The standing matter/energy dichotomy was transformed over the course of the nineteenth and early twentieth centuries. Mayer -(Julius Robert Mayer, 1842), Joule -(James Prescott Joule, 1843–1845), and Helmholtz -(Hermann von Helmholtz, 1847) established the conservation of energy as a unified principle bridging mechanical, thermal, and chemical domains; Faraday’s experimental program -(Michael Faraday, 1839–1855) and Maxwell -(James Clerk Maxwell, 1873) made the field a first-class physical entity alongside matter; Einstein’s -(Einstein, A., 1905) mass-energy equivalence supplied a precise relation between rest mass and rest energy. E = mc^2 states that a system with invariant rest mass has corresponding rest energy; it does not by itself settle whether matter and energy are identical, two ontological categories, or two aspects of a deeper ontology. Nor does it establish a single globally conserved scalar called “mass-energy” in every spacetime. Those remain interpretive and theory-dependent questions.

The structural reading is therefore not a result extracted directly from mass-energy equivalence. It is an interpretive proposal: persistent physical bearers and their changing capacities are both needed for the Physicality role the book calls Φ. Rest mass, fields, particles, and dynamical energy may realize those functions in different physical theories, but the structural argument does not identify any one of them with Φ or claim that physics has reduced them to two metaphysical aspects. It asks only that an adequate physical ontology provide something that persists through transitions and bears causal capacity across them.

Quantum field theory (the framework that recasts particles as quantized excitations of underlying fields defined at every point of spacetime) extends the convergence (Weinberg -(Weinberg, S., 1995); Redhead -(Michael Redhead, 1982) on QFT ontology; Wallace -(Wallace, D., 2012) on QFT and the Everettian-dynamical reading). Particles are quantized excitations of underlying fields, and the apparent dichotomy between localized matter and extended field is plausibly an artifact of looking at the same Substance at different scales of description. What persists and what transforms are the same Substance registered in its two aspects.

The QFT reading above (fields defined at every point of spacetime, particles as their quantized excitations) is one of two live readings of what the fundamental physical object is. The alternative, wavefunction realism (Albert -(David Z Albert, 2015); Ney -(Alyssa Ney, 2021); Ney and Albert -(Alyssa Ney and David Z. Albert (eds.), 2013)), takes the fundamental physical object to be a single quantum state (the universal wavefunction) defined on a very-high-dimensional configuration space, with 3+1 spacetime as emergent rather than fundamental. Whether the fundamental ontology is fields-on-spacetime or wavefunction-on-configuration-space is a live question in the foundations of quantum mechanics; the structural argument does not need to resolve it. What the argument requires is that the Substance / Φ role be filled: that something persist and bear Physicality through Time and Space. Either reading saturates the role. On the fields-on-spacetime reading, Substance is registered as the underlying fields of the standard model and general relativity, persisting at every point of the manifold. On the wavefunction-on-configuration-space reading, Substance is registered as the universal wavefunction, persisting in configuration space with 3+1 spacetime emergent from its structure. The argument constrains *that* a bearer obtains; it does not pick which kind of object plays the role. A structural-necessity claim about Substance should not stand or fall with whichever interpretive program closes the QFT-foundations literature.

A second, structurally adjacent dispute runs one layer up from the physics-foundational one: what kind of thing *bears* causal properties at all, regardless of which physical theory describes it. Four positions are active. Substance ontology (Aristotle, *Categories* 1–5; Lowe -(Lowe, E. J., 1998); Heil -(John Heil, 2003)) takes primary substances as the ontological primitives that bear properties (a horse, a galaxy, a field) with essential and accidental properties standing in dependence on the substance that has them. Trope theory (Williams -(Donald C. Williams, 1953); Campbell -(Keith Campbell, 1990); Maurin -(Anna-Sofia Maurin, 2002)) inverts the order: particularized properties (tropes) are the basic items, and what we call substances are compresent bundles of tropes, with no underlying bare bearer behind them. Bundle theory (Hume, *Treatise* 1.1.6 'Of modes and substances'; Russell -(Bertrand Russell, 1940); Van Cleve -(James Van Cleve, 1985) on bundle-theory variants) takes substances as bundles of co-instantiated universals: same denial of the bare substratum, different account of the property-relata. Process metaphysics (Whitehead -(Whitehead, A. N., 1929); Rescher -(Nicholas Rescher, 1996); Seibt -(Johanna Seibt, 2017)) takes processes (temporally extended activities) as basic, with what we call substances being relatively stable patterns within the underlying flow.

The structural argument is neutral across the four. Distinctness, Substantiality, and Persistence ([§1.4.3](<1.4.3 Substance.md>)) are role specifications (what any Φ-bearer must do), not commitments about what kind of thing plays the role. A primary substance carries causal properties and persists through transitions on the substance reading. A compresent bundle of tropes carries them and persists, by trope-grouping or trope-causation theories, on the trope reading. A bundle of co-instantiated universals carries them and persists, by the co-instantiation relation, on the bundle reading. A relatively stable process carries them and persists, by its own temporal extension and internal coherence, on the process reading. Each saturates the Φ-role differently; none fails to saturate it. The structural argument requires that *something* bear Physicality; it does not require that the bearer be of one ontological category rather than another. Which of the four is correct is a question for argument outside the modal-structural framework — total-theory comparison, parsimony, explanatory virtues, and the empirical fit of each reading with contemporary physics. The argument needs a bearer; it does not settle what kind of thing bears.


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← [7.4.1 What is Disputed](<7.4.1 What is Disputed.md>) | [7.4.3 What is Predicted](<7.4.3 What is Predicted.md>) →
