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Immanuel Kant

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Kant versus Hume on the Necessary Connection

Stuart Greenstreet finds that free will and determinism really do go together.

Samuel Johnson got to the core of philosophy's hardest problem in one line: “All theory is against the freedom of the will; all experience for it.” Theory denies the will is free because science doesn't exempt human beings from the causal laws which determine the behaviour of all other natural phenomena. Yet at any moment of decision our actual experience is of choosing freely what to do, with no hint of external constraint.

The concept of causality is that of a necessary connection between events. One event must always follow another in accordance with a universal rule. Free will, in contrast, entails the will having a distinct causality of its own. The will can make things happen which were not themselves compelled by prior causes. This ‘causality of reason', as Immanuel Kant called it, seems to evade the laws of science. It is a power spontaneously to originate actions. Each newly-willed action begins of itself, though it then unfolds according to nature's causal rules.

All theory dismisses Kant's idea of a distinct causality of reason as not just unscientific but embarrassingly mysterious. Professional philosophers generally scorn the idea of human beings having a causal power which could work independently of nature's laws. We are part of nature, and the whole of nature, they insist, is governed by laws that operate necessarily.

Hardline determinists should reflect on the fact that a necessary connection cannot be proved by empirical evidence or logically deduced. No one can be induced to believe that because a certain event has, so far, always followed another it will necessarily go on doing so. The problem of induction, as it is known, was exposed by David Hume in his Treatise of Human Nature (1739). Kant saw that Hume's argument is valid and was provoked by its astounding conclusion – that causal necessity has neither an empirical nor a logical foundation – into writing his Critique of Pure Reason (1781). The whole of this revolutionary work was, he wrote, an attempt to solve Hume's problem. Kant's solution was radical: the nature of the world as we experience it is dependent on the nature of our apparatus for experiencing. The “objects of the senses must conform to the constitution of our faculty of intuition.” This was Kant's ‘Copernican Revolution’.

Hume on the nature of cause

The problem of induction is that we are never given sensory data from which the idea of a necessary connection could be derived; nor may we assert that such a connection exists even though it cannot be observed. Hume's first principle is “That there is nothing in any object, considered in itself, which can afford us a reason for drawing a conclusion beyond it.” To this he adds the principle “That even after the observation of the frequent or constant conjunction of objects, we have no reason to draw any inference concerning any object beyond those of which we have had experience.” From the sole premise that one billiard ball is rolling towards another it would never be possible “to infer motion in the second ball from the motion and impulse of the first.” No relations of force or power can be detected in the actual phenomena (the billiard balls). We need a bridge to carry us from a true belief in one matter of fact (the first ball will hit the second) to a true belief in another (the second ball will move). By ‘bridge' I mean a middle premise which would claim, quoting Hume, “that instances of which we have had no experience, must resemble those of which we have had experience, and that the course of nature continues always uniformly the same.” But this is impossible to prove. No matter how often we have seen a ball move when hit by another, this evidence provides no logical grounds for inferring that future collisions will necessarily have the same result. The logical gap can never be bridged.

Yet we cannot doubt that every event has a cause. Without a causal connection – Hume's ‘cement of the universe’ – there would be no empirical world of ordinary experience or scientific study. We happen to live in that world, so we're certain there is a causal connection. We also know that this indispensable feature of our world is neither observable nor logically deducible. So how is it to be explained?

Hume had an answer: the mind associates ideas. Through observing the constant conjunction of events, the mind gets conditioned to expect this regularity to recur. In Hume's words: “the mind is carried by habit, upon the appearance of one event, to expect its usual attendant and to believe that it will exist.” It is in this habitual mental glide from an event to its constant follower that he discovered “the impression from which we form the idea of power or necessary connection.” The habit is so ingrained that we project the strength of the association into the phenomena themselves. We inject the idea of power into colliding billiard balls. We fell under the illusion that ‘necessary connection‘ is the name of a relation in which they actually stand to one another.

Hume, a tough empiricist, held that the content of every thought is derived, in the last analysis, from the experiences which warrant it. No belief can be certified as true except by reference to the sensory impressions which are its guarantee. This is why he felt he had solved the problem of induction when he hit upon an impression which he thought supported the idea of necessary connection – the constant conjunction of events. Empiricism restricts each person's knowledge of the world to knowledge of their own point of view. All knowledge thus becomes subjective and contingent. Kant, as we shall now see, threw out this doctrine.

