Necessity and Contingency

Plato inherits two rival accounts of the world, the changeless being of Parmenides and the blind necessity of the atomists, and adopts neither without qualification. In the Timaeus the making of the cosmos is presented as the joint work of two principles: nous, or intelligent craftsmanship, and anankē, or necessity. The demiurge does not command necessity but persuades it, shaping the receptacle toward such order as the materials will admit. Whatever is regular and beautiful in the cosmos is referred to reason; whatever is irregular, the residue of what Plato calls the wandering cause, is referred to necessity. The world so described is neither a deduction nor an accident, but a mixture in which the better principle prevails for the most part without prevailing altogether.

Two grades of being follow, and with them two modalities. The Forms are necessary in the strictest sense available to Plato: eternal, self-identical, incapable of being otherwise. The sensible cosmos is contingent, since it is made of becoming and of a matter that resists form. It does not follow that the sensible order is lawless. Book X of the Laws argues, against those who make soul a late product of body, that soul is self-moving and therefore prior to body, so that the ultimate sources of motion are intelligent rather than blind, the regularities of the heavens being cited as evidence. Whether an account of such a world can be knowledge in the full sense, or only what the Timaeus calls a likely story, Plato does not finally settle.

Necessity, as Plato employs the term, is not the necessity of the logician nor the determinism of the physicist. It signifies those resistant factors in nature which mind must overcome, or persuade to give way, if purpose is to prevail in the coming to be of anything. In this sense necessity stands opposed less to contingency than to intelligence, and it keeps company with chance, since blind necessity and blind chance alike exclude the operation of final causes. The consequences reach into ethics and politics. If the order of causes were indifferent to mind, the arguments of the Laws for divine providence and for the justice of punishment would want their foundation. The government of the world by such causes is treated more fully under the ideas of Fate and Chance, and the bearing of necessity upon human choice under the idea of Will.

The difficulty Plato leaves is one his successors work at. If the Forms are necessary and the sensible order is shot through with contingency, it becomes a question whether the cosmos can be demonstrated or only conjectured. Aristotle presses that question, holding that natural science attains demonstration concerning what happens always or for the most part, and that the contingency of particular events does not preclude necessity in the connections among their kinds.

Aristotle supplies the tradition with its working vocabulary. The necessary is that which cannot be otherwise; the contingent is that which admits of contraries; the impossible is that which cannot be at all. In Metaphysics V.5 he separates the senses in which a thing is called necessary: what compels by force, what is an indispensable condition of some end, what cannot be demonstrated otherwise, and what cannot not be. The last of these he takes to be primary, since "from this sense all its other meanings are somehow derived." Necessity in this primary sense belongs to the eternal, and most of all to the unmoved mover, whose actuality is unmixed with potency and therefore cannot fail.

That necessity should be found in the world does not, on Aristotle's view, require that everything in the world be necessary. In De Interpretatione 9 he states the problem of future contingents. If it is true today that a sea-battle will occur tomorrow, the battle appears to be necessary; if it is false, the battle appears to be impossible. Aristotle meets the difficulty by holding that of such propositions one must be true and the other false, but that we cannot say determinately which. Particular future events are in themselves contingent, and chance, as the Physics describes it, is the coincidence of causal lines which are independent and which need not have met. Nature admits of what happens for the most part as well as of what happens always.

Necessity is not confined in Aristotle to the analysis of propositions. In the order of being he tends to identify the possible with the perishable, with that which both comes to be and passes away, and to treat as necessary those substances which are exempt from generation and corruption. The celestial bodies, whose matter he supposes to admit change of place but not of substance, are necessary in this sense, eternal because always existing. In the order of demonstration a further distinction is required, between absolute necessity and the hypothetical necessity by which, if a thing is to come to be, certain conditions must be present. Aristotle observes that if all things happened of necessity men would neither deliberate nor take trouble, since the outcome would be the same however they acted. The bearing of these distinctions upon knowledge is treated under the ideas of Cause and Science, and upon action under the idea of Will.

The fourfold division of necessary, contingent, possible, and impossible passes from Aristotle into the vocabulary of every later discussion of the subject, and the sea-battle passage remains the place to which writers return when they ask whether the future is already settled. Aquinas takes up the distinction between absolute and conditional necessity in accounting for God's knowledge of future contingents, and Leibniz takes it up again in distinguishing what is certain from what is necessary.

