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Free Will
Mental Causation
James Symposium
David Hume
David Hume's Treatise of Human Nature, Book II, Part III, Sections I-II and Enquiry Concerning Human Understanding, Chapter VIII are said by many current philosophers to be the locus classicus of "compatibilism," the position that "free will" is compatible with strict physical determinism.

There is no doubt that Hume's reconciliation of freedom and necessity was a great influence on most analytic and logical empiricist philosophers, through John Stuart Mill, G. E. Moore, Bertrand Russell, A. J. Ayer, and Moritz Schlick, as well as physical scientists like Ernst Mach.

R. E. Hobart developed Hume's compatibilism in his landmark 1934 essay Free Will As Involving Determination And Inconceivable Without It. But Hobart did not deny chance, as did Hume. P. H. Nowell-Smith in 1948 strengthened the attack on chance. But Philippa Foot in her 1957 article Free Will As Involving Determinism (misquoting Hobart) cautioned that "determination" of the will, by reasons and motives for example, does not imply the kind of strict causal determinism back to the beginning of time that denies the existence of chance.

And we will show below that Hume himself did not like a universal causal determinism - it denies the possibility of liberty.

So what is it that distinguishes Hume's compatibilism from earlier compatibilists from Chrysippus to Thomas Hobbes?

Major differences between Hobbes and Hume can be traced to the work of empiricist philosophers John Locke and George Berkeley and the scientist Isaac Newton between Hobbes and Hume.

Locke's "Theory of Ideas," which limits human knowledge to that gathered through the senses (the mind starts as a blank slate with no innate ideas) was an enormous influence on Hume. Hume is often simply regarded as one of the three British empiricists who put knowledge of the "things themselves" with their "primary" qualities, beyond the reach of our perceptions. It is this standard view of Hume, as one denying unknowable concepts, particularly the notion of "causation," that inspired the positivists to declare such concepts "meaningless" and "metaphysical.

But Hume is much more complex, as a careful reading of the Treatise and especially the Enquiry concerning Human Understanding shows. Hume did not deny causation. He embraced it. But he did say that empirical methods could not logically prove its necessity, as observations only show a "constant conjunction" of events, a "regular succession" of A followed by B, which leads the mind to the inference of cause and effect. For Hume, causality is something humans naturally believe.

The skeptical Hume argued that we cannot logically prove causation and "matters of fact," as we can know and prove "relations of ideas" internal to mathematics and logical systems of thought. But Hume the naturalist said that we can have a natural belief in causation and in many matters of fact. Kant would later argue that such things were knowable as "synthetic" a priori.

Reasons and Passions
A major theme of Hume's work, perhaps his core contribution, is that "Reason" cannot motivate our Beliefs. Reason is an evaluative tool only. It is "Feeling" and "Passion" that motivates our "natural" beliefs, judgements, and actions.

Most earlier and later philosophers make the feelings and passions subject to reason. Hume turned this around and based his ideas of morality on sentiments and feelings. He denied that one could ever produce reasoned arguments to derive "ought" from "is," but that we naturally hold many of our moral beliefs simply based on our feelings and moral sentiments. And that only these Passions, not Reason, are capable of motivating us to action. In a most famous observation, he says...

I cannot forbear adding to these reasonings an observation, which may, perhaps, be found of some importance. In every system of morality, which I have hitherto met with, I have always remark'd, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surpriz'd to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, 'tis necessary that it shou'd be observ'd and explain'd; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it.
(Treatise of Human Nature, Selby-Bigge, p.469)

What is true in moral thinking is true in our physical understanding; we have a natural belief in causality, says Hume. Although it is not an empirically justified "idea" and thus not knowledge, we have a natural feeling about how one billiard ball causes a second one to move, even when watching a single example - "constant conjuctions" are not needed for this "natural" belief.

Similarly, we judge a person praiseworthy or blameworthy because we see the causal connection between a person's character, volition, and resulting actions. This agrees with Thomas Hobbes and with the later work of R.E.Hobart and Philippa Foot on "determination."