Kant's reaction to Hume

Hume convinced Kant that causal necessity has no logical or empirical explanation. Kant decided, as Hume had done, that we ourselves impose a causal connection on the world. But though its source is in our minds it is not, as Hume has claimed, a mental habit, a mere trick we fell for by seeing events constantly conjoined. Here is a key passage where Kant contradicts Hume. The concept of the causal relation, he wrote,

“positively requires that something A be such that something else B follow from it necessarily and in accordance with an absolutely universal rule, Appearances (ie what we perceive) certainly provide cases in which a rule is possible according to which something customarily occurs, but never that the result is necessary. [The causal relation has a property] that one absolutely cannot express empirically: namely, that the effect is not merely joined to the cause, but rather is posited through it and results from it. The strict universality of the rule is certainly not a property of empirical rules, which, through induction, can possess nothing but comparative universality.” Critique of Pure Reason, A91-2 / B123-4)

Empirical rules “possess nothing but comparative universality” because induction entitles us to say only that all events of type A observed so far have been followed by type B. Induction can never ground the strictly universal rule that all events of type A are followed by events of type B. Strict universality belongs to the mind's judgement of our perceptions, and this, Kant concludes, “indicates a particular source of knowledge for such, namely a faculty of a priori knowledge.” Hume may well assert that all knowledge is grounded in experience. But experience is not the simple concept that he supposed. It comes to us pre-structured. Space, time, substance and causality are not objects of sense in themselves. They are conditions to which our perceptions of objects must conform in order that we may have experience of them.

Kant’s universal principle of causality

In Kant’s model of the mind the causal relation is one of twelve ‘categories’ – concepts that are fundamental to all knowledge. They are mental functions which ‘make sense' out of our disparate perceptions by organising them into the form in which we experience the world. The categories in unison form ‘the understanding' – the faculty for making empirical judgements. Finally, the categories are ‘pure’ concepts, in the sense that they are not as Hume had claimed, derived from experience, but have their origin in the very constitution of the mind itself.

The judgement that A is not merely joined to B (by always preceding it) but actually causes B is grounded in an a priori source of knowledge – ie in the faculty of the understanding with its a priori conditions of objective knowledge. It is because the judgement is so grounded a priori, that we are entitled to assert the principle of causality: that all events of type A are universally and necessarily followed by events of type B.

This principle is ‘transcendental’ in this strictly technical sense: it is known to be true not from experience, but because it is a condition that must be fulfilled for empirical knowledge to be possible. Kant's next step is to distinguish particular empirical causal laws – those which scientists discover – from the causal principle. The very discovery of particular laws, he argues, takes place “in virtue of the original laws through which experience first become possible” – ie the categories of the understanding. Therefore even particular laws are not derived solely empirically. They are subsumed under the a priori concept of causality in such a way that they thereby become necessary and acquire more than inductive status. Particular laws are, in other words, grounded in the universal causal principle. The principle makes experience possible by injecting both necessity and strict universality into the particular causal laws of science.

We are now ready to identify the crucial difference between Hume and Kant on causal necessity: Hume works from world to mind, Kant from mind to world.

Reconciling freedom and necessity

Kant found a way to reconcile freedom and determinism. This final section is a bare outline of his argument. It is easier to follow if one recalls that the universal principle of causality asserts nothing about the nature of empirical phenomena as they are independently of how we experience them. It has nothing to do with the mechanistic determination of events, such as the dynamics of colliding billiard balls. The universal principle of causality is a condition of judgement. It applies to the operation of the mind, not to the behaviour of matter. The mind is ruled by this principle in all its judgements of the causal relations of empirical phenomena.

We cannot help reacting to other people as though they did what they did but could have done otherwise. We blame them if they choose to do A rather than B, and A is hurtful to us. We believe that their action, just by being an act of will, is free, and that they are responsible for it. The default position, surely, is that we are free. What needs to be explained is why anyone should think the opposite: that despite this ingrained belief we are actually unfree in our voluntary actions.

We would be unfree if the will, and hence our choices, were determined – as physical events are – by prior events. But not all necessity is of this mechanical kind. When you or I choose what to do we decide what action to take, and what determines an action is categorically different from what determines an event. When I explain why a friend did what she did I see her as determined not, like an object, by prior events, but by her character, Say she lied to me about something important. If I tried to explain why she lied, I would, Kant says,

“trace the empirical character of the action to its sources, finding these in defective education, bad company, in part also in the viciousness of a natural disposition insensitive to shame, in levity and thoughtlessness, not neglecting to take into account the occasional causes that may have intervened.” Critique of Pure Reason, B582)

Although, like a scientist, I am searching for an explanation which would have allowed me to predict the lie, I do not (nor would anyone) believe that my friend had to lie. As Kant says, we make her responsible:

“Our blame is based on a law of reason whereby we regard reason as a cause that irrespective of all the above mentioned empirical conditions (defective education, bad company, etc) could have determined, and ought to have determined, the agent to act otherwise.” (B583. My italics.)

If I could discover all the conditions which led her to lie, I would be wrong to claim that they made her lie. Yet even so, amazingly, I could have predicted the lie.

“... if we could exhaustively investigate all the appearances of men's wills, there would not be found a single human action which we could not predict with certainty and recognise as preceding necessarily from its antecedent conditions.” (B578)

Had I found a complete explanation of the lie I would have been able to predict it. Yet these sufficient conditions wouldn't show that the lie was necessitated in such a way that she was prevented from telling the truth. Hasn't Kant contradicted himself? How can a lie be both somehow necessitated and predictable, and yet the liar could have chosen not to lie? In what possible way could her will still be free if her actions are somehow necessitated and predictable?

