What, in a nutshell, is Hume's philosophy? The question has not readily produced answers. Studies of Hume's philosophy abound, but it is not uncommon for these to discuss Hume's arguments at great length without discerning a central thread, and thereby without providing an answer of the right kind. Part of the explanation for this state of affairs is the modern analytic philosopher's tendency to think of philosophy as a matter of arguments, and therefore of great philosophy as great arguments. So a great, dead philosopher proves himself so by the arguments contained within his works—rather than by the fundamental philosophical idea which his arguments are intended to serve. One consequence of analytic philosophy, then, is that it has tended to read the mighty dead by concentrating on the detail on the page, rather than stepping back to consider what might be the point or tendency of a given philosopher's corpus of arguments.

In the case of Hume there is a further factor. It has seemed to many of his critics that there just is no such fundamental idea underpinning his arguments, and so no neatly-summarisable outlook that can be called Hume's philosophy; unsurprisingly, then, they make no attempt to discover what they doubt to exist. The central cause of doubt is Hume's apparent inconsistencies, such that he presents not one philosophy, but many; or, alternatively, many fragments of possible philosophies, in the shape of arguments that, whatever their brilliance, do not all point in the same direction. Thus, for example, it is pointed out that, in the Treatise of Human Nature, the most widely-studied of his works, apparently-contradictory passages abound, sometimes in close proximity to each other. Moreover, some of the most popular interpretations of the dense pages of Treatise Book I render virtually all of Hume's subsequent writings—including the later Books of the Treatise itself—indefensible. Problems such as these drove the more charitable of past interpreters to despair, or at least to the view that Hume gave up philosophy after the arguments of Book I cut the ground from under his own intended literary career.Footnote ¹ The less charitable simply dismissed him as a charlatan.Footnote ²

Views expressing such perplexity or extremity are now rare. The principal antidote has been a series of excellent modern studies of Hume's corpus, which, in their various ways, discern a coherent and unified philosophical viewpoint therein. What is more, the bulk of these interpretations show an encouraging degree of convergence. If their more obvious differences are glossed over, a broad interpretative consensus is discernible: the once-common view that Hume's philosophical conclusions are destructively Pyrrhonian is resisted, and in its place is presented a picture of a moderately-sceptical philosophy with a naturalistic core (or, a broadly naturalistic philosophy with moderately sceptical implications).Footnote ³ This broadly-construed consensus will not be challenged here—but it will be given a fairly strong tweak. It will be argued that Hume's philosophy can be pinned down much more accurately: that it is, in a nutshell, sceptical materialism.

The claim will immediately invite two objections. The first will come from advocates of older views of Hume's thought, who see no reason to give way to new-fangled naturalist interpretations—and especially not this one. This objection is that, as was once commonly held, Hume is an Idealist, and therefore not any kind of materialist.Footnote ⁴ The second objection is internal rather external criticism: it is that scepticism and materialism are mutually exclusive categories, so their conjunction is absurd. These objections will be considered before moving on to the positive case.

Idealism or Realism?

The Idealist interpretation arises naturally from remarks Hume makes in a number of passages in the Treatise. These seem not only to invite, but to require, Idealism. A representative example is the following:

Here Hume seems to be affirming a version of Berkeley's Idealism: everything we experience is in the mind, in our imagination; and no matter how independent of us we suppose objects to be, they are in fact nothing more than our own conceptions, and as such are mental after all.

However, passages such as this cannot be taken to settle anything, because there is no shortage of other passages, especially in the first Enquiry but also in the Treatise, which point towards quite opposed conclusions. For example, just a few pages before the above quotation we read the following:

Here we see a resolutely realist view, in which the perceptions with which we must deal set a limit to our knowledge, because they cannot get us in touch with the real world that causes them. We must ‘content’ ourselves with what we can know, and must recognize that the ‘nature of bodies’ and their ‘secret causes’ all lie ‘beyond the reach of human understanding’.

Nor is this merely a momentary lapse on Hume's part. He returns to the point later, in the Appendix to the Treatise, which appeared with Book III in 1740. Commenting on the above passage, he notes:

Like the passage it glosses, this is also plainly realist: perceptions are appearances of real objects, and the reason why we should stick to the appearances is not because that is all there is, but because by so doing we can avoid making mistakes. In short, it is epistemological rather than metaphysical in intent. What we are encouraged to avoid is not belief in an external reality, but speculative claims about its underlying nature, its causes and effects.

As such, these two passages have a decidedly Newtonian (not Berkeleian) air. They repeat Newton's strictures against ‘hypotheses’, or occult principles—his insistence that we must keep to manifest principles if we are to hope for success in the conduct of enquiry.Footnote ⁸ In these passages, then, Hume is far from advocating any form of (metaphysical) Idealism, in which ideas (and the minds which have them) exhaust what there is. Instead, he is claiming, with Newton, that our perceptions are only appearances of a real world whose ‘nature and operations’ remain hidden from us, and that therefore progress in knowledge can only be made by keeping this firmly in mind; in short, by restricting our claims to the observable.