Lucretius, setting forth the doctrine of Epicurus, carries necessity down to the atomic level and then admits one exception to it. Whatever exists is atoms and void, and the atoms fall through the infinite by their own weight along parallel lines. If that were the whole account, the chains of causes would stretch backward without limit, and every motion of body and of mind would be settled by antecedents beyond the reach of the agent. This is fate in the sense the Stoics gave the word, and it is against fate so understood that the Epicurean argument is directed.

The exception is the clinamen, or swerve. At times and places which are not determined, atoms depart from the straight line of their fall by the least amount that can be spoken of. Lucretius assigns two offices to this small indeterminacy. In cosmology it permits the collisions from which worlds are formed, since bodies falling in parallel would never meet. In the account of living things it is offered as the physical ground of that will wrested from the fates by which animals move as they are minded to move rather than as they are pushed.

The Epicurean position is not that nature is lawless. Lucretius insists upon fixed compacts of nature which determine what each kind can bring forth and what it cannot, so that nothing arises from nothing and seeds of a given sort produce their own kind. What he denies is that the order so produced was contrived, or that the gods concern themselves with its administration. Necessity and chance are alike opposed, in his account, to purpose, and the argument that the world exhibits design is answered by pointing to the many respects in which nature serves men badly. The swerve is thus a narrow exception admitted into an otherwise lawful physics, and admitted for the sake of the mind rather than of the atoms. Whether an uncaused deviation can be the ground of a choice that is the agent's own is a question raised against him by later writers, and it belongs also to the discussions of Chance and of Will.

Lucretius is the earliest writer in the tradition to place contingency in the motions of matter rather than in soul or in form, and the swerve stands behind later attempts to ground freedom in an indeterminacy at the base of physics. Spinoza denies the premise outright, holding that nothing in nature is contingent. Hume grants the denial of chance while withdrawing necessity from things and assigning it to the customary transitions of the mind.

Plotinus takes up a question which Plato and Aristotle leave unsettled: whether the first principle is free, or whether what proceeds from it proceeds by necessity. Ennead VI.8, On Free Will and the Will of the One, considers the alternatives and declines both. To say that the One acts by necessity suggests a compulsion laid upon it from without, and nothing stands prior to it that might compel. To say that it acts contingently suggests that it might have failed to act, and there is nothing in it capable of failing. Its activity is identical with its nature, and that nature is self-constituting. The One is on this account the cause of itself, and the ordinary opposition of the necessary to the free does not apply to it.

From this self-determination the hypostases proceed, Intellect, Soul, and the sensible world, each less unified than what precedes it and each more exposed to multiplicity and to the mixed necessities of matter. The further the procession descends, the more fully events are bound into the chains of causes which the Greeks called heimarmenē. Even at the lowest level, however, soul retains something of the freedom of its source. The wise man ascends by detaching himself from compulsion and living according to intellect, which is subject to a necessity of another kind, the necessity of being what it most truly is.

The treatise On Fate shows the other side of the doctrine. Plotinus examines the several accounts by which events are made to follow necessarily, the motions of atoms, the influence of the stars, and the Stoic chain of causes, and finds each of them, taken alone, destructive of the soul's own agency. Fate governs so far as the soul is embodied and acts through the body; it does not reach the soul insofar as the soul acts from itself. A single cause, whether physical or astral, does not therefore account for all that happens, and providence is to be understood as an order to which souls contribute causes of their own. These questions are treated further under the ideas of Fate and God, and the derivation of the many from the one under One and Many.

In describing a being that is cause of itself, and in which freedom and necessity are not distinct, Plotinus supplies a formula which the later tradition puts to different uses. Augustine adapts it to the account of a simple God whose foreknowledge does not compel. Spinoza extends it, holding that the self-necessitating substance is nature itself, and that whatever follows from it follows with the same necessity by which it exists.

Augustine considers two propositions which have seemed to many incompatible: that God foreknows all events, and that men choose freely. Cicero, he reports, judged the two irreconcilable and gave up foreknowledge in order to keep freedom. Augustine declines that exchange. God's knowing beforehand does not, in his account, make anything necessary in the sense of compelling it; it registers what the creature will in fact do. "A thing does not therefore come to pass because God foreknew it, but because it was to come to pass, He foreknew it." The foreknowledge is infallible because the choice is what it is, and not because the will has been forced.

The argument turns on a distinction between two senses of necessity: the necessity of compulsion, which is inconsistent with freedom, and the necessity by which what will occur cannot fail to occur, which describes the event without constraining the agent. The will that turns toward God is necessitated in the second sense and not in the first, and the act remains its own. Against the Stoic doctrine of fate and against the astrologers, Augustine grants that there is an order of causes but denies that it is an order of compulsion. Some causes are voluntary, and voluntary causes are not the less causes for being free.

