Types of Modality

Formal systems are very general. They need not have anything to do with modality despite involving accessibility relations. Structurally, models of modal logic are just graphs like the one shown in Figure 2.2.1. Possible worlds are just nodes or vertices, abstracta lacking inherent meaning, and accessibility relations are sets of directed edges between nodes.

The rest of the semantics has no more intrinsic meaning. A truth function is a function taking two parameters, a proposition and a possible world, and mapping them to things called truth values. For any application, it is important to explicate the parts of a modal system and give some details about how models relate to the analysanda.

Logical systems can be helpful for creating explicit illustrations and analytic mechanisms.

In this sense, logical systems need not attend to propositions, possible worlds, truth values, accessibility relations, or even logic. Suppose, for instance, that one wants to develop a system for bags of colored marbles. Non-modal propositions represent colors and possible

worlds represent sizes. Colors can be manipulated by taking their complement with ¬ or their combination with ∧. The accessibility relation between sizes is understood as is larger than. The truth function may be partial, mapping size/color pairs to true just in case there is a marble of that color and size. The modal operator◇ is used to indicate that there is a larger marble. For instance, ◇red holds at 1.5cm if there is a red marble larger than 1.5cm.

There is nothing wrong with using modal systems to represent one thing as opposed to another, although it is desirable to avoid confusion no matter how a logical system is applied. The marble example is not evidently harmful, illustrating that modal systems can by interpreted in ways having little to do with modality or propositions. Anything that can be depicted using an accessibility relation may be called formally modal to distinguish the mere formality of accessibility in the technical sense from more modal senses of modal.

Formal systems can be applied in various ways and formally modal elements of a system need not be modal in other senses of the term. It may therefore be desirable to identify criteria for separating genuinely modal accessibility relations from simply formal ones.

As indicated in Section 2.1, genuine modalities like logical, metaphysical, and physical are explicable in terms of a set or sets of principles expressed as propositions. Such relations may be called propositionally modal. The structure of propositional modalities reduces to consistency with the laws characterizing those modalities. This is not to say that proposi-tional modalities themselves reduce to consistency. Rather, the accessibility relations used to illustrate those modalities can be dened using consistency.

Propositional modalities always unambiguously pick out an accessibility relation. Here is an explication for the formally inclined. Let Lp be the set of laws corresponding to a propositional modality. For a given language in which Lp is expressible, each model has a unique accessibility relation, Rp, such that:

(2.4) Rp = {⟨m, m^′⟩ ∣L^p is satised at m}²

2This definition assumes that moments are characterized by the set of propositions that are true there.

Again, this denition just states that the laws Lp characterize the modality, which in turn is represented by the accessibility relation Rp. For instance, consider physical modality.

Physical laws dictate what is physically possible or necessary. Nodes respecting physical principles can only physically access certain other nodes. As a more concrete example, stipulate a modality, Sisyphean modality, having a single law:

(2.5) The deceitful necessarily roll boulders.

(2.5) holds at moments at which no one is deceitful. (2.5) is also satised by moments at which there are deceitful individuals provided that those moments only access moments in which the deceitful persons roll boulders. The most inclusive accessibility relation satisfying those criteria represents the Sisyphean modality.

There are two cases of propositional modality. On the one hand, the set of laws may be node-independent. Logical possibility is the prime example here. Given that all nodes are consistent, each node relates to every other. The principles of logic are not world- or moment-dependent. Additionally, there may be no need to consider more than one set of physical laws. These laws are presumably the actual physical laws, although they do not have to be. On the other hand, it may be important to represent dierent logically possible physical laws, legal laws, moral rules, et cetera. It would thus be appropriate to refer to the laws at a world rather than the laws simpliciter. World- or moment-dependent accessibility relations will play an important role in this analysis (although there will be no need to employ node-dependent laws).

All propositional modalities are formal modalities. The converse is false; that is, not all formal modalities are propositionally modal. Unlike formal modality, propositional modality

It is also assumed that there is a moment corresponding to each set of propositions that is both consistent and closed under entailment.

Note that the definition does not directly require that Lp is also satisfied at m^′. Consider the case of physical laws. Standard physical laws seem to be physically necessary in that if φ ∈ Lp, then physically-necessarily:φ ∈ Lp. In this case, if Lp is satisfied at m and mRpm^′, then Lp is satisfied at m^′. However, it is possible that the current physical laws change; for instance, if there were another big bang and some constants change. So it is not in the nature of physical modality, and thus propositional modality generally, that the same laws must be satisfied at both nodes. Only the source node must satisfy the modality’s laws.

