A Treatise of Human Nature: Being an Attempt to Introduce the Experimental Method of Reasoning into Moral Subjects

1. Introduction

A Treatise of Human Nature: Being an Attempt to Introduce the Experimental Method of Reasoning into Moral Subjects is David Hume’s most systematic attempt to build a comprehensive “science of man.” Written when he was in his twenties, it combines an empiricist theory of mind with a naturalistic account of emotions and morals. Rather than beginning from metaphysical principles or religious doctrines, Hume starts from what he takes to be observable features of human experience—our sensations, feelings, habits, and everyday beliefs—and seeks to explain how these give rise to knowledge, character, and moral judgment.

The work is structured in three books—Of the Understanding, Of the Passions, and Of Morals—corresponding to cognition, motivation, and ethics. Hume’s guiding idea is that all three domains can be treated with the same kind of “experimental” method that he associates with Newtonian natural philosophy: describing phenomena, identifying constant patterns, and formulating explanatory principles grounded in experience rather than a priori speculation.

The Treatise is often read as both a continuation and a radicalization of British empiricism. It develops John Locke’s theory of ideas into a stricter copy principle, offers a skeptical analysis of causation and personal identity, and argues that morality rests on feeling rather than reason. At the same time, it aims to leave ordinary life and empirical science intact by distinguishing between what can be strictly justified by reason and what we are inevitably led to believe by custom and human nature.

Later in life, Hume presented shorter and more polished versions of many of the Treatise’s doctrines in his Enquiry Concerning Human Understanding and Enquiry Concerning the Principles of Morals. Nonetheless, the Treatise remains the fullest statement of his project to found all the “moral sciences” on an empirical psychology of human nature.

2. Historical Context

Hume’s Treatise emerged in the intellectual milieu of early eighteenth‑century Britain, shaped by scientific, philosophical, and religious developments.

Scientific and Philosophical Setting

The dominant scientific influence was Isaac Newton’s natural philosophy, which many thinkers saw as a model for all branches of knowledge. Hume’s subtitle—“introduce the experimental method of reasoning into moral subjects”—explicitly aligns his project with Newton’s empirical, law‑seeking approach.

In philosophy, Hume wrote against the background of British empiricism:

Hume inherits the empiricist focus on ideas and experience while pressing it toward more skeptical conclusions about causation, the self, and morality.

Religious and Moral Debates

The period was marked by tension between traditional Christian theology and emerging freethought and deism. Moral philosophers debated whether morality derives from divine commands, rational insight, or human sentiment. Rationalists such as Clarke defended the objectivity of moral truths accessible by reason; sentimentalists like Francis Hutcheson emphasized a moral sense or feeling of approval and disapproval. Hume’s sentimentalist ethics and his naturalistic explanations of religion intervene in these disputes.

Political and Social Context

The Treatise was written after the Glorious Revolution and Union of 1707, in a Britain grappling with questions of political authority, allegiance, and commerce. Hume’s analysis of justice, property, and political obligation as conventional and utility‑based participates in wider discussions about contract theory, sociability, and the foundations of government associated with thinkers such as Hobbes, Locke, and Pufendorf.

Commentators disagree on how far Hume should be seen as a radical underminer of religious and political orthodoxies, or as a moderate contributor to Enlightenment projects of reform, but most agree that the Treatise belongs squarely within the broader European Enlightenment’s attempt to apply reason and observation to human affairs.

3. Author and Composition

David Hume (1711–1776) was a Scottish philosopher, historian, and essayist. The Treatise is widely regarded as his earliest major philosophical work and the most ambitious statement of his “science of man.”

Hume’s Background and Aims

Born in Edinburgh and educated at the University of Edinburgh, Hume developed an early interest in classical literature and modern philosophy. By his own later account, he conceived “some time between 1729 and 1734” the project of a comprehensive system of the sciences of human nature. He sought to do for the mind and morals what Newton had done for the physical world, grounding them in observation and psychological principles.