A further question concerns necessity as applied to God himself. To say that God must live forever and must foreknow all things is not, on Augustine's view, to subject him to necessity, any more than his power is diminished by saying that he cannot die or be deceived. Such impossibilities belong to omnipotence rather than detract from it, for God is called omnipotent on account of doing what he wills, not on account of suffering what he does not will. The same care about terms governs his treatment of fate. He allows that the order of causes may be called fate if one wishes, provided it is understood that the will of God stands first in that order and that the wills of men are themselves causes within it. Against the astrologers he argues from the differing fortunes of twins born under one constellation. These matters are pursued further under the ideas of Fate, Providence, and Will.

The distinction between the necessity of compulsion and the necessity by which an event cannot fail passes from Augustine into the whole later literature of grace and freedom. Aquinas carries it further, holding that God not only foreknows contingent events but wills them to come about contingently, so that contingency is an effect of providence rather than a limit upon it. That extension Augustine makes available without himself drawing it.

In Aquinas the distinction between the necessary and the contingent does work in metaphysics, in theology, and in the demonstration of God's existence. The Third Way in Summa Theologica I, Q. 2 proceeds from the one to the other. We observe things which come to be and pass away; whatever is merely possible, given time enough, at some time is not; if all things were of this sort, then at some time nothing would have existed, and nothing would exist now. There must therefore be something whose existence is not received but belongs to it of itself. Such a being, in whom essence and existence are not distinct, is what all men call God, and creatures are contingent in being because before they existed they were possible and at every moment depend upon the power that preserves them.

That God exists necessarily does not, on this account, necessitate the creation. Against Avicenna, and by anticipation against Spinoza, Aquinas holds that God wills things other than himself freely, since his goodness is perfect and no perfection accrues to him from them. "God wills some things to be done necessarily, some contingently. To some effects He has attached necessary causes that cannot fail; but to others defectible and contingent causes, from which arise contingent effects, it being His will that they should happen contingently." Providence on this view is not opposed to contingency but is the reason there is any. As for the knowledge of what is still future, God knows contingent things not merely in their causes but as each is present to his eternity, as one who sees the whole road from a height sees at once all those travelling upon it.

Two further applications occupy him. In the analysis of the will he separates the necessity of coercion, which is altogether repugnant to the will, from the natural necessity by which the will desires an ultimate end, and from the conditional necessity by which, an end having been freely adopted, the one available means must be chosen. Where alternative means are available, or where the end itself need not be sought, the act of choice is not necessitated. In the analysis of knowledge he asks how there can be science of contingent things, and answers that a contingent thing may be considered either as contingent or as containing something necessary. That Socrates runs is contingent; that he moves if he runs is not. Natural science is accordingly concerned with such hypothetical necessities. These questions are treated at greater length under the ideas of Will and Science.

The position Aquinas leaves, a necessary being who freely wills a world in which some causes are defectible and some effects contingent, becomes the standard doctrine in the schools and the one against which the later rationalists define themselves. Spinoza denies its central claim, holding that God acts from the necessity of his own nature and could have produced no other order. Leibniz attempts to keep the freedom of the divine choice by referring it to a comparison among infinitely many possible worlds.

Spinoza takes the step the schoolmen refused. In Part I of the Ethics God and substance are identified, and substance is defined as that whose essence involves existence, which is necessary being in the strictest sense the tradition allows. Whatever else exists is a mode of this one substance, and every mode follows from the divine nature in the same way as it follows from the nature of a triangle that its three angles are equal to two right angles. The comparison is exact rather than ornamental, for on this view the causal relation is a relation of consequence, and what follows from the necessity of God's nature cannot be otherwise than it is.

Contingency is accordingly referred to the condition of the knower. "In nature there is nothing contingent, but all things are determined from the necessity of the divine nature to exist and act in a certain manner." To call a thing contingent is to report a defect in our knowledge of its causes rather than a character of the thing. Nor is God free in the sense the theologians intended, for "things could have been produced by God in no other manner and in no other order than they have been produced." God acts from his own nature and is determined by nothing outside himself, and in that sense alone is called free; but no other world was open to him, since no other world would have followed from what he is.

