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David Hume

David Hume at 300

Howard Darmstadter looks at the life and legacy of the incendiary tercentenarian.

In 1734, David Hume, a bookish 23-year-old Scotsman, abandoned conventional career options and went off to France to Think Things Over. Living frugally and devoting himself to study and writing, he returned after three years with a hefty manuscript under his arm. Published in three volumes in 1739 –40 as A Treatise of Human Nature, it attracted little attention. Reflecting on the event near the end of his life, Hume joked that it “fell still-born from the press.”

Hume soon rallied, going on to enjoy a long and successful career as an historian and political essayist (the accomplishments for which he was best known in his lifetime) and as an important contributor to the infant science of economics. But from time to time he returned to the Treatise, stripping out extraneous material and sharpening the arguments. The results he published as An Enquiry Concerning Human Understanding (1748) and An Enquiry Concerning the Principles of Morals (1751), which works brought his philosophical views to a wider audience.

Setting the Scene

Philosophers in the seventeenth and eighteenth centuries distinguished ‘moral’ explanations from ‘natural’ (or ‘mechanical’) explanations. In Hume’s day, ‘moral’ had somewhat the same meaning as ‘human’, as in the ‘moral sciences’, which encompassed not only philosophy but also psychology, politics and economics. In Hume’s sense, moral explanations concern the actions of thinking things – human beings, higher-order animals, angels, gods – while natural explanations are used for the actions of non-thinking things, such as rocks, plants, atoms and billiard balls. Moral explanations are generally phrased in terms of wants, and natural explanations in terms of mechanical forces. Thus we may explain why the chicken crossed the road by saying that it wanted to get to the other side: this is a moral explanation. But ‘it wanted to get to the other side’ is not an acceptable explanation for why the eight ball crossed the pool table. The eight ball is not a thinking thing, and its path requires a mechanical explanation in terms of impact, mass, velocity and other non-volitional causes.

For prescientific peoples, explanations of natural phenomena tend to be moral. The prescientific world-view sees many events in the natural world as caused by gods or other quasipeople with human-like motivations. Thus, a storm is caused by an angry god, or perhaps is itself an angry god. The world is thus imbued with meaning and purpose.

In the seventeenth century, the French philosopher and mathematician René Descartes attempted to limit the scope of moral explanation by dividing the world into two distinct kinds of stuff, mind and matter. For the world of mind, moral explanations were appropriate, whereas the material world required natural explanations. Descartes and his followers, the Cartesians, thought that we could see intuitively that for material objects, natural explanations in terms of pushes, pulls, and collisions had to be correct. In more formal terms, they held that we could be sure of the truth of such an explanation once we had a “clear and distinct idea” of the interaction. For example, from a clear and distinct idea of the cue ball hitting the eight ball, we would know what path the eight ball would take. The natural world thus retained a comforting human dimension, as its fundamental principles were directly accessible to reason.

Like Lord Nelson, Cartesianism achieved its greatest triumph and sustained its mortal injury at the same moment. The Cartesian Trafalgar was the publication in 1687 of Isaac Newton’s Principia Mathematica. For millennia, people had assumed that the heavenly bodies were governed by entirely different principles than earthly objects. But from three laws of motion and a law of gravitational attraction, Newton explained the motions of both celestial and terrestrial objects as parts of a single system. Newton showed that the planets’ orbits around the Sun and the fall of an apple to the Earth obeyed the same universal laws. Everything – the stars in their courses, the cannonball’s flight, the cycles of the tides – stepped to the same mechanical measures.

Cartesians fretted that Newton’s law of universal gravitation failed to be clear and distinct. How, asked the Cartesians, can the Sun act on the planets without some intervening mechanism? For the law of gravitation to work, it seemed that the Sun and the planets would have to somehow sense each other’s presence, and this sensing seemed to involve non-mechanical forces. But for Newton, it was enough that the law of gravity accurately predicted events: that the law might seem mysterious rather than ‘clear and distinct’ was not significant.

David Hume the Newtonian Moralist

Enter David Hume. Born May 7, 1711 of respectable parents in the Scottish Lowlands, his early life was outwardly uneventful. After leaving Edinburgh University, he at first contemplated a legal career, and briefly worked as a clerk for a Bristol merchant. But in his late teens Hume was seized by ideas that “opened up to me a new scene of thought.” He decided to become the Newton of the moral sciences.

Newton had shown that all of the material world was governed by the same mechanical laws. Hume’s great project was to base the study of man and society on similar universal principles. Indeed, the Treatise of Human Nature bore the subtitle Being an Attempt to Introduce the Experimental Method of Reasoning into Moral Subjects.