Is it possible to resolve these apparently-conflicting tendencies in Hume's thought? If we suppose him to be, at bottom, an Idealist, then resolution is not possible. Talk of what lies beyond the appearances of objects is then empty; indeed, it is muddle. So to affirm the Idealist interpretation means that one must accept that he is maddeningly inconsistent, as so many have supposed. (In fact, here we can see the source of the thought that he is inconsistent: it is the assumption of an underlying Idealism which leads to that conclusion.)

If, on the other hand, we suppose that Hume's philosophy is, at bottom, some species of realism, the contradictions can readily be made to disappear. This striking result is possible because, in Hume's day as in our own, there was and is an influential form of realism which implies that the objects of our knowledge are entirely distinct from the real things that cause them. This is the theory now known as representationalism; but in the early modern period it came to be known as ‘the way of ideas’ or ‘the theory of ideas’. It holds that real physical objects impinge, in various ways, on our sense-organs, thus causing us to have the perceptions we have of them; but, because the objects cause the perceptions, and because causes and effects are distinct occurrences,Footnote ⁹ the objects are quite distinct from the perceptions. Briefly (if not necessarily happily) put, the objects are in the world, the ideas are in the mind. Thus an uncomplicatedly realist account of the world and of perceptual processes can include, amongst its implications (or apparent implications), an idealist account of the objects of knowledge: metaphysical realism can imply epistemological idealism.

If we now look back at Hume's apparently (metaphysically) Idealist passage quoted above, we can see that it actually asserts no more than the epistemological idealism implied by the theory of ideas. Its point is that we can't go beyond our ideas, that our ideas are in our minds, and therefore that we can't get outside our minds: no matter how far afield we ‘send’ our thoughts, they in fact remain in our minds, in ‘the universe of the imagination’. The point is, in short, that the real world remains inaccessible to us—not that it does not exist.

Of course, to affirm such a view is immediately to face an objection: if the world is fully epistemologically inaccessible, then we have no grounds for holding it to be there at all. So it is, or at least seems, that the theory of ideas leads directly to fully-fledged metaphysical Idealism. This is precisely Berkeley's point. But the evidence for taking it also to be Hume's depends on taking his apparently-Idealist remarks as if genuine commitment to Idealism. However, what such remarks commit him to is, strictly speaking, nothing more than the theory of ideas; and, on internal grounds, he should be understood to mean no more than this, since, if we so understand him, his apparently-contradictory vacillations between Idealism and realism disappear, and a consistently realist outlook emerges.

This conclusion has found it difficult to achieve widespread support. The reason, I suspect, is because it is thought that to accept the theory of ideas is to make the Idealist conclusion unavoidable; that Hume did accept the theory; and therefore did not avoid the conclusion.Footnote ¹⁰ However, whatever the merits of this syllogism, it is not hard to see that Hume saw things in a more complex light. To mention a remark to which we shall return, he said of Berkeley's arguments that they ‘admit of no answer and produce no conviction’.Footnote ¹¹ In saying this he was not offering merely a psychological report, but making a philosophical point intended to raise doubts about Berkeley's Idealist conclusion. In short, it may be argued that Hume should have embraced Idealism, but our concern here is whether he did so in fact. It may be that, by building the theory of ideas into a realist philosophy, he maintained a position marked by a fatal flaw.Footnote ¹² But this is no reason to think that it was not a realist philosophy that he advanced.Footnote ¹³

Moreover, if it is accepted that he does advance a realist philosophy, then several further claims about the character of that philosophy become plausible. The first is that, because it is committed also to the epistemological idealism of the theory of ideas, Hume's philosophy is readily-describable as sceptical realism, because it affirms the existence of an external reality while leaving us in the dark about its essential nature. This darkness is not total, however; it remains possible to affirm that the external reality is physical reality, and so is made up of material things; and, further that these things conform to the principles of motion and connection, as Newton had shown. To accept this is then to open up a further possibility: if one can show the same, or strongly analogous, conformities to obtain in the mind which processes the ideas caused in it by the external reality, then it can reasonably be concluded that the nature of the mind is not of a kind alien to material reality, and so is itself also best understood to be material. This viewpoint, because it affirms materiality as the best explanation for mind as well as matter—while prescinding from any dogmatic claims about the essential nature of matter itself—could thus be called sceptical materialism, and it is what is meant here by the claim that Hume's philosophy is a sceptical materialist philosophy.