But on the surface, Hume's compatibilism contains a basic contradiction, one that calls into question the entire theory of causal mechanical determinism that was the basis of Hobbes' compatibilism. Hume showed that causality could not be proved logically from mere induction. For Hume, the necessity of causality was found in the human mind and in its natural beliefs. But if necessity and causality are in the mind and not in nature, how can that explain the work of Isaac Newton, which Hume claimed as his model for the experimental study of human nature?

To be sure, Hume was as cautious as Newton had been in claims to know the reality underlying observations and experiments. Empiricism must not "go beyond experience" to the "things themselves," as Kant would later call them.

Hume's academic skepticism about logical proofs has withstood the test of time. There is nothing that can be shown to be logically true of the physical world. Unfortunately for Hume, that includes causality and strict physical determinism. Newton himself was not convinced of the "truth" of strict physical determinism. It could not be proved on the basis of observations, which Newton understood always contain experimental errors.

Nevertheless, Hume was also skeptical about his skepticism (as any good skeptic ought to be) and said that just because we are unable to prove things like causality and an external world of objects, that did not prevent us from believing that such things really exist.

In Hume's study of Morals we find another skeptical/realist contradiction. He famously claims we cannot derive "ought" (logically and rationally) from "is." Yet he affirms the existence of "moral sentiments" which he says we know by direct observation and his experimental study of human nature - a "science of man."

Hume at first appears to state what G. E. Moore will later call the naturalistic fallacy, namely that "good" cannot be defined as corresponding to anything in nature. But then Hume proceeds to find moral sentiments in his human naturalism, studying man as a part of nature.

Hume has put strict limits on what we can know (or derive) from logical analysis. In this he anticipates Immanuel Kant's Critique of Practical Reason, where Kant says he must put limits on Reason and Knowledge to make room for belief in God, Freedom and Immortality.

Unlike Kant, who invented a metaphysical "noumenal" realm to locate our Freedom, Hume could not find God or Freedom in Kant's metaphysical sense. Kant called Hume's compatibilist freedom a "wretched subterfuge." William James called it a "quagmire of evasion."

Most of Hume's followers ignore his skeptical doubts about certain knowledge and causality. For example, many philosophers of science continue to claim they can put a sound epistemological foundation under physics, probing to find fault with quantum mechanical uncertainty. Many philosophers of mind continue to assert the truth of compatibilism, though under pressure from the success of indeterministic quantum physics, most now claim to be agnostic about the truth of physical determinism.

...there is but one kind of necessity, as there is but one kind of cause, and that the common distinction betwixt moral and physical necessity is without any foundation in nature. This clearly appears from the precedent explication of necessity. 'Tis the constant conjunction of objects, along with the determination of the mind, which constitutes a physical necessity: And the removal of these is the same thing with chance. As objects must either be conjoin'd or not, and as the mind must either be determin'd or not to pass from one object to another, 'tis impossible to admit of any medium betwixt chance and an absolute necessity. In weakening this conjunction and determination you do not change the nature of the necessity; since even in the operation of bodies, these have different degrees of constancy and force, without producing a different species of that relation.
(Treatise, Book I, Part I, Section XIV, p.171)

Of all the immediate effects of pain and pleasure, there is none more remarkable than the WILL. I desire it may be observ'd, that by the will, I mean nothing but the internal impression we feel and are conscious of, when we knowingly give rise to any new motion of our body, or new perception of our mind.