The vocabulary of freedom is retained and redefined. That thing is called free, Spinoza writes, which exists from the necessity of its own nature alone and is determined to action by itself alone; that thing is called necessary, or rather compelled, which is determined by another to existence and action in a fixed and prescribed manner. Freedom is thus opposed to compulsion and not to necessity, and among finite things it admits of degrees according as their actions follow from their own natures. On this footing the later parts of the Ethics treat human bondage as the condition of being determined chiefly from without, and human freedom as the condition of acting from adequate ideas. Final causes are set aside as figments which arise from men's ignorance of the causes of their own appetites. How far this doctrine of freedom differs from the older accounts is considered under the ideas of Liberty and Will.

Spinoza states the necessitarian alternative in its most complete form, and the tradition returns to him whenever it is asked whether a demand for sufficient reason leaves room for anything to have been otherwise. Leibniz accepts the demand and refuses the conclusion, holding that other worlds were possible and that this one was chosen. Hegel raises the contrary objection, that the substance of the Ethics is not false but incomplete, since it does not pass over into subject.

Leibniz attempts to hold the principle of sufficient reason without arriving at Spinoza's conclusion. He divides truths into two kinds. Truths of reasoning are necessary, their opposite implying a contradiction, and they hold in every possible world; logic, mathematics, and the axioms of metaphysics are of this sort. Truths of fact are contingent, their opposite being conceivable, and they hold in this world only; that Caesar crossed the Rubicon is an example. Every truth has its reason, but the reason for a truth of fact is not exhausted by any finite analysis, and it terminates at last in the will of God, who chose this world from among infinitely many.

The alternatives Spinoza denied are thus restored. Other worlds were possible, and God, acting by wisdom rather than by blind necessity, elected the one that yields the greatest variety together with the greatest order. The choice is morally necessary, in that a perfect being must choose the best, without being metaphysically necessary, since the worlds not chosen remain free of contradiction. Contingent things are on this account certain without being necessary, and their connection with the divine choice Leibniz calls hypothetical rather than absolute necessity, the distinction being the one Aristotle and the schoolmen had already employed.

The principle of sufficient reason does work outside theology as well. Leibniz argues from it against the vacuum and against absolute space and time, since God could have had no reason for placing the world in one part of an undifferentiated space rather than another, and he derives from it the identity of indiscernibles, two things differing in no respect leaving the difference of their positions without a reason. In the Theodicy the same principle governs the treatment of evil, which is admitted into the best world as the condition of goods not otherwise obtainable. The account of contingency itself is given in terms of analysis: in a necessary truth the resolution of predicate into subject comes to an end, whereas in a contingent truth it proceeds without end, and God alone surveys the whole of it. These matters bear upon the discussions under Cause, God, and Good and Evil.

Whether the position is stable has been asked from the first. If God, being perfect, must choose the best, it becomes a question whether the choice differs from the necessity Leibniz means to avoid, and whether a moral necessity that admits of no alternative in practice leaves the other worlds possible in more than name. Hume moves the discussion to different ground, denying that necessity is discoverable in things at all and referring it to the customary transitions of the mind.

Hume raises a question prior to the disputes between the rationalists, namely whether necessary connection is ever perceived. Examine any instance of causation, he proposes, and there will be found contiguity in place, succession in time, and constant conjunction in repeated cases, but no perceptible tie binding the one event to the other. "They seem conjoined, but never connected." Since the idea of necessity answers to no impression in the objects, he refers its origin to the mind, which, having been moved by custom from the appearance of one event to the expectation of its usual attendant, feels the transition and takes the felt transition for a quality in things.

The analysis is applied in two directions. In the account of nature it leaves the term necessity with no content beyond regularity together with the inference founded upon it, so that what is called the necessity we ascribe to matter is only the constant conjunction of similar objects and the consequent passage of the mind from one to the other. In the account of human action the same measure is used: motives are joined to actions with a uniformity as great as that observed among bodies, and whoever admits regularity as necessity in the one case can hardly refuse it in the other. The long dispute between liberty and necessity Hume accordingly treats as verbal, liberty meaning for him a power of acting or not acting according to the determinations of the will, and being opposed to constraint rather than to necessity.

Two consequences follow for the extent of demonstration. Since the contrary of every matter of fact remains conceivable, no proposition concerning existence can be demonstrated, and necessity in the strict sense is confined to relations of ideas, of which mathematics furnishes the examples. Hume nonetheless denies that anything happens by chance, chance having in his view no existence in the world, so that what goes by that name is only our ignorance of the real causes. The two denials are consistent in his account, since to say that nothing exists without a cause of its existence is not to say that we perceive any tie between cause and effect. What remains to causal reasoning is probability, graduated according to the uniformity of past experience. The bearing of this upon scientific inference is treated under the ideas of Cause, Induction, and Experience.

