Necessity

What could not have been otherwise - and the several very different reasons a thing might not have been able to be otherwise.

Theme: Being and reality
Tradition: Greek
Term: ἀνάγκη · anankē
Term fixed: c. 330 BC

In plain terms

Some things could not have been otherwise, and the interesting question is why not.

A bachelor is unmarried, and that is settled by what the word means. Two plus two is four, and that is not about any words in particular. Nothing travels faster than light, and that is a fact about this universe rather than about arithmetic. Water is H₂O, and that is a discovery - yet if it is true, it could not have been false, because being H₂O is what water is.

Four different kinds of must, and they are routinely confused with each other. The last is the one that upset the settled picture, because it is necessary and yet nobody could have worked it out from an armchair. Necessity and knowability came apart, and a great deal of twentieth-century philosophy is the consequence.

As a philosopher would put it

The standard division is between logical necessity, truth in virtue of form; conceptual or analytic necessity, truth in virtue of meaning; metaphysical necessity, what could not have been otherwise given the natures of things; and nomological necessity, what the laws of this world do not permit.

Kripke's Naming and Necessity is the pivot. Necessity is a metaphysical notion and apriority an epistemic one, and they cross: identity statements between rigid designators - water and H₂O, Hesperus and Phosphorus - are necessary if true and known only empirically, while the standard metre bar could fix the reference of one metre a priori without making its length necessary.

Against all of this stands the empiricist tradition. Hume finds no impression of necessary connection between events, only constant conjunction, which makes causal necessity a projection of habit. Quine attacks the analytic-synthetic distinction and with it the idea that any truth is immune from revision. On that view necessity is a status we grant to statements we have decided not to give up, and it can always be withdrawn.

Where you meet it

Until 1983 the metre was defined by a measurement: originally a fraction of the earth's meridian, later a platinum-iridium bar in Sèvres, then a number of wavelengths of krypton-86 light. The speed of light was something you measured in metres per second, and the figure kept improving.

At the 17th General Conference on Weights and Measures the relation was inverted. The metre became the distance light travels in vacuum in 1/299,792,458 of a second, which makes the speed of light exactly 299,792,458 metres per second by definition. It is no longer measured. It cannot come out differently, and an experiment that appeared to contradict it would be recording something about the apparatus.

The physics did not change. What changed was which quantity is held fixed. So a proposition that was contingent and empirical in 1982 was necessary and definitional in 1984, by a committee vote - which is precisely the phenomenon Quine described, performed in public with minutes.

What is actually disputed

Whether necessity is in the world or in our descriptions of it.

The conventionalist answer is that all necessity is at bottom definitional: we fix meanings, and what follows from them could not be otherwise because we have not left ourselves the option. Quine's version is stronger - even that distinction fails, and no statement is unrevisable in principle.

The realist answer is Kripke's: some necessities are discovered rather than stipulated, and no convention makes water H₂O. Between them sits the question of laws, where Humeans hold that laws are summaries of what happens and carry no necessity at all, and necessitarians hold that they govern. The metre case is uncomfortable for everyone, because it shows a genuine necessity manufactured by decision without any corresponding change in the world.

Commonly confused with

Determinism 1677 Determinism says the present fixes the future given the laws. It leaves the laws themselves contingent, so a determined world can be thoroughly contingent.

Defined against

Contingency What is necessary could not have been otherwise; what is contingent could. Every position on one is a position on the other

Philosophers

Aristotle The necessary distinguished from the merely always-true
David Hume No impression of necessary connection - only constant conjunction
Immanuel Kant Necessary truths that are nonetheless about the world, via the forms of experience
Willard Van Orman Quine No statement is immune from revision, including the analytic ones
Gottfried Wilhelm Leibniz Truths of reason and truths of fact, and the distinction between them

Works

Metaphysics The modal distinctions in their original form
A Treatise of Human Nature The attack on necessary connection
Critique of Pure Reason Synthetic a priori truths, and how they are possible
Naming and Necessity Necessity and apriority pulled apart
Monadology Necessary and contingent truths, and why the world is as it is

Schools

Peripatetic school (Aristotelianism) Where the vocabulary is set
British Empiricism Where necessity in nature is denied
German Idealism Where it is relocated into the structure of experience
Vienna Circle Where all necessity is made analytic
Cartesianism Where even the eternal truths are said to depend on a will

Related concepts

Possibility c. 330 BC · Contingency c. 1020 · Causation c. 330 BC · Truth c. 330 BC · Explanation 1948

Aristotle separates the necessary from the merely always-true; Kripke's 1970 lectures separate necessity from what can be known a priori, which had been assumed to coincide for two centuries.