'Tis universally acknowledg'd, that the operations of external bodies are necessary, and that in the communication of their motion, in their attraction, and mutual cohesion, there are not the least traces of indifference or liberty. Every object is determin'd by an absolute fate to a certain degree and direction of its motion, and can no more depart from that precise line, in which it moves, than it can convert itself into an angel, or spirit, or any superior substance. The actions, therefore, of matter are to be regarded as instances of necessary actions; and whatever is in this respect on the same footing with matter, must be acknowledg'd to be necessary. That we may know whether this be the case with the actions of the mind, we shall begin with examining matter, and considering on what the idea of a necessity in its operations are founded, and why we conclude one body or action to be the infallible cause of another. (Treatise, Book II, Part III, Section I, p.399-400)

I dare be positive no one will ever endeavour to refute these reasonings otherwise than by altering my definitions, and assigning a different meaning to the terms of cause, and effect, and necessity, and liberty, and chance. According to my definitions, necessity makes an essential part of causation; and consequently liberty, by removing necessity, removes also causes, and is the very same thing with chance.

The objection to chance is half of the standard argument against free will
As chance is commonly thought to imply a contradiction, and is at least directly contrary to experience, there are always the same arguments against liberty or free-will. If any one alters the definitions, I cannot pretend to argue with him, 'till I know the meaning he assigns to these terms.
In the Treatise Hume distinguishes the liberty of indifference (in which a random decision is made because the options are evenly balanced) from the liberty of spontaneity (freedom of action from external coercion or voluntarism, as Hobbes called it).
I believe we may assign the three following reasons for the prevalence of the doctrine of liberty, however absurd it may be in one sense, and unintelligible in any other. First, After we have perform'd any action; tho' we confess we were influenc'd by particular views and motives; 'tis difficult for us to persuade ourselves we were govern'd by necessity, and that 'twas utterly impossible for us to have acted otherwise; the idea of necessity seeming to imply something of force, and violence, and constraint, of which we are not sensible. Few are capable of distinguishing betwixt the liberty of spontaneity, as it is call'd in the schools, and the liberty of indifference; betwixt that which is oppos'd to violence, and that which means a negation of necessity and causes. (Treatise, Book II, Part III, Sections I-II, p.407)
Hume himself has difficulty distinguishing between these two liberties, or two senses of the word liberty. Today we can see that this is in part because the idea of absolute chance had become anathema to the scientists and mathematicians who developed the "calculus of probabilities." Probabilities describe merely human ignorance, not the existence of absolute chance.

In the Enquiry Concerning Human Understanding, Section VIII, "Of Liberty and Necessity," Hume attributes the lack of progress on this controversy to the lack of clear conceptual definitions and ambiguities in the words.

It might reasonably be expected in questions which have been canvassed and disputed with great eagerness, since the first origin of science, and philosophy, that the meaning of all the terms, at least, should have been agreed upon among the disputants; and our enquiries, in the course of two thousand years, been able to pass from words to the true and real subject of the controversy. For how easy may it seem to give exact definitions of the terms employed in reasoning, and make these definitions, not the mere sound of words, the object of future scrutiny and examination? But if we consider the matter more narrowly, we shall be apt to draw a quite opposite conclusion. From this circumstance alone, that a controversy has been long kept on foot, and remains still undecided, we may presume that there is some ambiguity in the expression, and that the disputants affix different ideas to the terms employed in the controversy...It is true, if men attempt the discussion of questions which lie entirely beyond the reach of human capacity, such as those concerning the origin of worlds, or the economy of the intellectual system or region of spirits, they may long beat the air in their fruitless contests, and never arrive at any determinate conclusion. But if the question regard any subject of common life and experience, nothing, one would think, could preserve the dispute so long undecided but some ambiguous expressions, which keep the antagonists still at a distance, and hinder them from grappling with each other. (p.80-1)

I hope, therefore, to make it appear that all men have ever agreed in the doctrine both of necessity and of liberty, according to any reasonable sense, which can be put on these terms; and that the whole controversy, has hitherto turned merely upon words. We shall begin with examining the doctrine of necessity. (p.81)

Hume redefines "necessity"
Hume himself has "affixed different ideas" to the terms in this ancient debate - both "indifference"/chance and "spontaneity"/determination in the Treatise and only the latter in the Enquiry. For a moment Hume seems to realize that "both" liberty and necessity are involved, but he soon decides that the only "reasonable sense" for liberty is to make it consistent with a uniform nature of necessity and causation.

