The Variety of Modalities

Modals are expressed by words such as "must", "have to", "ought to", "may", "can", "could", "should", "necessarily", "possibly". As a general rule, something is said to be possible if it is not necessary that it is not the case, and conversely, something is necessary if it is not possible that it is not the case, but beyond this, there is a variety of use. Here are a few distinctions between various kinds of possibilities. I will discuss in the next articles some criteria for disinguishing them, and attempt to reach a taxonomy.

A logical or conceptual possibility

anything that is not self-contradictory, that makes sense, that one can express or conceive without reaching a logical contradiction (for example respecting the fact that for any proposition, it must be that either p or not p"). Example: it is conceivable that aliens exist.

An epistemic possibility

anything that is compatible with what we know or with available evidence and what follows from it (such as "she must be at home"). Example: she might be sleeping.

A metaphysical possibility

anything that is compatible with the nature or identity or essence of things (that molecules of water are of the kind H2O). Example: the fundamental constants of the universe could have been different.

A natural possibility

anything that is compatible with the laws of nature (such as the law that heavy objects must fall towards the ground). This is the modality typically used in scientific explanations, or to analyse causation. Example: the ball could have broken the window.

A deontic possibility

anything that is compatible with norms (such as "she has to stay at work tonight"). Example: she is allowed to make a call.

A practical possibility

anything that is compatible with our aims and capacities (such as "we must use light materials to fly"). Example: we could use wood to build this plane.

Note that in all these cases necessity naturally comes first, in the sense that it is easier to define possibility in terms of compatibility with a source of necessity rather than to define necessity in terms of, say, what a set of a priori unrelated possibilities have in common. A given statement can be compatible with many sources of necessity, so it's not always clear in what sense we mean that something is possible, but when saying that something must be so, it is generally easier to see in virtue of what (although there is leeway in fixing the relevant background conditions in counterfactual talk, which is often a source of disagreement, this can generally be made explicit). One could argue, as Divers (2004) did some times ago, that as a matter of generality, a commitment to a mere possibility is not binding, while a commitment to a necessity does affect our inferences and actions (an exception, perhaps, is a practical possibility that would motivate us to act). For these reasons, this work will be primarily focused on understanding necessity

We could suspect that this order of primacy is reversed when it comes to modal epistemology, in the sense that we are directly acquainted with actualised possibilities, not necessities (see my Modal Empiricism). However, we are not concerned with modal epistemology here, but with having a conceptual grasp of what different kinds of modalities are, so our primary interest lies in sources of necessity. I will also put aside general scepticism about whether there really are constraints of necessity of the kind considered, because again, my aim is merely conceptual.

I will examine in the next posts some criteria for distinguishing various kinds of necessity conceptually: what exactly makes then different? My aim is to reach some kind of taxonomy, before examining its relation to modeling activities.