David Hume: Lecture five
The Principle of Sufficient Reason
Given Hume’s wider views about the nature and the intelligibility of causal connections between distinct events, it naturally followed that he did not feel that it was possible to demonstrate the principle of sufficient reason, according to which every new event needed to have some cause or other. Notwithstanding the fact that we might be accustomed to associate causes and effects in our minds, and to pass smoothly from one to the other in thought, they do still remain distinct objects. There is no contradiction in supposing an effect to exist without its usual cause, for this does not amount to supposing, of any one single thing, both that it does and does not exist.
Indeed, it is perfectly intelligible to suppose that something might come into existence without any cause whatsoever. In themselves, all objects are so wholly distinct from and logically independent of all others that the universe could very easily have been utterly chaotic, where things just popped uncaused into existence. On Hume’s analysis, all that this would require is that such a new object could not be fitted into some pre-existing pattern of constantly conjoined pairs of objects. Maybe the new one might not resemble anything that had gone before at all. Or else, if a place could be found for it in a class of resembling objects, it might yet be the case that there was no constant conjunction between the members of that class and the members of a second class of resembling objects.
It seems evident that, if all the scenes of nature were continually shifted in such a manner that no two events bore any resemblance to each other, but every object was entirely new, without any similitude to whatever had been seen before, we should never, in that case, have attained the least idea of necessity, or of a connexion among those objects. We might say, upon such a supposition, that one object or event has followed another; not that one was produced by the other. The relation of cause and effect must be utterly unknown to mankind. ( Enquiry , §8.1).
But, if such a hypothesis is intelligible, then its negation—that everything does require a sufficient cause—cannot be known a priori .
’Tis a general maxim in philosophy, that whatever begins to exist, must have a cause of existence . This is commonly taken for granted in all reasonings, without any proof given or demanded. ’Tis suppos’d to be founded on intuition, and to be one of those maxims, which tho’ they may be deny’d with the lips, ’tis impossible for men in the hearts really to doubt of.
But if we examine this maxim by the idea of knowledge above-explain’d, we shall discover in it no mark of any such intuitive certainty; but on the contrary shall find, that ’tis of a nature quite foreign to that species of conviction. [...]
But here is an argument, which proves at once, that the foregoing proposition is neither intuitively nor demonstrably certain. We can never demonstrate the necessity of a cause to every new existence, or new modification of existence, without shewing at the same time the impossibility there is, that any thing can ever begin to exist without some productive principle; and where the latter proposition cannot be prov’d, we must despair of ever being able to prove the former. Now that the latter proposition is utterly incapable of a demonstrative proof, we may satisfy ourselves by considering, that as all distinct ideas are separable from each other, and as the ideas of cause and effect are evidently distinct, ’twill be easy for us to conceive any object to be non-existent this moment, and existent the next, without conjoining to it the distinct idea of a cause or productive principle. The separation, therefore, of the idea of a cause from that of a beginning of existence, is plainly possible for the imagination; and consequently the actual separation of these objects is so far possible, that it implies no contradiction nor absurdity; and it therefore incapable of being refuted by any reasoning from mere ideas; without which ’tis impossible to demonstrate the necessity of a cause. ( Treatise ,
I.3.3).

But so what? Reason is the slave of the passions, and it has already been shown to be largely impotent. Human psychology, and human nature in general, are governed not by the dictates of the pure intellect, but rather by the non-rational habits of the imagination. And Hume believes that those do generate within us a belief in the principle of sufficient reason. In a letter of February 1754 (to John Stewart), Hume writes: ‘But allow me to tell you that I never asserted so absurd a Proposition as that anything might arise without a cause: I only maintain’d, that our Certainty of the
Falshood of that Proposition proceeded neither from Intuition nor Demonstration, but from another Source.’
As a matter of fact, our experience is extremely regular, displaying constant conjunctions all over the place. We couldn’t demonstrate a priori that it should be that way, since, logically, it doesn’t need to be. But, for whatever reason, it is that way.
And, moreover, human nature is such that we are disposed to respond to experience like that by building up a view of the world as a deterministic system of interlocking necessary connections. Again, human nature doesn’t logically need to be that way either: it just is . And, in virtue of this, a belief in the principle of sufficient reason, although not logically demonstrable, is humanly inescapable .
Necessity in human affairs (Enquiry, sect. 8, part 1)
Crucially, Hume feels that this determinism applies to human behaviour every bit as much as it does to the interactions of inanimate objects. Our experience reveals that human nature is itself extremely uniform, both in how people respond to external stimuli, and also in how their behaviour is connected to their own motivations within.
Just as we discover regularities in the way billiard balls tend to communicate motion into one another, so too do we discover regularities in the way people tend to behave in various circumstances. And our experience of such regularities leads us to form inferential habits in just the same way in both cases, and such habits enable us to get around in the world. If we could not predict how inanimate bodies would respond to certain impulses, we would never know how to act in relation to them. But, equally, if we could not predict how other people would respond to our behaviour, we would never know how to act in relation to them .