At this point, however, the relevant issue is not whether this interpretation is correct, but to show that it is no great jump from recognizing the most reasonable harmonization of Hume's apparently-contradictory remarks to arriving at a sceptical materialist interpretation of his philosophy. Indeed, this line of thought suggests that sceptical materialism might be a coherent philosophical position. Is that suggestion justified? This brings us to the second of the objections from which we began.

Is the Idea of Sceptical Materialism Self-Contradictory?

Is the very idea of sceptical materialism self-contradictory? The remarks above are meant to indicate why it might not be—and, indeed, to indicate what such a philosophy might be like—but it will be more useful at this point to step back from the interpretation of Hume himself, and to consider the question in general. I suggest that the reason why the term might seem self-contradictory is because of the twin thoughts that the sceptic denies the reality of the external world, and that the materialist both affirms its reality and, indeed, tells us what that reality is. In these senses of the terms, it is plain that they are incompatible positions. Equally plainly, however, if different senses are allowed, there need be no difficulty in the matter.

This is particularly relevant with respect to the term ‘scepticism’. It has a long history, and throughout most of that history it did not imply the denial of external reality. Indeed, as the very term ‘external reality’ indicates, scepticism thus understood reflects a philosophical outlook in which the standpoint of the individual subject is accorded not merely epistemological, but ontological, priority. This is the common understanding of Descartes' philosophy as developed in his famous method of doubt; and the conception of scepticism as the denial of external reality owes its career to the influence of the Cartesian philosophy.

But ‘scepticism’ is also the name of an ancient philosophical school, and for the members of that school, to be a sceptic was to be resistant to the knowledge claims made by the major philosophical schools: Platonism, Aristotelianism and Stoicism. These schools, whether rationalist or empiricist, made metaphysical claims about the nature of reality, and did so on the basis of epistemologies which explained how those conclusions were acquired. Although they differed on the details, what the epistemologies all provided was an account of how the mind came to grasp the intelligible natures—or essences—of things. Knowledge was understood as the grasping of the thing's essence.Footnote ¹⁴ The sceptics simply denied that any such grasping was possible: they denied that there was any way of getting past the appearances of things. They offered a range of arguments designed to show that our conceptions of things varied according to variable perceptions, and as such remained limited by those perceptions; that is, by appearances.Footnote ¹⁵ They concluded that we can live only according to opinion; that knowledge is impossible because it concerns essential natures; and essential natures, whatever they might be, remain beyond human reach.

Two features of this ancient scepticism are important here. The first is that it is essentially an epistemological position: the reality of the world is not its focus of attention, and, given its conception of knowledge as concerned with the nature, not the being, of the thing, epistemological doubts do not readily flow on to become metaphysical doubts. The second is that, thus construed, scepticism need have no deep quarrel with materialism. In fact, materialism may well provide the best explanation for why we are trapped at the level of appearances. If knowledge is the grasping of an intelligible essence, but perception is merely an interaction between two material objects, or worse, between their surfaces, it becomes unclear how intelligibility could be transferred from the object to the subject. Of course, it all depends on just what intelligibility is. In the main philosophical schools of the ancient world, it was understood in terms of the divine power of reason; in particular, in reason's ability to recognize itself. Thus knowledge of nature depended on the rational mind's ability to recognize the inner rationality of the natural order: that is, on its ability to recognize the essences themselves, and the logical relations that obtained between them. A short sketch of the main schools will bring this out.

Plato and Aristotle, for all their differences, agreed in dividing reality into matter and form, and identified reason with the latter.Footnote ¹⁶ So, according to their views, reason is sharply distinct from the materiality of things and the processes to which matter is subject. On these views, then, intelligibility, and thus knowledge, depends on denying the materialist's restriction of reality to material things. The Stoics did not oppose reason to materiality, but, nevertheless, ended up affirming the special nature and role of reason. They identified reason and thus intelligibility with something material: the element of fire, which permeates all living things (including the cosmos itself). To this extent, then, they treat reason as one part of the material world. Beyond this difference, however, they line up with Plato and Aristotle: reason is, on their account, a distinct and special reality, because fire is also: it is the vital force, the divine element permeating the cosmos; in its purest form it is self-moving soul.Footnote ¹⁷ So, even for the Stoics, reason is not just one kind of matter amongst others, but a distinctive and divine medium which, by permeating the cosmos, renders (ordinary, non-divine) matter intelligible. So, although they replace the Platonic and Aristotelian appeals to non-material form with a material cause, that material cause is so distinctive that they differ from their illustrious forebears far less than may initially appear.