Hume redefines the term "necessity" to describe the inference of the human mind that discovers causality in the regular succession of events, that postulates "uniformity of nature" to assume that the laws of nature will continue tomorrow to be the same as today, and even to describe the assumption that we can predict future behaviors of an agent based on our observations of the agent's habitual behaviors.

While this is true, modern uses of Hume's word "necessity" lead many philosophers to misunderstand Hume. Today we should say that the empirical observations of all these regularities only justify our assigning high probabilities to such predictions, and never the "certainty" that is associated with a physical causal determinism or a logical necessity. Hume's usage may be closer to the eighteenth-century use of the terms "moral necessity" or "moral certainty."

Indeed, now that quantum mechanics has shown that the laws of nature are fundamentally probabilistic, Hume's insight "that the common distinction betwixt moral and physical necessity is without any foundation in nature" is seen to be correct, though perhaps not exactly in the way that Hume hoped. There is evidence that Hume's "necessity" was only such a high probability.

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 these objects...Inference and reasoning concerning the operations of nature would, from that moment, be at an end; and the memory and senses remain the only canals, by which the knowledge of any real existence could possibly have access to the mind. Our idea, therefore, of necessity and causation arises entirely from the uniformity observable in the operations of nature, where similar objects are constantly conjoined together, and the mind is determined by custom to infer the one from the appearance of the must follow, that all mankind have ever agreed in the doctrine of necessity, and that they have hitherto disputed, merely for not understanding each other. (p.81-2)
Hume is cautious and circumspect. He knows that perfect uniformity has never been seen. Agents may act differently even in the same circumstances.
We must not, however, expect that this uniformity of human actions should be carried to such a length as that all men, in the same circumstances, will always act precisely in the same manner, without making any allowance for the diversity of characters, prejudices, and opinions. Such a uniformity in every particular, is found in no part of nature. On the contrary, from observing the variety of conduct in different men, we are enabled to form a greater variety of maxims, which still suppose a degree of uniformity and regularity. (p.85)
Despite the "variety of conduct in different men," Hume seriously over-exaggerates the workings of necessity in human affairs. He claims history and politics would not be sciences without it (they are of course not sciences in the sense Hume wants), and that morals would be undermined as well. Science and action of any kind demands necessity, he mistakenly claims, but the high probability of adequate determinism is good enough.
We do not have such reason
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?

Philosophers entertain many opinions
Nor have philosophers even entertained a different opinion from the people in this particular. For, not to mention that almost every action of their life supposes that opinion, there are even few of the speculative parts of learning to which it is not essential. What would become of history, had we not a dependence on the veracity of the historian according to the experience which we have had of mankind? How could politics be a science, if laws and forms of government had not a uniform influence upon society? Where would be the foundation of morals, if particular characters had no certain or determinate power to produce particular sentiments, and if these sentiments had no constant operation on actions? And with what pretence could we employ our criticism upon any poet or polite author, if we could not pronounce the conduct and sentiments of his actors either natural or unnatural to such characters, and in such circumstances? It seems almost impossible, therefore, to engage either in science or action of any kind without acknowledging the doctrine of necessity, and this inference from motive to voluntary actions, from characters to conduct. (p.89-90)
Hume's idea of the "nature" of a person is that we can reliably predict the behaviors of someone based on their past behaviors, because their actions are normally "determined" by their character and motives. This is the basis of the second stage in our two-stage model of free will.

A careful reading shows that Hume backs away from strict necessity and says the inferences are only probabilistic, with certainty only "more or less."

Above one half of human reasonings contain inferences of a similar nature, attended with more or less degrees of certainty proportioned to our experience of the usual conduct of mankind in such particular situations. (p.91)
Hume wonders how mankind continues to believe in absolute freedom (that some actions are not caused by existing character, that some motives might be new and free from prior habits) when they acknowledge (his kind of) necessity. He might have made the occasional freedom of an undetermined liberty one of those things that cannot be "known" but that humans naturally believe as a matter of moral sentiment.

