The PSR demand is met at [§4.1.3](<4.1.3 Principle of Sufficient Reason.md>) by showing that Nature is not a thing that grounds itself but what grounding consists in. A residual question remains: can an entity **within the modal domain** be explanatorily prior to real possibility, conferring content on the global range W that ◇ quantifies over? The answer runs by disjunctive elimination on the candidate's own modal status. It is deliberately narrower than the claim that every possible essence-prior or primitive grounding account has been refuted by contradiction.

The position pressed against is the family of grounding accounts that take possibility as a derivative phenomenon: neo-Fregean abstractionism on modal grounds (Hale, B., 2013), Lewisian reduction of modality to concrete possible worlds (Lewis, David, 1986), Williamsonian necessitism (Williamson, Timothy, 2013), dispositionalist grounding of possibility in the potentialities of actual objects (Vetter, Barbara, 2015), engaged on its own terms at [§4.1.12](<4.1.12 Dispositionalism about Modality.md>), and the theistic moves that locate modality in a divine intellect or divine nature (Adams, R. M., 1983); (Pruss, A. R., 2011); (Leftow, Brian, 2012); the last of these, pressed as an argument from modality's need of a ground to God's existence, is engaged on its own terms at [§5.1.8](<5.1.8 Modal-Grounding Argument.md>), whereas the result established here runs purely at the level of any candidate's modal status.

*Definition.* Let *source of possibility* name any X such that ◇p (for some non-empty range of p) holds in virtue of X. The "in virtue of" relation is left maximally general: causal, metaphysical, conceptual, intellectual, or explanatory grounding all qualify. The argument does not turn on which grounding relation is in view.

> [!premise]
> **Modal Exclusivity** · *Premises*
>
> 1. If X is a source of possibility, then ◇p holds in virtue of X (Definition).
>
> 1. Every candidate X has exactly one modal status: ¬◇X (impossible: true in no world), ◇X ∧ ¬□X (contingent: true in at least one world but not in every world), or □X (necessary: true in every world). The three cases are mutually exclusive and jointly exhaustive.

*The impossible case.* ¬◇X. If X is impossible, X has no instances and no relata. Nothing is the case in virtue of what is not the case. The impossible is not a source but the absence of one.

*The contingent case.* ◇X ∧ ¬□X. If X is possible without being necessary, ◇X is itself a fact within the modal field. The proposed source of ◇ presupposes ◇ in the specification of its own modal status. The field is not grounded by one of its own features.

*The necessary case.* □X. This horn must be met in its strongest form, the one that grants the most. That X is necessary does not by itself bar it from being explanatorily prior: grounding priority and modal status are distinct, and an item may be necessary without being explanatorily inert — essence need not reduce to any modal fact (Fine, Kit, 1994), and the necessitist may hold X necessary yet grounding (Williamson, Timothy, 2013). Concede the orthogonality and the modal classification alone does not close the case. Describing X as necessary uses □, but it does not follow that X's nature or grounding activity is ontologically posterior to the modal field; a Finean essentialist may hold that □X is true because of X's essence. What the classification shows is narrower: the modal description □X cannot itself explain the content of ◇ without circularity. Excluding X as an essence-prior source requires an additional theory of grounding, not the modal trilemma alone. The point is not that necessity precludes priority, but that priority *to ◇* is unavailable to anything whose own status is already modal.

**Modal Exclusivity, scoped.** *No candidate's modal status by itself explains the modal domain: impossible and contingent candidates fail as global sources, while a necessary candidate requires an additional, non-modal account of how its essence or grounding activity confers modal content. The trilemma exposes a circularity burden; it does not refute every essence-prior source by modal classification alone.*