In contrast to this broad consensus lies the uncompromising materialism of the philosophical atomists. On their view, everything reduces entirely to ‘atoms and the void’:Footnote ¹⁸ to affirm this slogan was to deny both formal realities and all claims in favour of any privileged kind of stuff, such as the Stoics' allegedly divine fire.Footnote ¹⁹ But what, on this view, can then remain of reason and intelligibility? The Epicurean response is striking: to dethrone reason by breaking the link between reason and knowledge. They held, instead, that the criterion of truth is sense—and that reason is entirely dependent on it.Footnote ²⁰ This means, in effect, that they redefined knowledge in terms of appearances, for what is sensed is appearance, not some hidden essence. This, in turn, suggests that their differences from the sceptics, in particular from the moderate or Academic sceptics, masked some significant affinities. For the sceptics' denial of knowledge began from accepting the Stoics' conception of knowledge as the grasping of essences: that denial was thus the denial that any such grasping was possible; it was not the denial of true beliefs concerning how things appear.Footnote ²¹ Plainly, such a view is not a long way distant from a theory which affirms knowledge but bases it in what is sensed: the primary difference between the sceptic and the Epicurean, from this angle, may be little more than the degree of pessimism or optimism about what can be established on this basis. It seems reasonable to conclude, then, that, in the ancient world, scepticism and the more reductive forms of materialism could be seen (and could see themselves) as allies: two different theories which agreed on starting-points if not on how far they could be taken, and which were united in their opposition to the allegedly appearance-overcoming knowledge-claims purveyed in the major philosophical schools.Footnote ²²

All of this might seem to be just ancient history, and irrelevant to explicating an early modern philosophical outlook. However, this would be a mistake, because it is falsely to suppose a great divide between the ancient and early modern intellectual worlds. The early modern philosophical world certainly thought of itself as a new beginning, engaged in forging a new foundation for the revolutionary changes in natural science. But that new beginning was not ex nihilo; it proceeded by adapting old tools to new purposes. In fact, it thought of itself, by and large, as the continuation of the Renaissance, as the cutting-edge of the revival of learning, ‘the revival of letters’.Footnote ²³ As such, it was soaked in the literature of the ancient world, and utilized the categories and concepts of ancient schools of thought in order to forge its new philosophies. Thus Descartes, for example, modelled the Meditations on First Philosophy on the allegory of the cave to signal his Platonic debts.Footnote ²⁴ Locke, to signal his opposition to such theses, employed recognizably Stoic turns of phrase, including his claim that the mind is initially like blank paper.Footnote ²⁵ He also defended the possibility of materialism by employing some recognizably Epicurean doctrines and arguments—for example, in his account of the nature of matter and of primary and secondary qualities, and in his (revised) explanation of action.Footnote ²⁶ Moreover, he wedded this quasi-materialist view to a species of scepticism, by denying that we can have access to the real essences of things; he even went so far as to class the attempt to gain philosophical knowledge of nature's essences as nothing but ‘lost labour’.Footnote ²⁷

It is also true of Hume. His familiarity with ancient authors is obvious to anyone who ventures beyond the Treatise, but even there its influence is discernible in its passage from (implicitly) materialist explanations to sceptical conclusions. An interpretative sketch of the argument of Book I of the Treatise will serve to bring these materialist aspects to the fore.

Treatise Book I: An Interpretative Outline

The Treatise opens with a very brief, but very bold, set of claims about the contents and functioning of the mind. Ideas are paler copies of vivid impressions. In fact, they differ only in degree of vividness. This means that all ideas are images, and that the mind is thus under ‘the empire of the imagination’.Footnote ²⁸ It also means that mental contents lack any mark of their truthfulness. Vivacity is not an empiricist criterion of truth, as clarity and distinctness is for the rationalists; it is merely a measure of perceptual intensity, and suggests a bodily origin.Footnote ²⁹ Although there are some ideas that do not depend on prior impressions—e.g. the missing shade of blue—these arise because of inertial tendencies in the mind, and so are not candidates for innate epistemic criteria. Ideas are not, then, reliable indicators of truth; and this unreliability is further implied by their being connected through associations, which are non-rational processes in the imagination rather than rational connections.

Ideas of space and time arise not by directly copying impressions of space and time, but by copying the order in which impressions present themselves. As such, they show how the theory deals with a potential hard case. But the further point is that they show that ideas are limited by their origination in impressions, and thereby impose limits on the sorts of theories we can accept. Like all ideas, ideas of space and time are images, and so they rule out ideas and theories in which non-imagist ideas are implied: such as the idea of a vacuum, and of theories of infinite divisibility. These cannot be imagined (pictured), and so cannot be entertained. The message is that, whatever the ultimate nature of reality, the mind has limits which theories cannot escape. It is also error-prone: the connecting of ideas by associations explains our vulnerability to errors in conceiving and in reasoning. Associations themselves may be supposed to be deviant motions in the brain.Footnote ³⁰

