The necessity of Nature is not a constraint on contingency. It is its precondition.

A proposition is contingent when it is possible and its negation is also possible (◇P ∧ ◇¬P). Contingency is a property of configurations within Nature (the necessary structure of T, S, and Φ), not of the structure itself; a contingent fact could have been otherwise.

Contingency is a defined modal status, but modal grammar alone does not guarantee that the status is occupied. A trivial modal field could contain only necessary truths. To establish actual contingency, the framework needs at least one proposition P for which both P and ¬P are possible. The claim carried here is therefore conditional and substantive: where Nature permits alternative configurations, contingency is available as their status; which configuration is actual remains contingent.

This is the positive formulation of the stage/play distinction established at [§2.2.1](<2.2.1 Change Is Actual.md>) and continued in [§2.4.6](<2.4.6 Succession Requires Temporality.md>) and [§2.6.6](<2.6.6 Modal Identity.md>), and grounded negatively in the analysis of ∅ at [§2.7](<2.7 Nothing to Consider.md>), where the attempt to specify a world without T, S, and Φ collapses into incoherence rather than naming a rival arrangement (re-established from independent materials at [§3.2](<3.2 Triconditional Necessity Positive Account.md>)). Without a necessary stage (without T, S, and Φ) there is no space of possibilities within which contingent facts could obtain. Contingency requires Temporality, Spatiality, and Physicality for alternatives to be genuinely open: causally structured succession (so that one configuration follows from another within a modal openness of alternatives: *being deterministically followed* and *being modally contingent* are compatible, since modal contingency concerns whether different outcomes are possible across worlds rather than whether outcomes are fixed by prior states within a world), relational extension (so that configurations are genuinely distinct), and causal persistence (so that causal chains are real rather than mere sequences of co-occurring events). The necessity of the structure of Reality is what makes the openness of the future (and therefore the reality of contingency) possible.

Necessity and contingency are therefore not in tension within this framework. They occupy different levels. Necessity belongs to the structural Conditions: the Conditions of possibility for any configuration whatever. Contingency belongs to the specific configurations enacted within those Conditions: which causal sequences occur, which relational positions are occupied, which particular interactions take place. The necessity of the stage does not determine the content of the play; it is what makes the play possible. And a play that is possible rather than predetermined is genuinely contingent: not despite the necessary stage but because of it.

The two claims hold together formally without conflict. Let F = T∧S∧Φ stand for the structural Conditions. The framework asserts □F at the structural level. Within the worlds where F holds (i.e., every world) it permits ◇p ∧ ◇¬p for any p describing which contingent configuration is occupied. The conjunction □F ∧ (◇p ∧ ◇¬p) is not a tension to resolve; it is the formal statement of what it is for necessary structure to host contingent configurations. □F fixes what holds in every world (the structural Conditions are instantiated); the contingent p ranges over what differs across the worlds where F holds. The Barcan-style question whether the domain of contingent individuals is itself fixed across worlds (taken up in Appendix A's *Domain Semantics and the Barcan Question*) does not arise for the structural necessity claim: necessity at the level of T, S, and Φ neither requires nor precludes a fixed contingent domain.

The reverse direction (that necessity-talk itself presupposes a non-trivial modal space) runs at [§3.2](<3.2 Triconditional Necessity Positive Account.md>), where the Way–Possibility Convertibility carries the bidirectional point.

> [!principle]
> **Modal Closure** · *Principle*
>
> The necessary structural Conditions admit no unnecessary alternative standing apart from them.

No parallel substrate, no rival ground, no external addition: everything unnecessary is a configuration within the Conditions, not a competitor to them. Closure constitutes the field within which contingency obtains rather than constraining it from outside. The principle restates what the Grounding Trilemma secured ([§2.8](<2.8 The Whole Line.md>)) and Theorem 7 completes ([§2.8.6](<2.8.6 Exclusivity of Nature.md>)); it pairs with Modal Exclusivity ([§4.1.10](<4.1.10 The Source of Possibility.md>)): nothing grounds the structure from above; nothing stands alongside it from outside.

A further precision follows from this distinction.

Contingency is not a property of things. It is a property of arrangements. What exists contingently is never a thing in isolation but always a specific configuration: this arrangement of causal relata at these positions within Nature, rather than some other. Nature itself is not contingent. The arrangement within it is.