In the Treatise, Hume tried to formulate the laws governing the succession of our thoughts. The result was a long and generally unconvincing exposition of numerous rules said to direct our mental life. But intertwined with this failed attempt at a complete theory of the mind, and at times buried by it, is Hume’s development of the startling implications of a scientific view of man. His two later Enquiries brought these implications powerfully to the fore.

Like most philosophers of his time, Hume conceived of thought as a flow of mental images. Seeing a tree, imagining a tree, or remembering a tree, were all thought to consist of our having a mental image, more vivid for the seen tree, less vivid for the imagined or remembered tree. A sentence like ‘The Earth is round’ would have a certain type of mental image as its meaning, and believing that the Earth is round necessarily involved a vivid mental image of that type. This theory also explained why certain beliefs were logically impossible. For example, a four-sided triangle was logically impossible (and a three-sided triangle logically necessary) because we could not form a mental image of a triangle that did not have three sides. (Try it.) Hume’s disturbing insight from this way of thinking about thinking, was that all our factual and moral beliefs can therefore only be justified in terms of the psychological laws that govern the succession of images in our minds.

Consider perhaps Hume’s most famous argument, which begins with the question, ‘What justification do we have for our factual beliefs?’ By ‘factual beliefs’, Hume meant those beliefs that we can imagine (that is, form a mental image of) and as a result of these images say are either true or false, occuring or not occuring. For example, when we see two billiard balls collide on a table, we believe that the impact of the first ball will cause the second to move in a particular direction. This belief is ‘factual’ because we can also imagine the second ball not moving at all, or returning in the direction from which the first ball came, or vanishing in a puff of smoke. Since we can imagine any of these things, they are all logically possible. Therefore, Hume concluded, there is nothing in the motion of the first ball from which we can logically infer the motion of the second. That we have an accurate belief as to how the second ball will move is not based on any logical deduction from the movement of the first, but from our past experience of seeing billiard balls collide.

But, Hume persists, what is our justification for drawing conclusions from experience? Only our belief that the future will be like the past. But this too is a factual belief. We can imagine that the future will not be like the past – for example, that tomorrow billiard balls will vanish upon being hit by other billiard balls. So our belief that the future will resemble the past is itself not based on any process of deductive reasoning, but solely on experience. So how is experience itself justified?

To justify anything, you give reasons. And you justify those reasons by giving still other reasons. This implies three possible structures for any chain of justification:

(1) Reasons go on forever, without repeating.

(2) Reasons go in a circle – that is, eventually a reason is repeated.

(3) The chain stops, with a final reason.

Structures (1) and (2) would plainly provide unsatisfactory justifications, which leaves structure (3). But if a chain of justification is to stop in a satisfying way, the last reason given must not require further justification. And since we can imagine the contrary of a factual belief, a factual belief cannot be a final reason. So a factual belief that the present is like the past cannot be the final justifying reason for any conclusions about the world.

For Hume, our beliefs about the motions of colliding billiard balls and other law-like natural behaviors are formed by the psychological principle of ‘habit’. Our minds are so constructed that having experienced a particular motion of one ball, constantly followed by a particular motion of a second, we form the image of the second motion whenever we’re presented with the image of the first. The more frequent and invariant the past conjunction of motions, the more vivid our present image of the second motion will be upon being presented with the first, and this vivid image is our belief that the second ball will move in a particular way. There is no decision to believe that the motion of the second ball will follow from the motion of the first. Rather, the belief is forced on us by the associational laws of thought.

Hume thus replaces moral explanations in terms of wants, with psychological laws that, like the principles of Newtonian mechanics, are not framed in terms of wants. The images forced upon us which constitute our most basic factual beliefs cannot be justified by reason, nor can they be escaped from. So rather than our controlling our own thoughts, Hume argues that our thoughts are controlled by unthinking forces. We are not the captains of our mental journey, merely passengers.

The Philosophical Psychologist

To see the emphasis Hume placed on our thought processes, consider a familiar philosophical problem. Seeing an external object – a tree, for example – involves a process that begins with the tree and flows, via reflected light, to our eyes, and then up our optic nerve to our brain, to produce a mental image of the tree, which is our experience of the tree. The tree is at one end of this chain of events, our mental image of the tree at the other. So how can we be sure that our mental image of the tree is like the tree itself? Descartes had a classic formulation of this problem: Could there be an evil demon who systematically gives us experiences (mental images) that are different from their causes? A modern version of this problem is, How do you know that you’re not just a brain in a vat, fed electrical impulses by a mad scientist, so that while you think you have a body and participate in a real world of people and objects, you are in reality only a player in a kind of cosmic video game? (You’ve seen the movie.)