Beliefs—and even judgements—are merely vivid ideas, not rational conclusions. Beliefs about the future arise from transferring past experiences to the future. This is an inertial tendency of the mind, which we call custom or habit.Footnote ³¹ Where past experience has been uniform, the future will be believed to be likewise uniform, and conforming events will therefore be thought to arise necessarily. The idea of causation, so central to all conclusions drawn from experience, thus arises instinctively, and depends on no insight into why things happen. Where uniformity in experience is lacking, the relative frequencies of contrary experiences will determine expectations through an opposition of forces,Footnote ³² and these we express as probabilities. Reason has nothing to do with these processes: any attempt to show that it does depends on a circular argument. So ‘reasoning’ in this sense (making inferences from experience) is really a kind of instinct. It is possessed by animals no less than by humans. The powers traditionally ascribed to the faculty of reason are entirely resolvable into either sense-perceptions or habits—and so into non-intellectual powers.Footnote ³³

Furthermore, if we were to rely on reason alone, we would find all our beliefs undermined, because caught in an infinite regress of ever-diminishing probabilities. We only escape total scepticism because the non-rational faculties of human nature oppose reason's tendencies. For example, our belief in the reality of the external world is due to ‘smooth transitions’ of the imagination (that is, to an inertial tendency). All our fleeting but resembling impressions are thereby connected to create the belief in enduring objects. In other ways, too, the attempt to rely on reason has borne false fruit: ancient science went astray by reading into nature the emotions we observe in ourselves; and modern science, because necessarily limited by imagist ideas, is unable to give an adequate account of the distinction on which it relies, between primary and secondary qualities.

Metaphysical disputes give comfort to materialism. Dualists' attempts to explain the nature of thought become enmired in paradoxes, so the materialist alternative—that thought is matter in motion—is the more plausible of the two.Footnote ³⁴ Our idea of the self, which may seem an obstacle to materialism because it seems to be the immediate experience of a non-material reality, is nothing more than a construct of experience, put together by associative processes. Finally, despite the optimism of providentialist metaphysicians, the human condition seems to be an unhappy one, since rational and other tendencies in human nature often come into conflict. In particular, the life of the rational enquirer seems doomed to misery: our rational capacities are weak, like a leaky vessel, ill-fitted to resist the storms and shoals of life. Nevertheless, the philosophical life, if pursued with the diffident spirit of the true sceptic, is to be preferred to superstition and its attendant vices.

This concludes the brief sketch of Book I of the Treatise. Its sometimes dramatically sceptical conclusions emphasise the limits and uncertainty of all that we claim to know: it is an extended attack on the much-vaunted rational powers of the human mind. Hume's aim is not, however, simply destructive; he does not mean us to conclude that, because some aspect of human nature is not based in reason, it is therefore without a justification of which it stands in need. To conclude thus would be to affirm precisely what he wants us to reject: the conception of the human being as the rational animal, the reason-governed animal—and so lost if reason should fail. His aim is, on the contrary, to shift our conception of ourselves away from that ancient model. He wants, in modern terms, to naturalize human nature by showing it to fit patterns of explanation consistent with a scientific materialism.

Faculty Psychology and Its Meaning

The clue to seeing Hume's arguments in this light is through recognizing the centrality of faculty psychology in early modern thought. Early modern philosophers divided up the mind's powers according to categories derived from Aristotle—sensation, memory, imagination, and reason (or intellect)—and they attributed precise meanings to these terms. The first three all concern images, broadly conceived;Footnote ³⁵ all depend on bodily organs or processes; in consequence, all three are shared by animals and humans alike. Reason, however, is unique to the human being: this is the meaning of the Aristotelian definition of the human as the rational animal. To the early modern rationalists, reason did not depend on images, and resided in the immaterial soul. It differed from other faculties in that its capacity to know—to discern the essences of things—showed it to be oriented to truth, rather than merely to utility, as were the other, bodily-based, faculties.Footnote ³⁶

Plainly, then, one obvious avenue for making a case for materialism would be to deny that there is any such distinct faculty of reason, and to argue that activities commonly attributed to such a faculty can be more plausibly explained as activities of one or other of the bodily-dependent faculties. This would also call in question the necessity of positing an immaterial part of the human being, and so the grounds for dualism and for confidence in human knowledge would be challenged together.Footnote ³⁷ In the early modern world, as in the ancient, to propose materialism was, simultaneously, to create room for sceptical doubts.Footnote ³⁸

For the early modern materialists, the preferred replacement for the faculty of reason was the imagination. This faculty was recognized, on all sides, to be dependent on bodily processes, and, unlike the passively receptive senses and memory, it was active in the necessary respect, being able to form new ideas from the raw materials supplied by sense and memory. These two features are visible in a standard definition of Hume's day, in Ephraim Chambers' Cyclopaedia of 1728. He defines the imagination as:

The writings of the major seventeenth-century philosophers—for example, Descartes, Hobbes, Malebranche and Leibniz—indicate a similar understanding of the term.Footnote ⁴⁰ So there is every reason to think Chambers' definition captured orthodoxy. This means that the imagination was recognized to be the mental activity produced by bodily processes, in contrast to the pure mental activity of the faculty of reason or intellect. The further implication is that the role it was allocated, in the various theories of the day, was a litmus test for those theories' materialist sympathies or hostilities. Thus Malebranche affirmed his Cartesianism by arguing for the necessity of ruling the senses and imagination by the intellect, whereas the materialists Hobbes and La Mettrie, despite their writings being separated by a full century, both held that the (so-called) intellect is really nothing but the imagination.Footnote ⁴¹

Against this background, Hume's strategy falls into place. His attack on the faculty of reason is a clear, if indirect, attack on Cartesian dualism. But he does not stop there; by offering alternative explanations in terms of the processes of the imagination he makes it sufficiently plain that he is making a case for materialism. This second, positive, aspect has already been noted. It is summed up in his remark, in the Abstract, that his philosophy places us under ‘the empire of the imagination’.Footnote ⁴² This means, of course, that we are not under ‘the empire of reason’, and so not the rational animals of philosophical tradition. But it also means that human thought and action is fully explicable by reference to, and so conceivably entirely dependent on, bodily processes. It amounts, then, to the claim that materialist-style explanations of the human mind leave nothing unexplained. The existence of an immaterial soul is not ruled out; but, if there is such a thing, it is left with nothing to do. From an explanatory point of view, the very idea is otiose; and if this is so, it is not clear what reason there could be to suppose that there is any such entity.Footnote ⁴³

Much the same message is implied by Hume's conclusion, at the very end of Book I, about the proper role of reason. He says: ‘where reason is lively, and mixes itself with some propensity, it ought to be assented to. Where it does not, it never can have any title to operate upon us.’Footnote ⁴⁴ The burden of this remark is to deny reason both autonomy and status, and so, again, to deny that it is a distinct faculty of the mind. If it does not yet reduce it to the status of slave—that has to wait until Book II—it plainly denies that reason is capable of independent functioning, let alone being the ruling faculty. So, whereas Locke had sought to show the possibility of materialism by ‘sensualising’ the faculty of intellect,Footnote ⁴⁵ Hume goes further and seeks to abolish it. When read through the lens of early modern faculty psychology, then, Book I of the Treatise of Human Nature is revealed as a sustained argument for the adequacy of materialist explanations for mental phenomena.

Nothing in Books II and II invite revision of this conclusion. In fact, their task is to work out the implications of the project for human action and social order. In Book II our reasoning capacity is subordinated to the internal mechanical responses to external stimuli: reason ‘is, and ought only to be the slave of the passions’.Footnote ⁴⁶ The passions themselves give rise to further ideas and impressions, so ideas are once again shown not to stand apart from impressions in some separate, higher, (rational) domain. Moreover, the changes are initiated by associations, so the account is once again consistent with materialism. The passions are the natural responses of a material being to external stimuli. Human reason, thus governed, responds to the exigencies of everyday circumstances, rather than to some overall telos of human life—to efficient rather than final causes.

Book III explains how this occurs. Hume there gives a historical account of the development of the human mind proceeding hand in hand with the development of necessary social institutions, the whole governed by the non-rational capacity for sympathetic identification with the concerns of others (a ‘moral sense’). This capacity is manifested directly in the case of natural virtues of benevolence, but it is also manifested indirectly—through an appreciation of its usefulness for human society, which usefulness is valued because of the sympathetic regard for the general good—in the steady development over time of the institutions of the artificial virtue of justice. The whole story is one of reason's limited role in reflecting on the concrete circumstances of life, its solutions surviving only in so far as they conform to the imperatives of our nature and circumstances. Once again, then, reason is cast as no more than a tool developed by a (clever) animal ruled by mechanical processes.Footnote ⁴⁷

Human Nature versus Human Understanding

If we locate Hume in the context in which he is more commonly placed, the Locke-Berkeley-Hume triad familiar to all students of epistemology, we can trace a different path to the same conclusion. That familiar context rightly brings out Hume's indebtedness to Berkeley's critique of Locke, and, by extension, of the modern scientific outlook that Locke was thought to have captured. But what, for him, was the message of that critique? His deployment of specific Berkeleian arguments against Lockean positions in parts of the Treatise has encouraged the thought that he intended merely to apply and extend the Berkeleian critique.Footnote ⁴⁸ This is, I think, to fail to see the forest for the trees; and drawing out the full significance of a remark in the first Enquiry is the best cure. Hume there makes a striking observation which we have already had occasion to note: he says that Berkeley's arguments ‘admit of no answer and produce no conviction’.Footnote ⁴⁹ That is, he claims that Berkeley's criticisms of Locke cannot be faulted on rational grounds, but that they nevertheless cannot persuade us of their truth. They show, in short, that human nature is too strong for reason. The problem is, how is that possible? It is this question that Berkeley's arguments raised for Hume. It is also a question for which there could be only one answer: the answer could only be that the human being is not the rational animal of philosophical tradition, but a being whose behaviour requires explanation by a causal psychology. So Hume's question becomes, what are the principles according to which the human mind functions—even in the face of irrefutable rational arguments? What is the nature of human nature?