*The self-grounding rejoinder.* A defender may concede the trilemma and posit that possibility grounds itself, that ◇ is its own ground. Two readings exhaust the move. On the deflationary reading, "self-grounding" is a notational variant of primitivism: ◇ is not derived from anything more basic, and the Principle stands. On the substantive reading, grounding is a reflexive relation on ◇, and the field grounds itself. Here the analytic literature on ground divides the relation in two (Fine, Kit, 2012). *Strict* ground is irreflexive: nothing strictly grounds itself (Schaffer, Jonathan, 2009); (Fine, Kit, 2012). So if the rejoinder means that ◇ strictly grounds ◇, irreflexivity rules it out directly. *Weak* ground is reflexive by construction (every fact weakly grounds itself) so ◇ does weakly ground ◇; but weak self-grounding is just the absence of any strict ground, which is primitivism stated in the grounding idiom rather than an alternative to it. Either way ◇ has no strict source. This is also what accommodates the minority dissent that admits reflexive or non-well-founded grounding (Jenkins, C. S., 2011): that dissent is a thesis about weak ground and the failure of well-foundedness, not a denial of strict-ground irreflexivity, so it is absorbed by the fork rather than set aside as a bare commitment. The substantive reading does not escape the trilemma; it renames its floor.

*The classical-theism rejoinder.* Aquinas-style classical theism does not propose God as a source of possibility distinct from God; on divine simplicity, God *is* the modal field (*ipsum esse subsistens* applied to modality). The move sidesteps the trilemma by collapsing the source/sourced distinction. The collapse delivers one of two results. On the relabeling reading, *God* renames "the primitive modal field," in which case the Principle stands and the relabeling is theological vocabulary rather than a modal-foundations counterexample. On the predicate-bearing reading, the modal field carries predicates classical theism attributes to God (will, intellect, agency), which is the engagement [§5.2](<5.2 Doctrines of the Divine Nature.md>) and [§5.1](<5.1 Theistic Arguments for a Divine Ground.md>) take up, not a counterexample to the present Principle.

*The transcendence rejoinder.* A defender may place X beyond modality altogether: neither possible, necessary, nor impossible. That proposal is not refuted merely by repeating the trilemma, because the trilemma classifies candidates whose modal status is already in view. The present result instead marks a burden. If X is said to explain W, the account must explain how an entity outside real possibility bears an explanatory relation to the domain of real possibilities without thereby being specified as one more item in that relation. A classical-theist or Finean essentialist may regard that relation as primitive or essence-prior. The chapter does not close that option by contradiction. It shows that it is no longer a modal-semantic explanation of possibility: it is an additional metaphysical posit whose credentials must be argued in its own theory of grounding. The theological version is engaged at [§5.1.8](<5.1.8 Modal-Grounding Argument.md>); the essence-priority question remains the live gloss identified below.

The same closure bears on the question [§2.6.1](<2.6.1 Three Conditions, One Nature.md>) and [§3.5.3](<3. Convergence.md>) defer to this section: whether the realizer biconditionals, □(T↔(S∧Φ)) and □(Φ↔(T∧S)), state a structural-functional necessity or a Finean real definition under another name. The two readings must be prised apart from what depends on them. The *type-upgrades* of [§3.5.3](<3. Convergence.md>) need only that the biconditionals are necessary, and they are necessary on either reading — a real definition is necessary if anything is — so □(T↔(S∧Φ)) transmits its necessity to the spatial and persistence types under modus ponens whichever reading is correct. The type-layer therefore closes even for a reader who keeps the Finean essence-prior gloss; nothing in the structural conclusion is hostage to the outcome.

What the elimination above underwrites is the *further* claim the modal-monist commitment ([§2.8.6](<2.8.6 Exclusivity of Nature.md>)) makes: that this necessity is not seated in the essence of an item standing prior to the modal field. That claim is not secured by declaring every standing a modal standing — the Finean is entitled to hold that essence, were it prior, would ground modal facts rather than stand among them. The 'occupying a role is itself a possibility' reply used against a *source of ◇* above does not carry over here: there the role at issue was the sourcing of ◇ itself, so standing in it was intra-modal by construction, whereas a realizer biconditional makes no claim to source ◇, and importing the move would beg the question. It is secured only to the extent the trilemma applies: any candidate put forward as the essence-bearing ground has its own modal status (□X, ◇X, or ¬◇X) and is to that extent specified from within the field rather than before it, leaving the essence-prior reading with no definiendum the elimination has not already located inside ◇.

