Early Modern Philosophy: David Hume

Painting of David Hume, by Allan Ramsay, 1754 / Scottish National Portrait Gallery

By Dr. Garth Kemerling / 11.12.2011
Professor of Philosophy
Capella University

Philosophy Pages

Life and Works

Soon after completing his studies at Edinburgh, Scottish philosopher David Hume began writing his comprehensive statement of the views he believed would contribute to philosophy no less than Newton’s had to science. But the public reception for the three books of his magisterial Treatise of Human Nature (1739) was less than cordial, and Hume abandoned his hopes of a philosophical career in order to support his family as a librarian, historian, diplomat, and political essayist, a course of action he described in the autobiographical My Own Life (1776). Hume’s Essays Moral and Political (1741-1742) found some success, and the multi-volume History of England(1754-1762) finally secured the modest livelihood for which he had hoped. Although he spent most of his life trying to produce more effective statements of his philosophical views, he did not live to see the firm establishment of his reputation by the criticisms of Kant and much later appreciation of the logical positivists.

The central themes of Book I of the Treatise receive a somewhat more accessible treatment in An Enquiry concerning Human Understanding (1748), a more popular summary of Hume’s empiricism. According to Hume, little human knowledge can be derived from the deductively certain relations of ideas. Since the causal interactions of physical objects are known to us only as inherently uncertain matters of fact, Hume argued, our belief that they exhibit any necessary connection (however explicable) can never be rationally justified, but must be acknowledged to rest only upon our acquired habits. In similar fashion, Hume argued that we cannot justify our natural beliefs in the reality of the self or the existence of an external world.  From all of this, he concluded that a severe (if mitigated)skepticism is the only defensible view of the world.

Hume recast the moral philosophy of the Treatise’s Book III in An Enquiry concerning the Principles of Morals (1751). In both texts Hume clearly maintained that human agency and moral obligation are best considered as functions of human passions rather than as the dictates of reason. In the posthumously published Dialogues concerning Natural Religion (1780), Hume discussed the possibility of arriving at certain knowledge of godthrough the application of reason and considered defense of a fideistic alternative.

Hume: Empiricist Naturalism

David Hume

Later in eighteenth century, Scottish philosopherDavid Hume sought to develop more fully the consequences of Locke’s cautious empiricism by applying the scientific methods of observation to a study of human nature itself. We cannot rely on the common-sense pronouncements of popular superstition, which illustrate human conduct without offering any illumination, Hume held, nor can we achieve any genuine progress by means of abstract metaphysicalspeculation, which imposes a spurious clarity upon profound issues. The alternative is to reject all easy answers, employing the negative results of philosophical skepticism as a legitimate place to start.

Stated more positively, Hume’s position is that since human beings do in fact live and function in the world, we should try to observe how they do so. The key principle to be applied to any investigation of our cognitive capacities is, then, an attempt to discover the causes of human belief. This attempt is neither the popular project of noticing and cataloging human beliefs nor the metaphysical effort to provide them with an infallible rational justification. According to Hume, the proper goal of philosophy is simply to explain why we believe what we do. His own attempt to achieve that goal was the focus of Book I of the Treatise of Human Nature and all of the firstEnquiry.


Hume’s analysis of human belief begins with a careful distinction among our mental contents: impressions are the direct, vivid, and forceful products of immediate experience; ideas are merely feeble copies of these original impressions. (Enquiry II) Thus, for example, the background color of the screen at which I am now looking is an impression, while my memory of the color of my mother’s hair is merely an idea. Since every idea must be derived from an antecedent impression, Hume supposed, it always makes sense to inquire into the origins of our ideas by asking from which impressions they are derived.

To this beginning, add the fact that each of our ideas and impressions is entirely separable from every other, on Hume’s view. The apparent connection of one idea to another is invariably the result of an association that we manufacture ourselves. (Enquiry III) We use our mental operations to link ideas to each other in one of three ways: resemblance, contiguity, or cause and effect. (This animal looks like that animal; this book is on that table; moving this switch turns off the light, for example.) Experience provides us with both the ideas themselves and our awareness of their association. All human beliefs (including those we regard as cases of knowledge) result from repeated applications of these simple associations.