I have frequently considered, what could possibly be the reason why all mankind, though they have ever, without hesitation, acknowledged the doctrine of necessity in their whole practice and reasoning, have yet discovered such a reluctance to acknowledge it in words, and have rather shown a propensity, in all ages, to profess the contrary opinion. The matter, I think, may be accounted for after the following manner. If we examine the operations of body, and the production of effects from their causes, we shall find that all our faculties can never carry us farther in our knowledge of this relation than barely to observe that particular objects are constantly conjoined together, and that the mind is carried, by a customary transition, from the appearance of one to the belief of the other.
Hume says we naturally believe in causation, even if it cannot be proved
But though this conclusion concerning human ignorance be the result of the strictest scrutiny of this subject, men still entertain a strong propensity to believe that they penetrate farther into the powers of nature, and perceive something like a necessary connexion between the cause and the effect. When again they turn their reflections towards the operations of their own minds, and feel no such connexion of the motive and the action; they are thence apt to suppose, that there is a difference between the effects which result from material force, and those which arise from thought and intelligence.

But being once convinced that we know nothing farther of causation of any kind than merely the constant conjunction of objects, and the consequent inference of the mind from one to another, and finding that these two circumstances are universally allowed to have place in voluntary actions; we may be more easily led to own the same necessity common to all causes. And though this reasoning may contradict the systems of many philosophers, in ascribing necessity to the determinations of the will, we shall find, upon reflection, that they dissent from it in words only, not in their real sentiment. Necessity, according to the sense in which it is here taken, has never yet been rejected, nor can ever, I think, be rejected by any philosopher. It may only, perhaps, be pretended that the mind can perceive, in the operations of matter, some farther connexion between the cause and effect; and a connexion that has not place in voluntary actions of intelligent beings. Now whether it be so or not, can only appear upon examination; and it is incumbent on these philosophers to make good their assertion, by defining or describing that necessity, and pointing it out to us in the operations of material causes. (p.92-3)

Hume hopes to "reconcile" freedom and necessity.

But to proceed in this reconciling project with regard to the question of liberty and necessity; the most contentious question of metaphysics, the most contentious science; it will not require many words to prove, that all mankind have ever agreed in the doctrine of liberty as well as in that of necessity, and that the whole dispute, in this respect also, has been hitherto merely verbal. For what is meant by liberty, when applied to voluntary actions?

In the two-stage model of free will, liberty is in the first stage, creating free thoughts, willed actions are of course adequately determined (not logically necessitated nor pre-determined by earlier events) by our reasons, motives, and for Hume especially, by our feelings
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; this 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.

Whatever definition we may give of liberty, we should be careful to observe two requisite circumstances; First, that it be consistent with plain matter of fact; secondly, that it be consistent with itself. If we observe these circumstances, and render our definition intelligible, I am persuaded that all mankind will be found of one opinion with regard to it. (p.95)

But since mankind has always combined chance and necessity in ideas of freedom, Hume must abolish chance
It is universally allowed that nothing exists without a cause of its existence, and that chance, when strictly examined, is a mere negative word, and means not any real power which has anywhere a being in nature. But it is pretended that some causes are necessary, some not necessary. Here then is the advantage of definitions. Let any one define a cause, without comprehending, as a part of the definition, a necessary connexion with its effect; and let him show distinctly the origin of the idea, expressed by the definition; and I shall readily give up the whole controversy. But if the foregoing explication of the matter be received, this must be absolutely impracticable. Had not objects a regular conjunction with each other, we should never have entertained any notion of cause and effect; and this regular conjunction produces that inference of the understanding, which is the only connexion, that we can have any comprehension of. Whoever attempts a definition of cause, exclusive of these circumstances, will be obliged either to employ unintelligible terms or such as are synonymous to the term which he endeavours to define. And if the definition above mentioned be admitted; liberty, when opposed to necessity, not to constraint, is the same thing with chance; which is universally allowed to have no existence. (p.96)