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Figure 2.2.2: A formal modality that is not propositional. The modality F is represented by an accessibility relation, RF, such that RF consists of one simple cycle. (a) and (b) depict two candidates for RF in otherwise identical models. This ambiguity shows that F is not propositionally modal.

necessarily involves what is true at nodes (possible worlds/moments). Formally modal rela-tions might have little or nothing to do with principles given that these rules are proposirela-tions represented by the object language. Put another way, propositional modality has to do with what is going in at nodes, their content. Formal modality is not so limited.

A formal modality that is not propositional is such that no set of laws is both necessary and sucient to characterize the modality. It is a simple matter to create a modality that is formal but not propositional. Let F be a modality with a corresponding accessibility relation R_F. RF is a subset of logical accessibility having just one simple cycle of nodes. The scenario is illustrated in Figure 2.2.2. There may be several options for RF in a given model, one of which must be chosen arbitrarily. F is not characterized by a set of principles.

The thin red line, the temporal relation of true-futurist theories, is supposed to be a formal modality that is not propositional. The thin red line is a linear subset of ATC accessibility.³ Whenever contingency plays a role, there is more than one possible thin red line but there is no special rule for prioritizing one timeline over others.

Although propositional modality is stricter than formal modality, propositional modality is still not enough to pick out all and only genuine modalities. A case in point is permissibility, which does not amount to any sense of genuine possibility. Even assuming that permissibil-ity is propositionally modal and that only possible acts are permissible, permissibilpermissibil-ity is not

3In the case of indexical true futurism, the arbitrariness stems from the assignment of timelines to nodes;

that is, the precedence of one thin red line over another.

necessary for any genuine possibility. Genuine modality ultimately stems from the object of analysis, the philosophical interpretation of the formal system. Physical possibility, for instance, can be represented in terms of consistency with propositions corresponding to the laws of nature. That the representation is of physical possibility depends on identifying the relation's characterizing propositions as the laws of nature, and that is something that must be done outside of the system. So propositional modality is not sucient for gen-uine modality. Nevertheless, familiar gengen-uine modalities are propositionally modal, making propositional modality an indicator of genuine modality.

An empiricist might insist that propositional modalities are the only genuine modalities.

Propositional modalities are characterized by a set of principles. Principles in that sense denotes propositions; but the term may also pick out mechanisms. Here is a candidate ex-ample. It is logically possible that some physical mechanism is entirely arbitrary, objectively random. There is a possible universe in which physical determinism holds except that there is a special, troublesome machine. This machine periodically outputs a binary digit, 0 or 1. The catch is that the number chosen by the machine is objectively random. The arbi-trariness of the selection process renders the mechanism impossible to describe using a law.

The machine ensures that the universe, which would otherwise be physically determined, is indeterministic. It is possible that the next number will be 0, and it is possible that the next number is 1. So there is a mechanism, a principle in the ontological sense, that signicantly alters the physical accessibility relation for that universe. Propositional modality cannot account for this accessibility relation because the indeterminism generated by the machine cannot be depicted by laws. A new sense of modality is required, ontological modality.

One might object that ontological modality is nonsense if taken apart from propositional modality. A genuine mechanism can always be captured by propositions in a suciently rich language. Objective randomness stems from an absence of mechanisms, of principles in the ontological sense, not their presence. The contrived example of the indeterministic machine is indeed representable as a physical modality. The propositions representing physical laws

must become as contrived as the objective principles themselves: some laws must contain clauses exempting the machine. These exemptions correspond to an absence of principles in the ontological sense.

If ontological modality can be explicated in terms of propositional modality, the former may be considered a subtype of the latter. Under such a taxonomy, propositional modalities may be divided into two groups, ontological and artifactual. Artifactual modalities stem from human artice or convention, including legal laws and mores. Ontological modalities, in this pacied sense, may simply be non-artifactual or they might be explicated positively.

This pacied notion of ontological modality is not used in this essay.

Even if all legitimate instances of ontological modality are reducible to propositional modality, the ontological sense of modality is nevertheless intensionally distinct from the propositional sense. As such, ontological modality is here added to the list of types of modal-ity. The arguments given later in this essay do not hinge on the legitimacy of ontological modality.

Finally, terms can be grammatically modal. Familiar grammatical modalities include could, would, should, can, might, and so forth. Interestingly, will and shall are also gram-matically modal, as discussed in Section 3.1. Ignoring terms like will and shall to avoid begging the question herethose terms are primary analysanda of this essayother stock grammatical modalities are propositionally modal and hence formally modal. Such terms are propositionally modal in that they can be represented by operators dened using propo-sitionally modal accessibility relations, although those accessibility relations may depend on the context of utterance. Can, for instance, might address logical or physical possibility.