Hume further distinguished between two sorts of belief. (Enquiry IV i)Relations of ideas are beliefs grounded wholly on associations formed within the mind; they are capable of demonstration because they have no external referent. Matters of fact are beliefs that claim to report the nature of existing things; they are always contingent. (This is Hume’s version of the a priori / a posteriori distinction.) Mathematical and logical knowledge relies upon relations of ideas; it is uncontroversial but uninformative. The interesting but problematic propositions of natural science depend upon matters of fact. Abstract metaphysics mistakenly (and fruitlessly) tries to achieve the certainty of the former with the content of the latter.

Matters of Fact

Since genuine information rests upon our belief in matters of fact, Humewas particularly concerned to explain their origin. Such beliefs can reach beyond the content of present sense-impressions and memory, Hume held, only by appealing to presumed connections of cause and effect. But since each idea is distinct and separable from every other, there is no self-evident relation; these connections can only be derived from our experience of similar cases. So the crucial question in epistemology is to ask exactly how it is possible for us to learn from experience. (Enquiry IV ii)

Here, Hume supposed, the most obvious point is a negative one: causal reasoning can never be justified rationally. In order to learn, we must suppose that our past experiences bear some relevance to present and future cases. But although we do indeed believe that the future will be like the past, the truth of that belief is not self-evident. In fact, it is always possible for nature to change, so inferences from past to future are never rationally certain. Thus, on Hume’s view, all beliefs in matters of fact are fundamentally non-rational. (Enquiry V i)

Consider Hume’s favorite example: our belief that the sun will rise tomorrow. Clearly, this is a matter of fact; it rests on our conviction that each sunrise is an effect caused by the rotation of the earth. But our belief in that causal relation is based on past observations, and our confidence that it will continue tomorrow cannot be justified by reference to the past. So we have no rational basis for believing that the sun will rise tomorrow. Yet we do believe it!

Belief as a Habit

Skepticism quite properly forbids us to speculate beyond the content of our present experience and memory, yet we find it entirely natural to believe much more than that. Hume held that these unjustifiable beliefs can be explained by reference to custom or habit. That’s how we learn from experience. When I observe the constant conjunction of events in my experience, I grow accustomed to associating them with each other. (Enquiry V ii) Although many past cases of sunrise do not guarantee the future of nature, my experience of them does get me used to the idea and produces in me an expectation that the sun will rise again tomorrow. I cannot prove that it will, but I feel that it must.

Remember that the association of ideas is a powerful natural process in which separate ideas come to be joined together in the mind. Of course they can be associated with each other by rational means, as they are in the relations of ideas that constitute mathematical knowledge. But even where this is possible, Hume argued, reason is a slow and inefficient guide, while the habits acquired by much repetition can produce a powerful conviction independently of reason. Although the truth of “9 × 12 = 108” can be established rationally in principle, most of us actually learned it by reciting our multiplication tables. In fact, what we call relative probability is, on Hume’s view, nothing more than a measure of the strength of conviction produced in us by our experience of regularity.

Our beliefs in matters of fact, then, arise from sentiment or feeling rather than from reason. For Hume, imagination and belief differ only in the degree of conviction with which their objects are anticipated. Although this positive answer may seem disappointing, Hume maintained that custom or habit is the great guide of life and the foundation of all natural science.

Necessary Connection

According to Hume, our belief that events are causally related is a custom or habit acquired by experience: having observed the regularity with which events of particular sorts occur together, we form the association of ideas that produces the habit of expecting the effect whenever we experience the cause. But something is missing from this account: we also believe that the cause somehow produces the effect. Even if this belief is unjustifiable, Hume must offer some explanation for the fact that we do hold it. His technique was to search for the original impression from which our idea of the necessary connection between cause and effect is copied. (Enquiry VII)

The idea does not arise from our objective experience of the events themselves. All we observe is that events of the “cause” type occur nearby and shortly before events of the “effect” type, and that this recurs with a regularity that can be described as a “constant conjunction.” Although this pattern of experience does encourage the formation of our habit of expecting the effect to follow the cause, it includes no impression of a necessary connection.

Nor do we acquire this impression (as Locke had supposed) from our own capacity for voluntary motion. Here the objective element of constant conjunction is rarely experienced, since the actions of our minds and bodies do not invariably submit to our voluntary control. And even if volition did always produce the intended movement, Hume argued, that would yield no notion of the connection between them. So there is no impression of causal power here, either.

Still, we do have the idea of a necessary connection, and it must come from somewhere. For a (non-justificatory) explanation, Hume refers us back to the formation of a custom or habit. Our (non-rational) expectation that the effect will follow the cause is accompanied by a strong feeling of conviction, and it is the impression of this feeling that is copied by our concept of a necessary connection between cause and effect. The force of causal necessity is just the strength of our sentiment in anticipating efficacious outcomes.

