systematic order and a history of growth and development. Ie: The story modern science again tells since the discovery of the Big Bang~Heat Death developmental arc of our own universe.
So as Peirce laid it out so nicely, existence is a tale of three developmental stages.
You have the primacy of a Firstness or Vagueness - the chaotic initial conditions that is a realm of fluctuations without order or dimension, and thus a state of maximum possible symmetry (or sameness, or indifference - the two being synonymous).
Then if you have a symmetry which is a state of maximal disorder, then you can have the symmetry-breaking which is the phase transition to some new state of regulating order. You have Secondness or duality in which an asymmetry breaks out, allowing the new thing of relationships. You have the emergence of a globally regular difference between parts and wholes, figures and grounds, events and contexts.
Or in terms of Big Bang physics, you get the symmetry-breaking in which local microscopic particles are reacting energetically with each other within the background of a relatively thermally empty spacetime vacuum. You get that vital distinction between events and contexts which yield the further thing of some actual possibility of a history. The past becomes a thing as an accumulation of all the little accidents that define the present. The future also becomes a thing as all the little accidents or degrees of freedom that remain unconstrained and so living possibilities - actions to be dissipated.
In short, from the total disorder of a vagueness, we get the emergence of a dualistic difference between the many aspects of being. There is the concrete difference between the past and the future, the void and its events, the laws of physics and the accidents provided by degrees of freedom.
And because all these "dualities" are actually asymmetries - the product of taking possibilities to their opposing or reciprocal limits - they are really nascent hierarchical structure. As the universe expands and cools, it becomes ever more cleanly separated into its local play of hot events against an empty and inactive void. The bland radiation bath of the Big Bang clears to become a structure of moving particles in a spacetime vacuum.
The classical realm that we then see as substantial is the bit that emerges right in the middle - where enough crud gets lumped together in a shared inertial frame to lose its quantum indeterminacy and behave how we expect canonical substance to behave. Gas clouds can gravitationally clump to form fusion stars. Stars become factories of heavy atoms. Crud at a higher level of self-stable organisation, which eventually gets clumped into planets and spawns further developmental possibilities like symbolically structured life and mind.
So monism - if it is understood in the usual reductionist fashion of finding something primary, a material root to existence - is always going to fail. It didn't ever work as metaphysics. And science has proved it fails as an approach to modelling nature.
The first step out of monism was always dualistic in being some kind of ur-story of reciprocality or symmetry breaking. You need two to tango. And that is why metaphysics was born out of the logic of dialectical reasoning. The modern scientific understanding of existence got going once metaphysics had nutted out all the useful dialectical distinctions - the unities of opposites - like atom~void, discrete~continuous, matter~form, flux~stasis, chance~necessity, one~many, mind~world, and so on.
The mistake then is get stuck with this oppositional stage of metaphysical thought - to do what these kinds of threads always do and obsess about "fixing things" by getting back to some kind of monism. Or worse yet, to enshrine a dualism of substances.
So if the metaphysics winds up in an opposition of the ideal and the material, then the choices are a) argue for materialism, b) argue for idealism, c) argue for the separate and disjoint reality of both of these realms of being.
But all this is standard issue "theology". A legacy of the scientific revolution colliding with the Church. Folk took sides on something 600 years ago and have not escaped the confines of that debate ever since.
There was always another alternative - the one that the Ancient Greeks already expressed and which modern science has again arrived at. And this is the triadic or hierarchical systems view. Existence is a process. Systematic order is what naturally develops from unbound chaos. We now have testable mathematical models of this kind of reality creating organisation. The emergence of substantial being via phase transitions or symmetry-breaking is just a routine thing for scientific theory these days.
So maybe we have the odd thing of modern science - as a systems view - clashing culturally with the received "classical materialist" physics that has become the new folk orthodoxy. Materialism has become the secular theology. Everyone then wants to show they are on the right side by "eliminating" anything that questions their ardent monism. It becomes impossible to understand the scientific revolution that took place in the 20th century because they are still locked in mental combat with 16th century theological doctrine.