The mutual dependence of men is so great in all societies that scarce any human action is entirely complete in itself, or is performed without some reference to the actions of others, which are requisite to make it answer fully the intentions of the agent. […] In proportion as men extend their dealings, and render their intercourse with others more complicated, they always comprehend, in their schemes of life, a greater variety of voluntary actions, which they expect, from the proper motives, to co-operate with their own. In all these conclusions they take their measures from past experience, in the same manner as in their reasonings concerning external objects; and firmly believe that men, as well as all the elements, are to continue in their operations, the same that they have ever found them. A manufacturer reckons upon the labour of his servants for the execution of any work as much as upon the tools which he employs, and would be equally surprised were his expectations disappointed. In short, this experimental inference and reasoning concerning the actions of others enters so much into human life that no man, while awake, is ever a moment without employing it. Have we not reason, therefore, to affirm that all mankind have always agreed in the doctrine of necessity according to the foregoing definition and explication of it? ( Enquiry , §8.1)
By and large, the inferential processes whereby we come to form predictions about future effects tend to run indifferently between human and non-human causes:

And indeed, when we consider how aptly natural and moral evidence link together, and form only one chain of argument, we shall make no scruple to allow that they are of the same nature, and derived from the same principles. A prisoner who has neither money nor interest, discovers the improbability of his escape, as well when he considers the obstinacy of his gaoler, as the walls and bars with which he is surrounded; and, in all attempts for his freedom, chooses rather to work upon the stone and iron of the one, than upon the inflexible nature of the other. The same prisoner, when conducted to the scaffold, foresees his death as certainly from the constancy and fidelity of his guards, as from the operation of the axe or wheel. His mind runs along a certain train of ideas: The refusal of the soldiers to consent to his escape; the action of the executioner; the separation of the head and body; bleeding, convulsive motions, and death. Here is a connected chain of natural causes and voluntary actions; but the mind feels no difference between them in passing from one link to another: Nor is less certain of the future event than if it were connected with the objects present to the memory or senses, by a train of causes, cemented together by what we are pleased to call a physical necessity.
The same experienced union has the same effect on the mind, whether the united objects be motives, volition, and actions; or figure and motion. We may change the names of things; but their nature and their operation on the understanding never change. ( Ibid .)
More succinctly, Hume observes: ‘A man who at noon leaves his purse full of gold on the pavement at Charing-Cross, may as well expect that it will fly away like a feather, as that he will find it untouched an hour after.’ ( Ibid ). We do not expect the purse to fly away like a feather, because we have become accustomed to imagine that heavy objects will, if unmolested, stay where they are, held in place by a law of universal gravitation. But, at the same time, we do not expect it to remain untouched, because we have become accustomed to imagine that valuable objects like that, left unattended in full view of the public, will sooner or later tempt some passer-by to have away with them. The laws that govern human behaviour won’t be the same as those that govern purely physical interactions, but they will have the same general form. In neither case can pure reason discover any necessary connections a priori at all. But, in both cases, the regularities in our past experience will lead us to feel such connections, to pass smoothly in thought between those events that we have become accustomed to regard as causes and effects, and to expect new instances of the same kinds of events to be similarly conjoined in the future too. Our experiences of interacting with other people lead us to conclude that their behaviour is deterministic and predictable—and it’s a good thing too, because otherwise we would have no way of judging how best to conduct ourselves in relation to them.
Liberty
If ‘liberty’ or ‘free will’ is supposed to characterise spontaneous actions that stand outside the necessary, deterministic course of events, it will follow that there can be no such thing—or, at any rate, that our actual experience will lead us to believe that there is no such thing. Our experience might have been different: but, as it is, it leads us to regard human actions as necessarily connected to prior events, every bit as much as the behaviour of inanimate bodies. Moreover, it should be observed that the necessary connections that govern human behaviour cannot merely apply to the links between their internal volitions and the outward actions that result from these. When dealing with other people, we have no direct experience of their mental states, but only of their outward actions and the surrounding circumstances. And yet we can still predict their behaviour. Through analogy with what we discover within our own selves, we might perhaps be inclined to postulate an internal volition as an intermediary in the translation of one set of outwardly observable events into another: but the necessary connections that we actually draw must be between those outwardly observable events themselves. So, if it is thought that liberty might at least be

consistent with a necessary connection between a volition and the ensuing action, just as long as that volition itself arose spontaneously within the mind without being necessarily connected with any earlier physical events, our experience again leads us to believe that there is no such thing.
But Hume does not conclude that there is no such thing as liberty. (The expression
‘free will’ does not really feature in his discussions at all. One might compare Locke’s dismissal of the latter notion as resting on a mere category-mistake. See Locke’s
Essay , II.21.14). Everyone believes that liberty does exist, at least in some cases.