The Self

In a notorious passage of the Treatise, Hume offered a similar account of the belief in the reality of the self. Here there is the ordinary human supposition that lies behind our use of first-personal pronouns. Upon this relatively simple foundation, philosophers have erected the notion of an immaterial substance, a mind or soul that persists through time on its own. Hume’s question is, “From what antecedent impression does the idea of the self arise?”

Hume pointed out that we do not have an impression of the self. No matter how closely I attend to my own experience, no matter how fully I notice the mental operations presently occurring “in my mind,” I am never directly aware of “I.” What I do experience is a succession of separate and individual ideas, associated with each other by relations of resemblance and causality. Although these relations may be extended through time by memory, there is no evidence of any substantial ground for their coherence. The persistent self and the immortal soul are philosophical fictions.

To suppose otherwise, Hume held, is to commit a category mistake: the self is just a bundle of perceptions, like the railroad cars in a train; to look for a self beyond the ideas would be like looking for a train beyond the cars. Our idea of a persistent self is simply a result of the human habit of attributing continued existence to any collection of associated parts. Like our idea of the necessary connection of cause with effect, belief in our own reality as substantial selves is natural, but unjustifiable.

External World

Another perfectly ordinary feature of human cognition is our belief in the reality of the external world. As I write this lesson, I readily suppose that my fingers are touching a keyboard, that the sun is shining outside and that the radio is playing a Clapton song. In Hume’s skeptical philosophy, what is the status of these beliefs?

The primitive human belief, Hume noted, is that we actually see (and hear, etc.) the physical objects themselves. But modern philosophy and science have persuaded us that this is not literally true. According to representationalists, we are directly aware of ideas, which must in turn be causally produced in our minds by external objects. The problem is that on this view we can never know that there really are physical objects that produce our sensory ideas.

We cannot rely on causal reasoning to convince us that there are external objects, Hume argued, since (as we have just seen) such reasoning arises from our observation of a constant conjunction between causes and effects. But according to the representationalist philosophy, we have no direct experience of the presumed cause! If we know objects only by means of ideas, then we cannot use those ideas to establish a causal connection between the things and the objects they are supposed to represent.

In fact, Hume supposed, our belief in the reality of an external world is entirely non-rational. (Enquiry XII i) It cannot be supported either as a relation of ideas or even as a matter of fact. Although it is utterly unjustifiable, however, belief in the external world is natural and unavoidable. We are in the habit of supposing that our ideas have external referents, even though we can have no real evidence for doing so. Representationalism thusly implodes: the ideas, originally introduced as intermediaries between perceivers and things, end up absorbing both, rendering everything but themselves superfluous.

Mitigated Skepticism

Where does this leave us? Hume believed himself to be carrying out the empiricist program with rigorous consistency. Locke honestly proposed the possibility of deriving knowledge from experience, but did not carry it far enough. Bayle and Berkeley noticed further implications. Now Hume has shown that empiricism inevitably leads to an utter and total skepticism.

According to Hume, knowledge of pure mathematics is secure because it rests only on the relations of ideas, without presuming anything about the world. Experimental observations (conducted without any assumption of the existence of material objects) permit us to use our experience in forming useful habits. Any other epistemological effort, especially if it involves the pretense of achieving useful abstract knowledge, is meaningless and unreliable.

The most reasonable position, Hume held, is a “mitigated” skepticism that humbly accepts the limitations of human knowledge while pursuing the legitimate aims of math and science. (Enquiry XII 3) In our non-philosophical moments, of course, we will be thrown back upon the natural beliefs of everyday life, no matter how lacking in rational justification we know them to be.

Hume: Morality and Religion

Grounds for Morality

Having examined the epistemological basis for Hume’s naturalism, we are ready to consider its application to human conduct. In morality as in all else, Hume supposed, our beliefs and actions are the products of custom or habit. Since all of our most scientific beliefs have exactly the same foundation, this account preserves the natural dignity of moral judgments.

Hume devoted the second book of the Treatise to an account of the human passions and a discussion of their role in the operation of the human will. It is our feelings or sentiments, Hume claimed, that exert practical influence over human volition and action. Observation does reveal a constant conjunction between having a motive (not a reason) for acting and performing the action in question. Hence, with the same reliability that characterizes our belief in any causal relation, on Hume’s view, we further believe that our feelings have the power to result in actions.