Hume spent extended periods in France (notably at La Flèche) during the early 1730s, where he completed much of the Treatise. Biographical scholarship commonly associates this French sojourn with the intensive composition phase of the work, when Hume lived as a private scholar and refined his arguments.

Composition and Internal Development

The Treatise was composed as a multi‑book project from the outset, though scholars debate how fully Hume had planned all three books before publication of the first two. The composition likely proceeded roughly in the order of the published books: Book 1 on understanding, then Book 2 on the passions, with Book 3 on morals following.

Some interpreters, such as Norman Kemp Smith, argue that Hume’s views evolved even within the Treatise, pointing to tensions between more skeptical sections (especially in Book 1, Part 4) and more naturalistic, constructive passages elsewhere. Others, like Don Garrett, suggest a greater overall systematic unity and see apparent shifts as part of a deliberate dialectic between skeptical doubt and natural belief.

Hume later distanced himself from the youthful style of the Treatise, calling it a work of “nonage,” yet he never explicitly rejected its main doctrines. Instead, he recast many of its arguments in shorter, stylistically polished works, leading to ongoing debate about whether the Treatise or the later Enquiries should be taken as the best guide to his considered philosophy.

4. Publication and Textual History

The Treatise appeared anonymously in London in three volumes, published by John Noon:

Hume did not include a formal dedication, and the title page identified only the work, not its author. Contemporary sources suggest that he hoped anonymity would mitigate theological or political backlash and allow the work to be judged on its philosophical merits.

Early Reception and Hume’s Response

The Treatise sold poorly, and reviews were sparse. Hume later wrote that it “fell dead‑born from the press.” Some early notices expressed concern about its perceived skepticism and irreligion, particularly in its treatment of miracles, causation, and the soul, though these topics are more fully developed in the later Enquiry Concerning Human Understanding.

In response to the disappointing reception, Hume reworked significant portions of Book 1 into the Enquiry Concerning Human Understanding (1748) and of Book 3 into the Enquiry Concerning the Principles of Morals (1751). He omitted many of the most technical discussions and some of the more radical skeptical passages. This has generated a long‑standing scholarly discussion about the relation between the Treatise and the Enquiries and which texts best represent Hume’s mature views.

Textual Transmission and Editions

No autograph manuscript of the Treatise is known to survive; the work is transmitted through printed copies only. The eighteenth‑century editions provide the basic textual foundation, with later scholarly editors collating variants and reconstructing the most reliable text.

Key modern editions include:

Textual debates focus on issues such as the significance of Hume’s own minor revisions, the relation between the Treatise and his later writings, and how to interpret ambiguous or inconsistent terminology within the work itself.

5. Structure and Organization of the Treatise

The Treatise is carefully structured into three main books, each with parts, sections, and sub‑sections. Hume presents this architecture as reflecting the logical order of inquiry in a “science of man.”

Overall Division

Hume begins with the understanding because he holds that questions about knowledge and ideas must be settled before those concerning passions and morals, which presuppose some account of the mind’s operations.

Internal Organization

Within each book, Hume uses a nested structure:

• Books → Parts → Sections → (Subsections)

For example, Book 1 is divided into four parts:

1. Of ideas, their origin, composition, connexion, abstraction, &c.
2. Of the ideas of space and time
3. Of knowledge and probability
4. Of the sceptical and other systems of philosophy

Book 2 is divided into parts on pride and humility, love and hatred, and the will and direct passions. Book 3 is divided into parts on virtue and vice in general, on justice and injustice, and on the other virtues and vices.

Thematic Progression

Many commentators see a deliberate progression across the books:

On this view, the theory of ideas and belief underpins the analysis of passions, which in turn underpins the account of moral evaluation and political institutions. Others argue that the books can be read more independently, though most agree that Hume intends significant cross‑references and dependencies among them.

The highly articulated structure, combined with Hume’s frequent “recapitulations” and preview passages, is often interpreted as evidence for a systematic and architectonic aim, even where the argument exhibits internal tensions or abrupt transitions.

6. Book 1: Of the Understanding

Book 1 develops Hume’s theory of cognition and sets out many of his most famous epistemological and metaphysical claims.