Which laws Nature happens to realize, which geometry measures its extension, which constants calibrate its interactions: all contingent relative to the structure. Each is one specification among those the structure permits, not one the structure requires.

One strand of the argument from contingency asks why this arrangement rather than another. The structural account answers that strand directly: Nature is necessary; the arrangement is one of indefinitely many Nature permits but none of which it requires. A stronger strand, pressed by Rasmussen's Foundation Argument, asks instead why any contingent concrete reality exists at all. That version is not narrowed away but dissolved at its root: since Nature is necessary, there is no contingent reality-as-such left over to demand an external foundation, and it meets the structural-terminus reply at full strength in [§5.1](<5.1 Theistic Arguments for a Divine Ground.md>). What requires explanation is therefore not why anything exists (Nature necessarily exists) but why this configuration rather than another, which is a question internal to structure, not a question about it.

What if the laws themselves evolve?

Smolin -(Smolin, Lee, 2013); (Unger, Roberto Mangabeira; Smolin, Lee, 2014) presses the strongest version of this distinction from the opposite direction: not that the structure is contingent, but that it is so thoroughly temporal that even the laws of nature evolve. On his temporal naturalism, nothing is timelessly necessary: the laws themselves change over cosmological timescales, and the very idea of timeless necessity is a Platonic holdover. Pressed hard, the challenge targets the □ in □(T∧S∧Φ) rather than the T/S/Φ: if every regularity is local to an epoch, then any putative necessity attaches to whatever happens to be invariant within that epoch, not to a structural ground holding across all of cosmological time.

The challenge is genuine. The structural response: the evolution of laws presupposes T/S/Φ. For a law to change, there must be a before-state and an after-state of the nomological configuration (T), a distinction between the law that was and the law that is (S), and a relatum that persists through the transition (Φ). Smolin's temporal naturalism converges with the structural picture at the level of what makes evolution itself possible. The laws may change. The structural preconditions for change do not.

Smolin has the direction right (time is more fundamental than the laws) but the structural account specifies what that fundamentality consists in.

*What closes.* This resolution also closes Aquinas's Third Way (the argument from contingency) which infers a necessary being from the existence of contingent things. Aquinas's reasoning, reconstructed: if every being were contingent, then at some past time nothing would have existed; but if nothing had existed, nothing could now exist (since something cannot come from nothing); therefore there must be a necessary being. The inference is valid in form, though the reconstruction's first conditional carries the argument's famous gap: each contingent thing failing at some time does not yield a time when all fail together. The pressure that something must be necessary survives the gap, earned here from the structural materials of §[§2.5](<2.5 Triconditional Necessity.md>)–2.7 rather than from that conditional. What the inference misidentifies is its target. The something-cannot-come-from-nothing premise is structural (∅ admits no transition out of itself, [§2.7.2](<2.7.2 Incoherence of ∅.md>)), and what the impossibility of ∅ requires is not a necessary *being* but a necessary *structure* (T, S, and Φ together) within which contingency is possible. The Third Way's pressure that something must be necessary lands; what is necessary is the structural ground of any possible state of affairs, not an individual being among possibles.[^2-8-4-necessity-precedes-contingency-ch4]

[^2-8-4-necessity-precedes-contingency-ch4]: The necessity-as-precondition-of-contingency claim and Modal Closure are engaged in Chapter 4. Rasmussen's Foundation Argument and the cosmological-argument-from-contingency family (Avicenna, Leibniz, Clarke, Rasmussen) meet the structural-terminus reply at full strength ([§5.1](<5.1 Theistic Arguments for a Divine Ground.md>)), the present section having closed Aquinas's Third Way inline as the historical ancestor. Williamson's necessitism is the closest contemporary ally on universal necessity but with a different scope ([§4.4.6](<4.4.6 Necessitism.md>)), Lewis takes up modal realism and contingency across worlds ([§4.4.1](<4.4.1 Modal Realism.md>)), modal collapse is the worry that necessity-as-precondition leaves no genuine contingency ([§4.4.9](<4.4.9 Modal Collapse Objection.md>)), and Modal Exclusivity is what Modal Closure pairs with structurally ([§4.1.10](<4.1.10 The Source of Possibility.md>)).


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← [2.8.3 Not Physicalism](<2.8.3 Not Physicalism.md>) | [2.8.5 Necessity of Nature](<2.8.5 Necessity of Nature.md>) →
