The forward conditional is stated as a necessary conditional: **□(◇C → (T∧S∧Φ))**. Not merely true in every possible world: structurally necessary. No possible world could satisfy the antecedent without the consequent. Three aspects establish this, and they converge on the same structural fact. One scope inheritance is restated here rather than silently absorbed: the T and S conjuncts hold without qualification, while the Φ conjunct inherits the conditional scope [§2.4.5](<2.4.5 Forward Conditional.md>) assigns it — invariant conditional on the causal reading of exchange, defended at [§2.4.8](<2.4.8 Exchange Requires Physicality.md>) and secured against the Humean alternative at [§4.1.4](<4.1.4 Humean Contingency.md>).

The *structural aspect:* modal facts are structural facts about Reality. The definitions (T as causally structured succession, S as relational extension, Φ as causal persistence) are not descriptions of contingent physical processes. They are what it structurally means to host change. A world in which change is possible but succession is absent is not a world with unusual physics. It is a world in which change is incoherent, which means it is not a world in which change is possible. The Δ argument of [§2.4.5](<2.4.5 Forward Conditional.md>) grounds this as framework-invariant: any theory that describes a Reality where change is possible is bound by the same structural constraints.

The *conceptual aspect:* to deny the conditional is to assert that a concept applies in the absence of its own defining conditions. Change without succession. Movement without extension. Interaction without persistence. This is not an unusual possible world. It is a contradiction in terms, in the same sense that a square circle is: not because every world happens to exclude it, but because its denial is not a coherent description of any world.

The *essential aspect:* the definitions are constitutive of essences, not merely reference-fixing descriptions. There is no deeper essence of change hiding behind the definitions. The concept and the essence coincide, not at the level of language but at the level of structural Conditions. A world where change lacks its essential structural requirements is a world where change is something other than change.

(Fine, Kit, 1994) argues that essence is prior to modality rather than derivable from it: the essence of a thing is what that thing is by virtue of its own nature, and necessities about the thing follow from its essence rather than grounding it. The essential aspect above endorses Fine's priority claim for the case of T, S, and Φ. The definitions in §[§1.3](<1.3 Change.md>)–1.4 are not modal stipulations from which the essences of change, Temporality, Spatiality, and Physicality follow as a consequence; they are direct specifications of what those phenomena are. The necessity □(◇C → (T∧S∧Φ)) is derivative: it holds because of what change, T, S, and Φ essentially are, not because a modal axiom has been stipulated to make it hold. This is why denying the conditional does not describe an unusual world: it describes no world, because the antecedent's own content already requires the consequent.

*What converges.* The three aspects are not independent paths to the same destination. They are the same structural fact viewed from every direction at which necessity has content. The forward conditional is not a fact about this universe. It is a fact about what any universe must be. And the fact is level-invariant in the sense established at [§2.4.3](<2.4.3 Demand Instantiation.md>): whether a given Reality is described physically, as content, or as mental activity, the conditional runs through all three descriptions without remainder, because what it requires (succession, distinction, exchange) is not a feature of any description but of what any description is a description of.[^2-4-9-forward-conditional-necessity-ch4]

[^2-4-9-forward-conditional-necessity-ch4]: The necessity claim is challenged on several fronts in Chapter 4: the Principle of Sufficient Reason offers a rival ground of necessity ([§4.1.3](<4.1.3 Principle of Sufficient Reason.md>)); Quine denies the analytic–essential distinction the essential aspect relies on ([§4.1.6](<4.1.6 Quinean Modal Skepticism.md>)); conventionalism treats necessity as stipulated rather than structural ([§4.1.7](<4.1.7 Modal Conventionalism.md>)); the source-of-possibility question asks whether possibility has an independent source ([§4.1.10](<4.1.10 The Source of Possibility.md>)); and Williamson's necessitism stands as the over-strong rival ([§4.4.6](<4.4.6 Necessitism.md>)).


---

← [2.4.8 Exchange Requires Physicality](<2.4.8 Exchange Requires Physicality.md>) | [2.4.10 Sufficiency of Structure](<2.4.10 Sufficiency of Structure.md>) →