For Hume, there was no such thing as chance. Human ignorance leads to all our ideas of probability. This was the view of all the great mathematicians who developed the calculus of probabilities - Abraham de Moivre before Hume and Pierre-Simon Laplace after him. And, following de Moivre, Hume called chance a mere word.
Though there be no such thing as Chance in the world; our ignorance of the real cause of any event has the same influence on the understanding, and begets a like species of belief or opinion.
(Enquiry, Book VI, Of Probability, p. 56)
The only proper object of hatred or vengeance is a person or creature, endowed with thought and consciousness; and when any criminal or injurious actions excite that passion, it is only by their relation to the person, or connexion with him. 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. According to the principle, therefore, which denies necessity, and consequently causes, a man is as pure and untainted, after having committed the most horrid crime, as at the first moment of his birth, nor is his character anywise concerned in his actions, since they are not derived from it, and the wickedness of the one can never be used as a proof of the depravity of the other.

Men are not blamed for such actions as they perform ignorantly and casually, whatever may be the consequences. Why? but because the principles of these actions are only momentary, and terminate in them alone. Men are less blamed for such actions as they perform hastily and unpremeditatedly than for such as proceed from deliberation. For what reason? but because a hasty temper, though a constant cause or principle in the mind, operates only by intervals, and infects not the whole character. Again, repentance wipes off every crime, if attended with a reformation of life and manners. How is this to be accounted for? but by asserting that actions render a person criminal merely as they are proofs of criminal principles in the mind; and when, by an alteration of these principles, they cease to be just proofs, they likewise cease to be criminal. But, except upon the doctrine of necessity, they never were just proofs, and consequently never were criminal.

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.

Hume on the Determinism Objection
Most compatibilists and determinists since Hobbes and Hume never mention the fact that a causal chain of events going back before our birth would not provide the kind of liberty they are looking for. But Hume frankly admits that such a causal chain would be a serious objection to his theory.
I pretend not to have obviated or removed all objections to this theory, with regard to necessity and liberty. I can foresee other objections, derived from topics which have not here been treated of. It may be said, for instance, that, if voluntary actions be subjected to the same laws of necessity with the operations of matter, there is a continued chain of necessary causes, pre-ordained and pre-determined, reaching from the original cause of all to every single volition, of every human creature. No contingency anywhere in the universe; no indifference; no liberty. While we act, we are, at the same time, acted upon.
To escape this objection, we might imagine that Hume wanted some kind of agent-causal freedom in voluntarist acts.
Hume's Natural Belief in Free Will
Although Hume was aware that we can not prove causality from any number of empirical examples of constant conjunction (just as we cannot logically prove the existence of external things), he nevertheless asserts that we have a natural belief in causality and external things.

Might Hume have similarly defended a natural belief in free will and its moral companion - responsibility? Since most of his contemporaries who defended free will were theists, might Hume have thrown the free will baby out with the theistic bath water?

Hume's Association of Ideas

18. It is evident that there is a principle of connexion between the different thoughts or ideas of the mind, and that, in their appearance to the memory or imagination, they introduce each other with a certain degree of method and regularity. In our more serious thinking or discourse this is so observable that any particular thought, which breaks in upon the regular tract or chain of ideas, is immediately remarked and rejected. And even in our wildest and most wandering reveries, nay in our very dreams, we shall find, if we reflect, that the imagination ran not altogether at adventures, but that there was still a connexion upheld among the different ideas, which succeeded each other. Were the loosest and freest conversation to be transcribed, there would immediately be observed something which connected it in all its transitions. Or where this is wanting, the person who broke the thread of discourse might still inform you, that there had secretly revolved in his mind a succession of thought, which had gradually led him from the subject of conversation. Among different languages, even where we cannot suspect the least connexion or communication, it is found, that the words, expressive of ideas, the most compounded, do yet nearly correspond to each other: a certain proof that the simple ideas, comprehended in the compound ones, were bound together by some universal principle, which had an equal influence on all mankind.