This is the question to which the (appropriately-titled) Treatise of Human Nature offers an answer. The title indicates a shift of focus when compared to the works of his distinguished Anglophone forebears: Locke's Essay on Human Understanding and Berkeley's Treatise concerning the Principles of Human Knowledge both focus precisely on the rational, knowledge-directed functions of the mind, and, by this fact, and despite the limits they place on its contents, implicitly affirm the priority of the rational faculty. That priority is, however, Hume's target, and the title of the Treatise of Human Nature signals the fact.Footnote ⁵⁰

Experimental Philosophy and Materialist Explanations

The Treatise also declares itself on the side of ‘the experimental method of reasoning’, promising to extend this method into ‘moral subjects’—into, as we would now say, the human sciences. The work's Introduction makes clear what this means. The experimentalist is not simply committed to a certain kind of method, but is so committed because nature's ultimate principles are inaccessible. In short, the experimentalist denies that we can have knowledge of essences; we must therefore restrict ourselves, in our enquiries, to manifest principles. Experimentalism is thus a kind of practical scepticism, since, like scepticism, it denies that we can discover (hidden) essences. (Newton's famous attack on ‘hypotheses’ is one expression of this denial.Footnote ⁵¹) This means immediately that, if Hume is to be faithful to his professed experimentalism, he must resist commitment to philosophical dogma, materialist dogma included. He must therefore resist the temptation to explain manifest or observable features of the human mind by reference to unobservable motions of matter. Of course, if he has no materialist sympathies, he has no temptation to resist. So it is particularly striking that he does give in to temptation at one point, and even describes himself as doing so: the remarkable passage where he explains errors in reasoning by indirect motions of the animal spirits in the brain, motions which cause them to ‘rummage’ the wrong cell and thereby to call up the wrong idea.Footnote ⁵²

This might, of course, be dismissed as a mere isolated incident. The problem, however, is that the evidence of materialist sympathy is not reducible to examples, but is systematic. This is because a philosophy which restricts itself to manifest principles does not thereby rule out echoes of dogmatic philosophy. In fact, its choice of manifest principles may be thoroughly tendentious: the principles to which it appeals may be recognizable as the kind of principles characteristic of a particular philosophical school, or even of a representative of that school. This is true in Hume's case. As already pointed out, the ‘empire of the imagination’ he establishes has a powerful, if indefinite, materialist air. But the evidence does not stop there, because the specifics of Hume's philosophical explanations are also recognizably materialist.

This could be illustrated in a number of ways. One is indirect: by showing that Hume's critical engagement with Cartesian dualism, in the shape of Malebranche's Search after Truth, results in Hume's taking over important features of Malebranche's account of the (defective, bodily) imagination, and building them into his account of the natural workings of the human mind.Footnote ⁵³ The other is direct: by showing that Hume's positive doctrines faithfully reflect the details of known materialists. Thus one could examine La Mettrie's Machine Man, which gives a good idea of the main themes of materialist writing in Hume's day. (The book appeared in 1747, i.e. between the publication dates of the Treatise and first Enquiry.) La Mettrie's argument—at least, where it is more than a repetition of Lucretius—is that a human being is a machine that can move itself, ‘a machine which winds itself up’, because the mental powers by which it thinks and acts are all processes of the imagination. Even in its moral convictions, the human being reveals itself to be a machine, since these are simply inner feelings which belong to the imagination.Footnote ⁵⁴ There is a discernibly Humean air about such views; and, although the work appeared too late to be an influence on the Treatise, it reflects a materialist orthodoxy which was probably also represented in the samizdat materialist literature in circulation during Hume's time in France when composing the Treatise.Footnote ⁵⁵

There is no need, however, to pursue such indirect or obscure sources. There is a much better source readily available in Hume's own backyard. The fact is that the central features of Hume's account of mental functioning are all traceable directly to the materialist psychology of Thomas Hobbes. The resemblances are so striking that it seems possible to describe the Treatise as a sceptical revision of the Hobbesian project. In accordance with the requirements of experimental scepticism, Hume deletes the overt appeals to motions in the body, but he nevertheless accepts the basic Hobbesian picture of human functioning. In large part, he even follows Hobbes's order of exposition.Footnote ⁵⁶ This can be brought out by a thumb-nail sketch of the opening chapters of Leviathan.