A committed essentialist can decline that last step; if so, the disagreement is over the gloss on a necessity both sides grant, not over the necessity the biconditionals transmit. The structural-functional reading is the one this account adopts and the one the modal monism of [§2.8.6](<2.8.6 Exclusivity of Nature.md>) requires: the biconditionals record what any medium hosting succession must be, the structural feature reality *is* rather than an essence some entity *has*.

Nor does the presupposition improve on iteration. Let the defender universalize: every possibility is sourced. Then the sourcing (itself a modal fact) is a further possibility, and by the defender's own thesis it requires a source; so, in turn, does the possibility of that. The chain forks. Either some link goes un-sourced, which concedes the Principle outright, or none does, and the sourcing relation never steps outside the field it was meant to ground. The regress is vicious not because it fails to terminate but because no stage of it is ever prior to ◇; the explanatory debt the account incurred is never discharged. Its one exit (that ◇ grounds its own possibility, closing the chain in a loop) is already foreclosed: a strict-grounding loop is barred by the irreflexivity of strict ground, and a weak self-grounding loop collapses into primitivism, as the self-grounding rejoinder established. What stands is a dilemma whose floor is the one-step presupposition: primitivism, or a regress that never leaves the field, with self-grounding shut.

The exotic placement of X buys no exemption from this. The via negativa is addressed at [§5.2.2](<5.2.2 Via Negativa.md>), on its own register. The Principle holds at the relation level against any candidate placeable at any level.

*What is established.* No candidate whose own status is stated in modal terms can be a thing-style source of the modal domain it already inhabits. The impossible and contingent horns fail directly; the necessary horn remains inside the domain whose source it was meant to supply. The result blocks internal modal grounds — divine intellect, concrete worlds, abstractions, or necessary entities specified as modal occupants. It does not settle whether a committed essentialist or transcendence theorist can posit an essence-prior ground; that is a further metaphysical cost and a live dispute, not a contradiction the present trilemma alone supplies. The Necessity of Nature identifies the structural content of the global domain; Modal Exclusivity establishes that no modal item within it confers that domain from outside. Its lateral counterpart, Modal Closure ([§2.8.4](<2.8.4 Necessity Precedes Contingency.md>)), records that no unnecessary alternative stands alongside it from outside.

*The impossibility pole.* The same elimination runs on any candidate source of impossibility. Let a *source of impossibility* be any X such that ¬◇p holds in virtue of X; the trilemma on X's own modal status is unchanged. An impossible X has no relata in virtue of which anything could hold. A contingent X is itself a fact within the field and presupposes the modality it would confer. A necessary X cannot explain impossibility merely by being described with □; whether its essence can ground modal exclusion remains the same live grounding question as on the possibility side. Modal Exclusivity is therefore bilateral only as a diagnostic: modal status alone does not source either ◇ or ¬◇. The stronger claim that both poles are primitive requires the structural-functional account adopted here, not the trilemma by itself. The field is sourceless at both poles. This closes the residual voluntarist retreat that grants possibility its primitiveness but relocates the necessary truths, or the standing of the impossible, in a divine intellect or will (the created-truths tradition; theistic activism on the necessary truths, (Leftow, Brian, 2012)); that inference is met on its own terms at [§5.1.8](<5.1.8 Modal-Grounding Argument.md>), while the result here holds at the level of any candidate's modal status. What Way–Possibility Convertibility ([§3.2.5](<3. Convergence.md>)) records on the possible side has its dual on the impossible side: to be impossible is for there to be no way to be, and the absence of a way is not conferred by an antecedent ground but is, like its positive twin, a structural feature reality is rather than has.


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← [4.1.9 Wittgensteinian Quietism](<4.1.9 Wittgensteinian Quietism.md>) | [4.1.11 Pizzi's Impossibility of Nothingness](<4.1.11 Pizzi's Impossibility of Nothingness.md>) →
