Modality and Essence
The possible worlds apparatus systematises modal talk without saying where modality comes from. Lewis's realism answers by reduction: necessity is truth at all worlds, and the worlds are just more concrete reality. The ersatzer answers by taking modality as primitive and building the worlds out of it. Neither explains why the necessary truths are the ones they are.
This page takes up the accounts that try to. There are three: essence, convention, and powers — and they disagree about the direction of explanation rather than about the facts.
Fine's reversal
The standard order runs: modality first, essence defined from it. A property is essential to if must have it. Fine's counterexamples — Socrates and his singleton, Socrates and — show the definition is extensionally wrong, and his diagnosis is that the order of explanation is backwards.
On the essentialist theory of modality, essence is primitive, and:
A proposition is necessary just in case it is true in virtue of the natures of some things.
The necessity of "Socrates is human" flows from the nature of Socrates; the necessity of " contains Socrates" flows from the nature of the set; the necessity of "" flows from the natures of the numbers. Every necessary truth traces to some entity's real definition.
Two consequences make this attractive:
Modality is explained rather than described. The worlds account says the necessary truths are those true at all worlds, which is correct and unilluminating — it does not say why those worlds are the ones there are. Essence says what makes a truth necessary, and locates the source in the things themselves.
No worlds are required. If the modal facts flow from natures, the ontology of possible worlds is unnecessary for modality, whatever else it may be useful for. This is a substantial economy for anyone unwilling to pay Lewis's price and unsatisfied with primitive modality.
The objections are that essence is no less obscure than modality — trading one primitive for another is not obviously progress — and that "true in virtue of the nature of" needs an account of its own, which is where the notion of grounding enters.
Conventionalism and deflationism
A quite different diagnosis: modal facts are not out in the world at all.
Conventionalism (Sidelle) holds that necessity has its source in our conventions of individuation. The necessary a posteriori is a compound: an empirical discovery (this stuff is ) plus an analytic principle about what makes something the kind it is (a substance is essentially of the chemical kind it belongs to). The empirical part is contingent, the modal part is conventional, and nothing in reality is irreducibly modal. Kripke's results are thereby accommodated without a world containing necessities.
Modal deflationism (Thomasson) extends her easy ontology programme to modality: modal claims express the semantic rules of our terms in the object-language. "Necessarily bachelors are unmarried" does not describe a modal feature of reality but articulates a rule for using "bachelor," said in a form that looks descriptive.
The standing objection to both is that they seem to make the modal facts depend on us, so that before there were speakers nothing was necessary or possible. Both reply that this misreads the position: the conventions determine the content of the modal claim, and what is thereby said may be perfectly objective and about the world before there were speakers. Whether that reply works is the same question that arose against predicate nominalism, and it has the same shape.
Modal epistemology
How do we find out what is possible? The question presses whatever the metaphysics, and the standard answers have known failures.
Conceivability entails possibility. The traditional criterion, and Kripke's results are its refutation. It is perfectly conceivable — in the sense of involving no detectable incoherence — that water is not ; but it is impossible. So conceivability is at best defeasible evidence, and the interesting question becomes when it fails.
Two-dimensional semantics (Chalmers) is the systematic repair. Distinguish two propositions associated with a sentence: the primary intension, evaluated at worlds considered as actual, and the secondary, at worlds considered as counterfactual. It is primarily possible that water is not — the world could have turned out that way, and had it, "water" would have named something else — but not secondarily possible. Conceivability tracks primary possibility reliably; the necessary a posteriori is exactly the case where the two come apart. This rescues conceivability as a guide, at the price of an elaborate semantic apparatus and a contested account of what the primary intension is.
Counterfactual reasoning (Williamson). Modal knowledge is not a special faculty but a by-product of the ordinary capacity to evaluate counterfactuals, which is plainly useful and plainly reliable. Necessity is definable from the counterfactual conditional: is necessary iff counterfactually implies a contradiction. Since we obviously do know some counterfactuals, modal knowledge is demystified. The objection is that our counterfactual competence is best attested in mundane cases and the metaphysical applications are far from mundane.
Essence-based epistemology. If Fine is right, knowing what is possible is knowing what things are — so modal knowledge reduces to knowledge of natures, which for natural kinds is empirical and for mathematical objects is whatever mathematical knowledge is. This is attractively continuous with ordinary inquiry, and it inherits whatever difficulties those inquiries have.
Where this sits
The page is about the status question applied to a tool rather than to an inventory: the whole section states its theses modally, and it matters whether those statements are about mind-independent features of reality, about the natures of things, or about our conventions.
The three answers line up with three positions the section has already met. The essentialist route makes modality flow from what things are, which is the position with the widest current influence and the one from which grounding and fundamentality descend. The conventionalist route is deflationary and belongs with easy ontology. The third route, not yet examined, locates modality in the properties themselves: if properties are powers, then what things can do is built into what they are, and the modal structure of the world needs no further source. That is the subject of the next page on dispositions and powers.