Hobbes limits mental contents to what comes through the senses. Sense-perceptions cause motions in the body which are, for us, feelings and ideas. Ideas are internal motions caused by external motions, and so are quite distinct from the objects or motions that cause them. Internal bodily motions make us attribute our ideas to an external source. The imagination is the repository of images that linger in the body: it is ‘decaying’ sense. (So its contents are paler copies of sense-perceptions.) What is called ‘the understanding’ is not a distinct faculty of reason, but simply those imaginings due to words. It is, in other words, a power of the imagination; it is common to man and beast. Thought is connected imaginings. Transitions of thoughts reflect past sequences of sensations (past experience): present sensations are associated by the mind with past ones, and so a present sensation prompts the imagination to call up the whole string of sensations that followed the same sensation in the past. This generates expectations about the future. ‘Trains of thought’ even when unguided are not ‘wild ranging of the mind’, but connected by associations. Guided thoughts are governed not by reason but by desires, and concern causes and effects. When concerned with the future, they depend on the thought that like events will follow like actions—they are suppositions about the future based on experience of the past. Reasoning is just a calculative power; it is not a special faculty that equips us for discovering hidden truths, but a fallible human skill developed through experience. Human actions begin in passions, internal motions in the body caused by internal or external motions, which therefore display predictable patterns. Reason is a ‘scout’ for passion—its eyes and ears—and so its servant.Footnote ⁵⁷

Hume's theory of basic human function is a refinement of this Hobbesian picture. He adopts nearly all the main themes of Hobbesian psychology, even to the extent of building his account of our inductive practices and our conceptions of cause and probability out of Hobbes's account of the origin of expectations from the tendency to transfer past experience to the future. Moreover, Hobbes affirms a non-rational source for our belief in enduring external objects, and similarly subordinates reason to passion. In short, the basic outlines of Hume's account of the workings of the mind are traceable to Hobbesian materialist psychology.

Hobbesian Materialism and Humean Scepticism

Hume's experimentalism was sufficient reason for him to rule out Hobbes's dogmatism; but it is striking that a careful reader of Hobbes would have found reasons internal to Hobbes's own theory for doing the same thing. So it is possible that Hume found a second reason, provided by Hobbes himself, for resisting Hobbesian dogmatism. This is because Hobbes had insisted, in typical materialist vein, that, because there is no distinct faculty of reason oriented to discovering hidden truths or essences, the traditional idea of ‘right reason’ must be discarded. There is, he said, no such thing as ‘right reason’; there is only your reasoning and mine. However, he had simultaneously treated his own philosophy as if it were simply true—as if it were right reason itself! So he failed to face up to the implication of his own position: that his own arguments, no matter how cogent they seemed to him, were only his arguments, and so could not claim to possess some final authority. So a sympathizer with Hobbesian views alert to this problem could be expected to rewrite Hobbes's theory by purging it of its dogmatism—by accepting, indeed, that, by the theory's own lights, its central doctrines simply could not be known to be true.

One way in which such a sympathizer could proceed would be by showing the adequacy of Hobbes's materialistic styles of explanation, all the while avoiding appeal to any explicitly materialist premises. This would generate a non-rationalist account of human psychology applied to a range of theoretical and practical issues: in short, a new treatise of human nature. The ‘anxiety of influence’ might then discourage explicit references to Hobbes's work, but later in life the author might acknowledge that the main problem with Hobbes's philosophy was his inconsistent attitude to reason. So it harmonizes nicely with this suggestion that, in the final volume of his History of England, Hume assesses Hobbes's achievement in just these terms. Hobbes's philosophy, he observes, ‘partakes nothing of the spirit of scepticism; but is as positive and dogmatical as if human reason, and his reason in particular, could attain a thorough conviction in these subjects’.Footnote ⁵⁸

This remark may reveal not only Hume's differences from Hobbes, but also his indebtedness to him. Hobbes saw that, if materialism is true, there is no special faculty of reason that orients us to truth. He also saw that, if there is no such special faculty, there is no ‘right reason’—no authoritative account of how things are. What Hume saw, but Hobbes missed, was that this implication applied to the materialist doctrine itself: that if materialism is true, it cannot be known to be true. He saw, in short, a fresh instance of the ancient connection between materialism and scepticism. He thus realized that the best and indeed only case for materialism is by means of an experimental philosophy which, in sceptical vein, eschews any claims to metaphysical truth, but which nevertheless makes an indirect case for it by showing the adequacy of materialist styles of explanation. Hume's philosophy is designed to satisfy just these requirements, and so it is in this sense that it can be described as sceptical materialism.