19. Though it be too obvious to escape observation, that different ideas are connected together; I do not find that any philosopher has attempted to enumerate or class all the principles of association; a subject, however, that seems worthy of curiosity. To me, there appear to be only three principles of connexion among ideas, namely, Resemblance, Contiguity in time or place, and Cause or Effect.

That these principles serve to connect ideas will not, I believe, be much doubted. A picture naturally leads our thoughts to the original: the mention of one apartment in a building naturally introduces an enquiry or discourse concerning the others: and if we think of a wound, we can scarcely forbear reflecting on the pain which follows it. But that this enumeration is complete, and that there are no other principles of association except these, may be difficult to prove to the satisfaction of the reader, or even to a man's own satisfaction. All we can do, in such cases, is to run over several instances, and examine carefully the principle which binds the different thoughts to each other, never stopping till we render the principle as general as possible. The more instances we examine, and the more care we employ, the more assurance shall we acquire, that the enumeration, which we form from the whole, is complete and entire.

Hume's Projectivism

It is a common observation, that the mind has a great propensity to spread itself on external objects, and to conjoin with them any internal impressions, which they occasion, and which always make their appearance at the same time that these objects discover themselves to the senses. Thus as certain sounds and smells are always found to attend certain visible objects, we naturally imagine a conjunction, even in place, betwixt the objects and qualities, though the qualities be of such a nature as to admit of no such conjunction, and really exist no where. But of this more fully hereafter.. Meanwhile it is sufficient to observe, that the same propensity is the reason, why we suppose necessity and power to lie in the objects we consider, not in our mind that considers them; notwithstanding it is not possible for us to form the most distant idea of that quality, when it is not taken for the determination of the mind, to pass from the idea of an object to that of its usual attendant.

Excerpts from the Works of David Hume
Treatise of Human Nature, Book II, Part III - The Will and Direct Passions (1739)

Of Liberty and Necessity, Chapter VIII, Enquiry Concerning Human Understanding (1750)

Letter from a Gentleman (1745)

For Scholars
Hume's Natural Belief in Free Will
In his Freedom and Belief, Galen Strawson writes
It is interesting that Hume does not in the case of freedom explicitly adopt his characteristic double position: that of inevitable philosophical scepticism about true responsibility on the one hand, and equally inevitable commitment to natural belief in true responsibility on the other hand. It is especially interesting because the case for scepticism about true responsibility is essentially stronger than the case for scepticism about, say, the existence of the exteranal world. For in the latter case what philosophy establishes is only that we cannot know that the external world does exist, not that we can know that it does not exist. Whereas in the case of responsibility the stronger conclusion does seem available. The reason Hume does not explicitly adopt the double position in this case is perhaps one of caution — although it is at least as much a desire to indulge in some heavy irony at the expense of theists: for he states the deeper objection to belief in true responsibility in indirect, theological terms, when he could equally well have stated it in terms of godless determinism.37

37 Cf. Enquiry, pp. 99-103. It could be argued that although Hume does not explicitly adopt his double position, it is there in essentials, connected to his moral subjectivism. Hume was surely aware of the sense in which true responsibility is impossible, God or no God. And he was, surely, aware of our deep commitment to belief in true responsibility, for it is built in to our natural disposition to praise and blame and to distinguish vice and virtue in actions, and these distinctions are "founded in the natural sentiments of the human mind, [which are] not to be controuled by any philosophical theory or speculation whatsoever" (p. 103).

Thus Hume is a source for P. F. Strawson famous thesis
He would certainly have agreed that we cannot give up belief in true responsibility, even if true responsibility is impossible, and not (like the existence of the material objects) just unprovable: "it is a point, which we must take for granted in all our [practical] reasonings" to adapt what he says about belief in material objects (Treatise 1.4.2, p. 187).

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