At one level, of course, this entails that we are determined to act as we do. Our feelings or sentiments produce our actions with the same degree of causal necessity, the same habitual expectation that the future will resemble the past, as that by which the rotation of the earth causes the sun to rise. (Like Locke, Hume denied that determination of this sort is relevant to our moral freedom; only when my actions are observed to be the effects of some cause outside myself could I decline to accept my own responsibility for them.) So a proper science of human nature will account for human actions, as well as for human beliefs, by reference to the natural formation of habitual associations with human feelings.

Clearly, rationality had no place in this account of morality. Although reason may judge relations of ideas and matters of fact, its most vivid outcomes never compel us to act as even the weakest of feelings may do. No compilation of facts, however complete or reliable, ever entails a moral obligation or results in action. “Reason is, and ought to be, only the slave of the passions,” Hume held. All human actions flow naturally from human feelings, without any interference from human reason.

Moral Sentiment

It does not follow that all actions are of equal value. On Hume’s view, the judgments and recommendations of traditional morality arise not from reason, but from a moral sense. As a straightforward matter of fact (discoverable by experience), virtue is always accompanied by a feeling of pleasure, and vice by a feeling of pain. Thus, we praise an instance of virtuous action precisely because it arouses in us a pleasant feeling, and we avoid committing a vicious action because we anticipate that doing so would produce pain. Our feelings provide a natural guide for moral conduct.

Hume worked out the details of this account in Book III of the Treatise. The ideas of benevolence, utility, and justice arouse our deepest and most pervasive feelings, he maintained, and these feelings in turn motivate us toward actions of moral worth. I offer assistance to those in need because it makes me feel good to do so, and I am fair in my dealings with others because it would make me feel bad if I were not. All of morality rests firmly upon the natural human inclination to seek pleasure and avoid pain.

This noncognitive derivation of morality from emotion rather than from reason may seem hopelessly subjective at first glance, but remember that on Hume’s view our confidence in causal efficacy has a similar source. I do what is morally right in the same way that I believe there is an external world—by following my natural inclinations in the absence of rational evidence. Thus, Hume regarded himself as having provided morality with a status no less significant in human life than that of natural science.

God’s Existence

Finally, we pause for a quick look at Hume’s views on religion. In his own time, he was often regarded as a great enemy of organized religion. The posthumously published Dialogues offer an extended treatment of the intellectual interchanges among facile orthodoxy, natural theology, and philosophical skepticism. There Hume took great care to expose what he believed to be the great mistake of trying to prove that god exists.

The newly-popular argument from design supposes that the order and beauty of the universe reflect the greatness and demonstrate the reality of its ultimate cause. Hume noted that since this analogical argument claims to infer a cause from presumed effects, it must be grounded as a matter of fact on the experience of a constant conjunction. But since in fact we have not observed repeated instances of gods creating universes, we cannot have formed the habit of associating our experience of the one with our inferences about the other. No causal relationship can ever be established from the observation of a unique example.

What is more, Hume argued that even if it were possible to engage in causal reasoning in this case, it could not warrant the intended conclusion. The presumed cause must always be supposed to be proportional to the observed effect, so the manifest imperfections of this world could never support belief in the perfection of its creator. The argument from design is a two-edged sword, as likely to persuade us of the frailty or malevolence as of the power and benevolence of the presumed cause of the world as we know it.


Nor did Hume suppose that references to the miraculous would provide a rational basis for religion. In this case, we do have the experience of constant conjunction to establish the “laws of nature” of which any purported miracle is a violation, and we have only the testimony of witnesses to establish the fact of the miracle itself. Since this testimony and the motives of the witnesses who offer it are always open to question, Hume argued, we will believe that the miracle occurred only when the possibility of false testimony seems an even greater violation of the natural order.

Some scholars suppose that the final paragraph of the essay “On Miracles” (Inquiry Section X) and the closing words of the Dialogues reflect Hume’s acceptance of religious fideism, the notion that religion is properly a matter of faith, not reason. On this view, a fideistic Hume could hold that belief in the existence of god or the immortality of the soul is no less natural than belief in the existence of bodies or the persistence of the self. An alternative interpretation, however, accepts the lengthy rejection of religious orthodoxy as sincere while attributing the brief, moderate endings as a half-hearted effort to take the edge off. Certainly Hume’s influence on the philosophy of religion has been primarily of the latter sort.