But Hume does not attempt to answer Descartes’ problem. Hume’s vantage-point is always that of a psychologist attempting to explain human behavior. The psychologist accepts that he and his subjects inhabit a common world of people and material objects. It is from within this common world that the psychologist attempts to discover the laws of thought. We can speculate as to whether this common world really (ie observer-independently) exists as we imagine it, but for Hume such speculations are idle. People always assume that such a world exists, and the psychological imperative to make this assumption settles the question for Hume. As he says in the Enquiry Concerning Human Understanding, skeptical principles “may flourish and triumph in the schools, where it is indeed difficult, if not impossible, to refute them. But as soon as they leave the shade, and by the presence of the real objects, which actuate our passions and sentiments, are put in opposition to the more powerful principles of our nature, they vanish like smoke, and leave the most determined skeptic in the same condition as other mortals” (from Part II of Section XII).

The Humanist

Just as our reasonings concerning matters of fact rest on a principle of association of ideas, so there can be no ‘ultimate’ justification for our moral beliefs, beyond psychological laws. This is summarised in Hume’s infamous law that ‘you can’t get an ought from an is’ [See Hume on Is and Ought in this issue – Ed].

Hume’s attempt to base morality on psychological principles begins with a conventional premise: humans are motivated by pains and pleasures. But Hume insists that humans are innately social: we take pleasure in the pleasure of others, and feel pain at others’ pain. This ‘principle of humanity’ is the foundation of Hume’s ethical theory. It is his gravitational principle. It’s our motivation for what we might call our ‘moral behavior’. Hume supports it with numerous examples drawn from everyday life, but disdains any attempt to explain it.

Hume never doubts that all people are united in possessing the same psychology, in particular, the principle of humanity. Our fellow-feeling can be extended to anyone with whom we have contact: “An Englishman in Italy is a friend, a European in China, and perhaps a man would be beloved as such were we to meet him in the moon.” Of course, the empathy shown in the principle of humanity does not extend to humankind generally, but only to people with whom we have contact: most strongly to family members and close friends, less to acquaintances, still less to those with whom contact is intermittent, and hardly at all to strangers. Hume’s principle thus fails to explain how people can live at peace in complex societies where they must interact with and depend upon relative strangers. Instead, since large societies are necessary to maximize human pleasure – a basic human motivation to Hume – in this case people use their reasoning ability to invent systems of legal rules and institutions:

“Two neighbors may agree to drain a meadow, which they possess in common, because ‘tis easy for them to know each others mind; and each must perceive that the immediate consequence of his failing in his part is the abandoning of the whole project. But ‘tis very difficult, and indeed impossible, that a thousand persons should agree in any such action… Political society easily remedies … these inconveniences. Thus bridges are built; harbors opened; ramparts raised; canals formed; fleets equipped; and armies disciplined everywhere by the care of government, which, though composed of men subject to all human infirmities, becomes, by one of the finest and most subtle inventions imaginable, a composition which is, in some measure, exempted from all these infirmities.”

Treatise of Human Nature, Book III, Part II, section vii.

Yet while reason finds the means for individuals to achieve their ends, those ends are not set by reason, but by irresistible mental tendencies which Hume calls ‘sentiments’ or ‘passions’. “Reason is, and ought only to be, the slave of the passions, and can never pretend to any other office than to serve and obey them” Hume claimed.

Life and Death

Hume’s philosophy might seem the creation of that stock comic character, ‘the dour Scot’. In fact, in personality Hume was closer to another cliché, ‘the jolly fat man’. Gregarious, open and generous, an engaging correspondent and conversationalist, he had a wide circle of friends in Scotland. However, his ungainly bulk, thick Scottish brogue, and heterodox views left him undervalued in London.

In middle age Hume spent over two years in Paris as private secretary to the British ambassador. Hume’s written works had preceded him, and he was lionized in the salons, receiving the respect and favor that eluded him in London.

In Paris Hume conceived a passion for the attractive and intelligent Comtesse de Boufflers. The Comtesse had admired Hume’s work before they met, and their relationship blossomed upon his arrival in France. The Comte, from whom the Comtesse had long been separated, did not pose an obstacle; but the Comtesse was also a former mistress of the Prince de Conti. When the Comte died shortly before Hume’s return to Britain, the Comtesse thought she might become the Princesse de Conti. It didn’t happen, but the Comtesse’s manoeuvrings dimmed Hume’s ardor. After leaving France, Hume never saw her again. But they maintained an affectionate, although intermittent, correspondence until his death: one of his last letters was to her. We know little of Hume’s other attachments.

