Why theories of explanation stop where necessity begins.

Contemporary metaphysics prides itself on explanation. We are encouraged to ask what depends on what, what grounds what, what explains what—and to keep climbing until the structure of reality comes into view. But something curious happens as that ascent continues. Again and again, explanation is welcomed right up to the point where it would begin to suggest necessity. At that moment, the climb stops.

These stopping points are not accidental. They recur across otherwise very different theories, and they appear in strikingly similar places. Explanation is allowed to run—sometimes impressively far—so long as it does not threaten contingency. When it does, constraints quietly appear. I call these constraints modal firewalls: principled-sounding limits on explanation that permit local intelligibility while preventing explanatory ascent from spilling over into necessity.

These firewalls shape vast regions of contemporary philosophy. They don’t just mark the limits of explanation; they reveal where the discipline becomes strategically protective. And the question is whether those limits are principled—or simply installed to keep necessity at bay.

Grounding: Explanation with a Controlled Burn

No contemporary tool has shaped metaphysics more than grounding. It promises dependence without reduction, explanation without elimination, structure without collapse. And it offers one of the cleanest illustrations of firewall logic.

The basic picture is familiar: some facts obtain in virtue of others. A set exists in virtue of its members; a disjunction is true in virtue of a true disjunct; an act is wrong (perhaps) in virtue of harm or lack of consent. So far, grounding behaves exactly as advertised.

The tension appears when we ask how far grounding extends.

If grounding is unrestricted, explanatory ascent becomes hard to contain. Once the base is fixed, explanation propagates upward—and with it, modality. Necessary grounds yield necessary grounded facts. Iterate the structure, and necessity spreads like fire through dry brush.

This is the moment grounding theory reaches for the firebreak.

We are told that grounding must be restricted. It bottoms out. It is asymmetric. It does not range across all domains. It does not ground everything that looks groundable. Crucially, these limits are not themselves grounded or explained by the relata. They are imposed to preserve a desired modal profile—to stop the spread.

This is not a complaint about any particular theorist. It is a structural observation. Grounding is permitted to burn hot within carefully cleared perimeters, but when it threatens to carry us from intelligibility into necessity, the line is drawn.

That is why grounding is the paradigmatic modern firewall: explicitly explanatory, explicitly metaphysical, and explicitly curtailed for modal reasons.

(Representative figures include Kit Fine, Jonathan Schaffer, and Shamik Dasgupta.)

Essence: Explanation That Must Not Explain Itself

Essence is often presented as grounding’s metaphysical backbone. Where grounding explains dependence, essence explains necessity. A thing has the modal profile it does because of what it is.

This looks like genuine explanatory progress—up to a point.

Essence is allowed to tell us why certain facts are necessary: why Socrates is necessarily human, why water is necessarily H₂O, why some identities could not have failed. But the moment we ask the next question—why this essence rather than another—the ladder is pulled away.

Essences, we are told, are not explained. They are not grounded. They are not fixed by anything deeper. To demand an explanation of an essence is, on this view, to misunderstand the very idea of essence.

This is not an accidental silence. It is a deliberate stopping rule. If essences themselves were open to explanation, modal ascent would resume: the explanation of an essence would inherit modal force, and the pressure toward necessity would reappear at a higher level.

Essence thus functions as an explanatory stop‑sign. It secures necessity, but it must not itself be explained. The firewall is clean, principled‑sounding, and structurally indispensable for anyone who wants contingency preserved at the level of kinds and individuals.

Normative Autonomy: A Softer Firewall

In contemporary metaethics—especially among non-naturalists—it is standard to insist on the autonomy of the normative. Normative facts are explained by reasons, not by natural or metaphysical facts. What one ought to do is determined by reasons; what reasons there are is not settled by physics, chemistry, or ontology.

This autonomy is often motivated by familiar considerations. Even if all the natural facts are fixed—who suffered, who consented, who benefited—it can still seem an open question whether an action was wrong. Moral properties are therefore taken to resist reduction to natural or metaphysical ones.

So far, this may sound like a familiar dialectical stalemate. But the explanatory structure deserves closer attention.

Consider a simple case. An action causes intense, unnecessary suffering to a conscious being. Most moral theories agree that this fact counts strongly against the action. But now ask the next question: why does suffering generate reasons at all? Why does it have normative force rather than merely describing a state of the world?

At this point, explanation reliably stops. We are told that suffering just is reason-giving, or that normativity is sui generis, or that reasons are primitive features of practical reality. Attempts to explain normative force in terms of metaphysical structure, rational agency, or the nature of value are treated with suspicion—often dismissed as category mistakes.

This is not because such explanations are incoherent. It is because allowing them would invite explanatory unification across domains. If facts about suffering had their normative force in virtue of deeper metaphysical facts, then normative necessity would begin to track metaphysical necessity. The space between “how things are” and “how one ought to act” would narrow.

