The chapter's modal spine arrives here at its named result. Its steps are already in place: the Cartesian witness establishes the possibility of change, the fixed global domain of real possibility lifts that possibility to necessity, and the Structural Requirement identifies what every change-permitting world realizes. The Triconditional Identity supplies an ontological reading of the joint structure but is not needed to validate the modal derivation itself. What those steps have discharged can now be stated as a theorem.

> [!theorem]
> **Necessity of Nature** · **T6** · *Theorem 6*
>
> Temporality, Spatiality, and Physicality hold of necessity: the structural conditions required for change are necessary. Nature, named at [§2.6.10](<2.6.10 Triconditional Identity.md>) as the Triconditional Identity apprehended through three readings, is necessary. □(T∧S∧Φ).

The theorem reads both ways. *The Necessity of Nature* is also *the nature of necessity*: by Theorem 5 ([§2.6.10](<2.6.10 Triconditional Identity.md>)) the modal constraint just is T, S, and Φ, so the genitive collapses in either direction. The Conditioned-Nature ([§2.6.12](<2.6.12 Necessity's Nature.md>)) derives the collapse and generalizes it: every *nature of X* terminates in a structural feature of N, with necessity as the limit case.

Two clarifications about the scope of the necessity claim. *Nature* in Theorem 5 is not a proper name for a concrete individual that exists necessarily. It is a structurally fixed term: its reference is fixed by the joint co-instantiation of T, S, and Φ, and once fixed it holds across modal evaluation, because any possible Reality is evaluated within the structural Conditions that make possibility evaluable at all. The machinery is Kripke's -(Kripke, Saul A., 1980): reference-fixing for a term and *rigid designation* across modal evaluation. The refinement here is that the referent is structural rather than concrete (T∧S∧Φ as a co-instantiation pattern, not a particular individual) so the designation holds rigidly in the Kripkean sense while the bearer is not a being among possible others. Theorem 6 is unlike Kripke's necessary *a posteriori* identities. Water = H₂O is discovered through empirical investigation of a natural kind; T6 is argued from a Cartesian witness, fixed-domain modal semantics, and constitutive structural requirements. Its necessity claim is therefore structural and a priori in the book's intended sense, while its empirical realizations remain open to investigation. Nature is the structural-condition complex (what any possible Reality must instantiate to be a Reality) not a necessary being among possible others.

And the necessity attaches to the structural Conditions, not to any contingent occupant within Nature. The claim is *de dicto* with respect to the modal constraint (necessarily, any possible Reality instantiates T, S, and Φ) not *de re* necessity of this universe, this configuration, this law-pattern, or any token state Nature happens to host. Contingency within necessity is preserved: Nature is necessary; what fills it is not. The same level-distinction is developed from the contingency side at [§2.8.4](<2.8.4 Necessity Precedes Contingency.md>) (the question *why this configuration rather than another* is internal to Nature, not directed at Nature) and that result and the present one are two sides of one scope: the necessary structural Conditions and the contingent configurations they permit to vary.

The distinction has a classic illustration. Quine -(Quine, W. V. O., 1960) noticed that necessity can appear to shift with description. One person may be picked out as a mathematician or as a cyclist, and predicates central to one description need not be essential under the other. The point does not require the outdated assumption that every cyclist is two-legged; it is that substituting co-referential descriptions inside modal contexts can change the apparent modal claim. Quine drew a deflationary lesson: necessity can look like a feature of how we pick something out rather than of the thing itself.

The lesson is well taken, as far as it reaches. What it leaves unsaid is that under every description the person answers to, whether mathematician, cyclist, parent, or passer-by, something does not vary. To ride at all there must be a surface to ride on, a bike to ride, and the time to ride it: a place, a thing, and a duration, which are Spatiality, Physicality, and Temporality under other names. Vary the description as often as you like and those three remain. Quine showed that necessity floats free of any particular predicate. What his example also shows, once carried a step further, is that it does not float free of everything. Beneath the predicates that come and go lies one set of conditions that no description of anything real can shed. The necessity is *de dicto*, a necessity of the conditions and not of the bearer, exactly as the cyclist suggests. It is not for that reason merely verbal, because these conditions are what any description whatever must already presuppose.

*Against an uncaused initial-state stopping point.* A naturalist may treat the initial condition or causal order as uncaused and decline an explanation beyond the natural order. That is a live rival to the demand for an external necessary ground, but it should not be attributed to Oppy as a doctrine that a particular concrete initial state exists of necessity. Oppy's comparative naturalism is assessed on its own total-theory terms in the later engagement ([§2.9.3](<2.9.3 Total-Theory Comparison.md>) introduces the comparative framework; Chapter 4 conducts the comparison). The present contrast concerns the kind of stopping point at issue: an uncaused or brute initial configuration remains a particular arrangement and so leaves the internal question [§2.8.4](<2.8.4 Necessity Precedes Contingency.md>) presses, why this configuration rather than another? The Necessity of Nature locates necessity one level down, in the structural Conditions any configuration must instantiate, not in any configuration that instantiates them. The disagreement is between the occupant and the structure, the scene and the stage; it is not a claim that Oppy himself necessitates the occupant.


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← [2.8.4 Necessity Precedes Contingency](<2.8.4 Necessity Precedes Contingency.md>) | [2.8.6 Exclusivity of Nature](<2.8.6 Exclusivity of Nature.md>) →