Hume’s original philosophical publication ceased around age forty, and most of his other writings were completed by his early fifties, more than a decade before his death in 1776. To his publisher, who earnestly solicited him to complete his History of England, Hume replied, “I must decline not only this offer, but all others of a literary nature, for four reasons: Because I’m too old, too fat, too lazy, and too rich.”

Hume approached his own death with a cheerful calm that bordered on disinterest. A few months before his death, he composed a brief autobiography in which he described his situation:

“In spring 1775, I was struck with a disorder in my bowels, which at first gave me no alarm, but has since, as I apprehend it, become mortal and incurable. I now reckon upon a speedy dissolution. I have suffered very little pain from my disorder; and what is more strange, have, notwithstanding the great decline of my person, never suffered a moment’s abatement of my spirits; insomuch, that were I to name a period of my life which I should most choose to pass over again, I might be tempted to point to this later period. … It is difficult to be more detached from life than I am at present.”

David Hume, My Own Life, penultimate paragraph.

The Religious Skeptic

Hume had become a religious skeptic in his teens, and remained so until he died. The manuscript for the Treatise originally contained a chapter, ‘Of Miracles’, which argued that “no testimony for any kind of miracle has ever amounted to a probability, much less to a proof.” [Again, see this issue.] Hume was prevailed upon to remove the chapter from the Treatise, but he included it in the first Enquiry. Hume’s initial hesitation is understandable: as recently as 1696, a young man had been executed in Edinburgh for blasphemy. Scotland last hanged a witch when Hume was seventeen.

At about forty, Hume composed the Dialogues Concerning Natural Religion, which substantially broadened his attack on standard pieties. Again Hume was prevailed upon not to publish, and the Dialogues only appeared after his death. In his will Hume asked his good friend Adam Smith to see to its publication, but Smith knew a hot potato when he saw one, and declined. Publication was eventually overseen by Hume’s nephew.

The Dialogues are presented as conversations between three friends: Demea, a religious traditionalist; Cleanthes, a proponent of the new ‘natural religion’ based on the argument from design; and Philo, a skeptic, who stands in for Hume.

The argument from design proceeds by analogy. Nature seems to work much like a machine, and just as a machine must have a designer, so must the world. Philo’s primary counter-argument against such ‘intelligent design’ is that we can only reason from experience, and while we have experienced the making of machines, we have no experience of the making of worlds from which we can draw a good inference.

Readers who accept Philo’s skepticism may find themselves unprepared for the Dialogues’ final chapter. There Cleanthes points out that all attacks on the argument from design are merely negative: they “start doubts and difficulties” but can at best only cause us to suspend our judgment. Although humans can sometimes adopt a new system of belief on the basis of such doubts, they cannot maintain it when it is opposed by a “theory supported by strong and obvious reason, by natural propensity, and by early education.” Thus, Hume’s negative argument against divine design cannot beat arguments for God which have already been established in peoples’ minds. In other words, you can’t beat something with nothing – a point that Philo, the presumed skeptic, immediately seconds:

“That the works of Nature bear a great analogy to the productions of art is evident … From this enquiry, the legitimate conclusion is that the causes [of the natural world] have also an analogy; and if we are not contented with calling the first and supreme cause a GOD or DEITY, but desire to vary the expression; what can we call him but MIND or THOUGHT, to which he is justly supposed to bear a considerable resemblance?” (Part XII)

When we open the Dialogues today, we know how the story of design ends: with Darwin’s Origin of Species, published a hundred years later. With evolutionary theory at hand, it’s easy to see how logically flimsy the argument from design really is; but while Hume was able to demolish the design argument’s logical pretensions, he knew that he had no positive theory to offer in its stead. Indeed, our belief in God seemed to Hume to resemble our belief in the reality of the external world: we cannot adequately answer the skeptical arguments about God or external objects, but we cannot help believing in Him or them. Hume’s real achievement in the Dialogues is his decoupling of the argument from design from any religious consequences. The analogy from which the argument derives its force is so weak that while we may find it persuades us to believe in God (or Mind or Thought), the analogy does not imply any particular religious practices.

Hume always considered organized religion a malign influence. He remained a Newtonian in matters religious as well as scientific – all our reasonings must proceed from experience:

“When we run over libraries, persuaded of these principles, what havoc must we make? If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.”

Enquiry Concerning Human Understanding, Section XII, Part III

Howard Darmstadter is an Adjunct Professor at the Benjamin N. Cardozo School of Law of Yeshiva University, New York.