That’s just one of those natural beliefs that are common to mankind as such, and
Hume has no quarrel with any such beliefs: his goal is merely to explain where they come from, and to clear up any confusions that earlier philosophers might have fallen into about their content. As far as Hume is concerned, liberty and necessity, when rightly understood, cannot be incompatible with one another, because everyone agrees that they are both real. For a philosopher to imagine some incompatibility between them must mean that he is confused about what one or both actually amount to—he cannot be using the terms with the meaning that their ordinary usage determines them to have.
For Hume, liberty is simply a matter of doing what we want to do, because we want to do it. It cannot signify a complete absence of causal determination for an action.
Every action must have some cause or other. What sets a free action apart from the rest is that it should have a special kind of cause, namely a volition within the agent, such that, if the agent had willed to do something else instead, he would have done so.
That volition must itself have been necessitated by some still earlier cause, and the causal chain must lead back out of the agent’s will, and ultimately into purely physical circumstances over which the agent had no voluntary control. But all of that other stuff is irrelevant to the liberty of the eventual action. All that matters there is that the agent is not prevented from doing what he has chosen to do, regardless of what might have made him choose to do it.
For what is meant by liberty, when applied to voluntary actions? We cannot surely mean that actions have so little connexion with motives, inclinations, and circumstances, that one does not follow with a certain degree of uniformity from the other, and that one affords no inference by which we can conclude the existence of the other. For these are plain and acknowledged matters of fact. By liberty, then, we can only mean a power of acting or not acting, according to the determinations of the will; that is, if we choose to remain at rest, we may; if we choose to move, we also may. Now this hypothetical liberty is universally allowed to belong to every one who is not a prisoner and in chains.
Here, then, is no subject of dispute. ( Ibid ).
Indeed, Hume goes further and suggests that liberty is not only compatible with necessity, but that it positively depends on necessity. A lack of necessary determination for an action, far from establishing its liberty, would in fact eliminate it.
Where an action is entirely spontaneous, and not necessarily connected with any earlier event at all, that’s just random chance. And Hume does not believe that there are any truly random events of this kind: but, even if there were, they are not the ones that we would regard as free actions. A chance event, even one that involves a human being, is something that happens to him, not something that he does . The subject of this chance event cannot be held responsible for it, because it was wholly outside his control.

Suppose that you have a close friend, such that your many years of intimate acquaintance with them have given you so precise a knowledge of their character that you can accurately predict how they are likely to behave in various scenarios. Over and above the general laws that you know to govern human behaviour at large, you have an even more detailed and personalised grip on the laws that govern this individual’s behaviour. That is to say, you have become accustomed to identify an interwoven cluster of necessary connections that govern precisely how their external circumstances actually or hypothetically determine their actions. Now, in any new situation that crops up, if your friend does indeed behave in the way you would have predicted, it would never occur to you to suspect that they were doing anything other than acting freely. But suppose that they were to start to behave in a radically out-ofcharacter way, one that does not cohere with your predictions, or fit into the pattern of necessary connections that you have come to associate with this person. This would surely lead you suspect that they were not acting freely. Maybe it was just a random fit, something over which they had no control. Or maybe they were being secretly coerced into doing that went against everything they believed in. Or maybe they had simply gone mad. As Hume observes: ‘’Tis commonly allow’d that mad-men have no liberty. But were we to judge by their actions, these have less regularity and constancy than the actions of wise-men, and consequently are farther remov’d from necessity.’
( Treatise , II.3.1). Either an action conforms with a person’s character—i.e. it is necessitated by their character, a character which was, in its turn, formed by circumstances outside their control—or it does not. It is only in the first case that we regard the action as free.
Liberty and Morality (Enquiry, sect. 8, pt. 2).
Hume feels that liberty is essential to morality: we only praise or blame people for the things that they do freely. But this, again, serves to demonstrate that liberty depends on necessity, rather than on its absence. Rather than simply blaming the action as such, it is the agent that we blame and punish for the action. Consequently, we must be able to trace our way inferentially back from the action to its cause in the agent’s character, and this can only be by following a necessary connection.
Actions are, by their very nature, temporary and perishing; and where they proceed not from some cause in the character and disposition of the person who performed them, they can neither redound to his honour, if good; nor infamy, if evil. The actions themselves may be blameable; they may be contrary to all the rules of morality and religion: But the person is not answerable for them; and as they proceeded from nothing in him that is durable and constant, and leave nothing of that nature behind them, it is impossible he can, upon their account, become the object of punishment or vengeance. […] It will be equally easy to prove, and from the same arguments, that liberty , according to that definition above mentioned, in which all men agree, is also essential to morality, and that no human actions, where it is wanting, are susceptible of any moral qualities, or can be the objects either of approbation or dislike. For as actions are objects of our moral sentiment, so far only as they are indications of the internal character, passions, and affections; it is impossible that they can give rise either to praise or blame, where they proceed not from these principles, but are derived altogether from external violence. ( Enquiry , §8.2).