The phrase ‘necessitated in such a way’ hints that human actions are not compelled as physical events. The picture of a science-like investigation into the ‘empirical conditions' sufficient for the telling of a lie is misleading. The conditions sufficient for any human action are not fixed states of affairs, not hard facts like the facts of physics, but psychological facts, facts of character, The best we can expect from an investigation of these facts is “an estimate concerning the subjective principle of the man's will” i.e., an assessment of the traits of character which have motivated a person's choices up to now. The sufficient conditions we cite of someone's actions are ‘necessary’ not because actions are predetermined but because such conditions must be empirically discoverable. They are discoverable, Kant claims, because human actions, just like physical events, must be systematically connected. Our actions take a law-like path because they are ruled by the causality of reason:

“Reason does not here follow the order of things, as they present themselves in appearance (ie. as compelled by the causal principle), but frames for itself with perfect spontaneity an order of its own according to ideas.... And at the same time reason also presupposes that it can have causality in regard to all these actions (ie the causality of reason) since otherwise no empirical effects could be expected from its ideas.” (B576)

If human actions were nothing but physical events – merely the by-products of physical processes – they would be subject to natural laws which determine physical events. But actions are not determined indirectly as physical events. They themselves fall under a set of rules in their very character as actions:

“Reason though it be, it must nonetheless exhibit an empirical character. For every cause presupposes a rule according to which certain appearances follow as effects; and every rule requires uniformity in its effects.” (B576-71)

All of Kant's explanations, so far, still leave a crucial question unanswered. The causality of reason presupposes a rule whereby one willed action systematically follows another. So actions can be explained by tracing them to their sources. How, then, is the will able to escape an endless causal chain and initiate a new series of actions, one which begins purely of itself? We must look more deeply into Kant's startling claim that if we could exhaustively investigate all the appearances, [ie empirical evidence] of men's wills we would see that every human action had followed from its prior conditions and could have been predicted.

He does not mean that it is possible to predict a person's action at any given moment in advance of it being performed. After it has been performed, and has entered a causal series, the action will be explicable within that framework. Given what we then know we could have predicted the action. The causal series does not precede that action, but includes it. Kant's meaning is easier to grasp in the light of his claim that human actions are necessitated by facts about character. Until a person has finally decided how to act (say, whether or not to lie) her choice remains open. She could have a change of heart. Therefore until she has acted, any estimate of her character is underdetermined by the data. Every moral choice she makes as she conducts her life yields more data about her essential character. It is the action she actually performs which enables us to see, retrospectively, how it fitted into a pattern and could have been predicted. We will be in exactly the same position at her next point of decision, and so on.

Earlier, I stressed the distinction between transcendental principles of causality (which must exist for experience to be possible) and the natural causality of material things (particular empirical causal laws discovered by science). Transcendental causality is not an event in the worldly chain of events. It is, in Kant's words, “the ground which determines the causality of natural things to an effect in accordance with their proper laws.” Transcendental causality stands behind all events and outside of time. It is non-temporal. Now recall the argument: physical events have empirical causes; human actions are not determined indirectly as physical events; they conform as actions to the rules of the transcendental causality of reason. Now maybe we can make sense of the claim that the will is spontaneously able to originate actions. This is possible, Kant says, because

“[the] first beginning of what we are here speaking is not a beginning in time but in causality. If, for instance, I at this moment arise from my chair, in complete freedom, without being necessarily determined thereto by the influence of natural causes, a new series, with all its natural consequences ad infinitum, has its absolute beginning in this event, although as regards time [it] is only the continuation of a preceding series.” (B748-9)

An act of the will, says Kant, ‘follows upon' natural causes “but without arising out of them”. Although human actions follow the train of natural events they arise from the transcendental causality of reason. They are not, therefore, time-bound. ‘First beginnings' – spontaneous human actions – are possible at any point in our lives because they respond to rules which exist outside of the temporal order. They can occur without interrupting nature's causal chain.

© Stuart Greenstreet 2005

Stuart Greenstreet, a business manager by day, began philosophy in the evenings at Birkbeck College in the 1980s, and has kept at it ever since because he's ‘afraid to stop’. He's currently doing postgraduate philosophy with the Open University.

Note: I have drawn on Michael Rosen's analysis of Kant's solution to the problem of free will in his paper ‘Kant's Anti-determinism’, which anyone engaged with the problem should read. The paper is in the Proceedings of the Aristotelian Society, New Series, Vol LXXXIX, 1988/89, pp.125-141.

David Hume: How we experience the world is conditioned by the world

If one event always follows another we believe the first causes the second. But it is impossible to prove, empirically or logically, that the second event happened because the first did. Causal necessity is an illusion: “The mind is carried by habit, upon the appearance of one event, to expect its usual attendant, and to believe that it will exist.” By force of habit we project necessity on to constant conjunction.

Immanuel Kant: How we experience the world is conditioned by the mind

It is true that a causal connection cannot be proved. But this connection is not, as Hume claimed, a mental habit which we derived empirically from impressions of constantly conjoined events. Causal necessity is an a priori mental precondition of all possible experience. The mind, via the understanding, applies it categorically and universally to every judgement of objects of sense.


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