The effect of Hume's analysis upon later philosophy is measured chiefly by the answers it drew. Kant accepts the argument that no necessary connection can be extracted from impressions and rejects the conclusion, holding that necessity is contributed by the understanding as a condition of there being experience of objects at all. Others accept the reduction and treat the laws of nature as summaries of observed regularity, a position which recurs in the later discussions of Science and Hypothesis.

Kant answers Hume by changing the place where necessity is looked for. That necessary connection cannot be drawn out of experience Kant grants; that there is therefore none he denies. Necessity belongs among the categories which the understanding brings to experience as conditions of its possibility. In the Second Analogy he argues that a perceived succession counts as an objective event, rather than as a wandering of the imagination, only insofar as it is represented as following a rule, so that without the category of cause there would be no experience of a world of objects at all. Necessity in nature is thus retained, but as a character of appearances and not of things as they are in themselves.

The same limitation is then applied to the proofs of a necessary being. The categories, necessity among them, have legitimate employment only within possible experience, and when applied to the unconditioned they yield illusion rather than knowledge. The ontological argument fails, in Kant's account, because being is not a real predicate and adds nothing to the concept of a thing; the cosmological and physico-theological arguments are said to rest at bottom upon the ontological and to fall with it. In the Antinomies reason is shown to be able to prove both that a necessary being belongs to the world and that none can, which Kant takes as evidence that the question lies beyond its competence.

Necessity appears in the practical philosophy under another aspect. The moral law commands categorically, that is, without reference to any end which might be adopted or given up, and its necessity is therefore neither the natural necessity of events nor the conditional necessity of means to a chosen purpose. A rational being is bound by it as by a law it gives itself, and freedom in this sense is self-legislation rather than the absence of law. The third antinomy prepares the way for the doctrine by showing that causality according to nature and causality through freedom may both be maintained without contradiction if the first is referred to appearances and the second to things in themselves. Whether the distinction will bear the weight placed upon it has been disputed since. The connected questions are treated under the ideas of Duty, Liberty, and Will.

The settlement Kant proposes assigns necessity in nature to the conditions of possible experience, withdraws necessary being from the reach of theoretical proof, and leaves freedom as a postulate which the speculative understanding can neither establish nor refute. His successors have divided over each of the three. Hegel holds the arrangement unstable, since a necessity restricted to appearances and a freedom restricted to the unknowable are, on his view, abstractions which the development of the categories itself overcomes.

Hegel declines to treat necessity and contingency as fixed opposites. In the Doctrine of Essence in the Science of Logic they are reciprocal moments of actuality. The merely possible is abstract and empty; the merely contingent is what happens to be and might not have been; the merely necessary is what must be and has no development. Actuality is the truth of all three, being the contingent which has become necessary by working itself out through its own conditions. What is rational is actual and what is actual is rational, because the real, traced to its grounds, shows itself to be self-determining.

Two objections are entered against his predecessors. Spinoza's necessity, in Hegel's account, remains an abstract substance in which the modes are absorbed, and it fails to pass over into subjectivity and hence into freedom. Kant's necessity is a category imposed upon appearances which are supposed alien to thought, whereas for Hegel the categories are the self-development of the real. Contingency is not on this account abolished; it is retained as the element through which necessity works itself out. History furnishes the example, accident and blind event serving as the means by which what Hegel calls the cunning of reason brings the rational into actuality.

Freedom, so understood, is not independence of necessity but the recognition of necessity as one's own. Necessity does not become freedom by vanishing, Hegel writes, but by having its inner identity brought to light; to understand why what is must be, and to will it as the working out of spirit, is to be free. He does not extend the claim to every detail of the world. Nature, in his phrase, is unable to hold fast to the concept in its particulars, and contingency has its proper place there in the irregularity of individual things, which philosophy is not required to deduce. The demand that thought exhibit the necessity of every empirical fact he regards as a misunderstanding of what a science of the concept undertakes. These questions are connected with the discussions under Nature, History, and Liberty.

Hegel brings the classical opposition to a close by denying that its terms are fixed, treating necessity and contingency as moments which pass into one another in the development of actuality. What such a treatment leaves to the merely accidental has been the standing objection to it. Whether history retains any sense in which it could have gone otherwise, once its course is exhibited as the unfolding of reason, is a question his successors have continued to press, including those who reverse his idealism while keeping the logic by which he argued.