Normative autonomy functions here as a firewall. It permits rich internal explanation—reasons explain obligations, values explain reasons—while blocking explanatory ascent into metaphysics or ontology. The restriction is not arbitrary, but it is strategic. It preserves the distinctness of the normative domain by preventing necessity from spreading across it.

Unlike grounding or essence, this firewall is rhetorically gentle. It presents itself as respect rather than restraint. But structurally, it serves the same role: explanation is allowed to operate, but only so far as it does not threaten modal unification.

Primitive Modality: The Firewall Without Apology

Some philosophers dispense with all explanatory machinery and say, simply: modality is primitive. Necessity and possibility are basic features of reality. They do not require explanation.

This view—associated with figures like Timothy Williamson—is often presented as anti‑reductionist and theoretically modest. But in the present context, its significance is different.

Primitive modality is the most forthright firewall available. It does not suggest that explanation almost continues but must be stopped. It denies that explanatory ascent even begins. Modal structure is taken as given. Full stop.

The virtue of this stance is its honesty. The cost is bruteness. Why these modal facts rather than others? Why this logic, this accessibility relation, this space of possibilities? There is no answer—and the view insists that none is needed.

If unexplained modality already feels acceptable, this will seem unproblematic. But it is a paradigmatic case of explanation being blocked at the base to prevent further questions from arising.

Laws of Nature: Contingency at the Base of Physics

A parallel structure appears in philosophy of science.

On Humean views, laws of nature do not govern the world; they summarize patterns in the distribution of particular facts. The laws could have been otherwise because the mosaic could have been otherwise.

This secures contingency at the fundamental level. Even a complete account of the actual laws does not explain why those laws obtain rather than others.

But this result is purchased by treating the base distribution of qualities as brute. There is no explanation of why the mosaic has the structure it does—any such explanation would threaten to propagate modal force upward.

Once again, explanation is permitted locally—laws explain counterfactuals—but prohibited globally. The firewall preserves contingency by denying that explanation can reach the base.

The Contingentist’s Best Reply

At this point, a sophisticated contingentist will object.

Of course explanation has limits. Demanding that everything be explained is itself a substantive metaphysical commitment—often associated with the Principle of Sufficient Reason. One can reject that commitment without inconsistency. Explanation ends where it ends. There is no obligation to keep pushing, and no reason to treat unexplained stopping points as philosophically suspect by default.

This is a serious reply, and it deserves to be taken seriously.

Contingentists do not reject explanation wholesale. On the contrary, they rely on it extensively. Grounding is invoked to illuminate dependence, essence to explain modal profiles, laws to support counterfactuals, and reasons to structure normativity. Explanation is not treated as a fragile practice that must be carefully rationed. It is allowed to run—often aggressively—when it delivers intelligibility without threatening core commitments.

Nowhere is this clearer than in the natural sciences. Few philosophers resist explanatory descent from chemistry to physics, or from thermodynamics to microstructure. We routinely explain why chemical regularities hold by appeal to molecular structure, why macroscopic behavior emerges by appeal to underlying distributions, and why apparent alternatives disappear once deeper structure is revealed. These explanations are welcomed even when they eliminate intuitive possibilities. No one insists that chemical laws must remain explanatorily autonomous lest they collapse into physical necessity. The explanatory ascent is permitted to continue precisely because no threat of modal collapse is perceived.

What distinguishes the earlier cases is not a general discomfort with explanation, but the specific fact that further explanation would begin to align necessity across domains. Grounding is curtailed when it would transmit necessity upward. Essence is declared inexplicable when explaining it would threaten contingency. Normativity is insulated when metaphysical explanation would collapse moral necessity into metaphysical necessity. Primitive modality is invoked when further explanation would reopen the ascent entirely.

The stopping points are therefore not random. They cluster at precisely those locations where explanation would begin to erode modal slack. This does not show that the stopping points are illegitimate. But it does undermine the idea that they are merely neutral denials of explanatory ambition. They look instead like targeted restraints—introduced not because explanation has gone too far in general, but because it has begun to point in a particular direction.

That is where the real dialectical pressure lies.

Connecting the Dots

Modal firewalls are not philosophical mistakes. They are pressure points—places where our explanatory practices strain against a background commitment to contingency. They reveal how much of contemporary metaphysics is already organized around allowing explanation to proceed up to necessity, and no further.

Recognizing this does not by itself refute contingentism. But it reframes the debate. If explanation is already trusted across domains whenever it increases intelligibility, then resistance at the point where necessity comes into view requires special justification. Appeals to autonomy, primitivity, or brute stopping points may be defensible—but they are not cost-free. They represent decisions about where explanation must be halted, and why.

Arguments for necessitarianism do not impose alien standards on metaphysics. They ask us to take our existing explanatory ambitions seriously, and to follow them without installing outcome-driven restraints. Whether one ultimately accepts that invitation remains an open question. But the terrain on which the question is decided looks different once the firewalls come into view.

Parmenides is not refuted. He is managed.

Related Essays

Ancient Modal Firewalls

Aquinas and the Modal Firewall

Middle Knowledge: Anatomy of a Modal Firewall