Perceptions, Ideas, and Association

Hume begins by classifying all perceptions into impressions (lively perceptions such as sensations and emotions) and ideas (faint images of these in thinking). He formulates the copy principle, arguing that every simple idea is derived from a corresponding simple impression. From this starting point he examines how complex ideas are formed and how ideas are connected by association—specifically resemblance, contiguity in time or place, and cause and effect.

Space, Time, and Abstract Ideas

In Part 2, Hume treats space and time as products of the imagination operating on discrete impressions. He challenges the doctrine of infinite divisibility and argues for “indivisible minima” of extension and duration. He also criticizes traditional accounts of abstract ideas, contending that what we call general ideas are particular ideas used in a general way through habit and custom.

Causation, Induction, and Belief

Part 3 introduces Hume’s analysis of knowledge and probability. He distinguishes relations of ideas (discoverable by mere thought, such as mathematics) from matters of fact (known only through experience)—a distinction often called Hume’s fork. He then argues that our idea of causal connection arises not from any perceived necessary tie between events but from the mind’s habit, formed by repeated constant conjunctions, of moving from one idea to another. This leads to his well‑known problem of induction, questioning the rational justification for expecting the future to resemble the past. Belief is explained as a lively or forceful idea produced by custom.

Skepticism and the Self

Part 4 examines the implications of this theory for our beliefs in external objects, personal identity, and the limits of reason. Hume contends that the idea of a persisting self cannot be traced to any single impression; the self is instead a bundle of perceptions related by resemblance and causation. He also critiques “sceptical and other systems of philosophy,” arguing that both extreme skepticism and rationalist metaphysics falter, and suggests a mitigated skepticism grounded in common life and natural belief.

7. Book 2: Of the Passions

Book 2 offers a systematic psychology of the passions, or emotions, and their role in human motivation and character.

Impressions of Reflection and the Passion Taxonomy

Hume distinguishes impressions of sensation (arising directly from bodily conditions or external objects) from impressions of reflection, which arise from prior perceptions and include the passions. He further divides passions into direct passions (such as desire, aversion, hope, fear) and indirect passions (notably pride, humility, love, hatred), which involve more complex cognitive and associative structures.

Pride, Humility, Love, and Hatred

In Parts 1 and 2, Hume analyzes pride and humility (self‑directed) and love and hatred (other‑directed) as arising from a double relation of impressions and ideas. A pleasing or painful quality (e.g., beauty, wealth, virtue) associated with a person by causal or other relations gives rise to an impression (pleasure or pain) and an idea (of self or other), which jointly generate these passions. He applies this model to phenomena such as family pride, national prejudice, and social rivalry, offering naturalistic explanations for complex social emotions.

Sympathy and Social Psychology

A central mechanism in Book 2 is sympathy, through which we come to share the feelings of others. Hume uses sympathy to explain the spread and modulation of passions in social contexts and to account for the construction of stable character traits. Sympathy also serves as an important bridge between personal passions and the more generalized sentiments involved in moral judgment, preparing the ground for Book 3.

Will, Motivation, and Freedom

Part 3 turns to the will and direct passions. Hume argues that the will is an internal impression that immediately precedes action and that reason alone cannot motivate; instead, it serves to inform and guide the passions. He defends a compatibilist view of freedom and necessity in human action: our actions are causally determined by character and circumstances, yet we can meaningfully distinguish free from constrained actions by their relation to agents’ stable motives and voluntary control. This analysis underlies his later account of moral responsibility and blame.

8. Book 3: Of Morals

Book 3 extends Hume’s empirical psychology to ethics, developing a sentimentalist account of virtue, vice, justice, and political obligation.

Moral Sentiments and the Basis of Virtue

In Part 1, Hume argues that moral distinctions are not derived from reason alone but from moral sentiments—feelings of approval and disapproval we experience when contemplating characters and actions from a “common point of view.” He maintains that what we call virtue is what is naturally pleasing or useful to ourselves and others, while vice is what is harmful or disagreeable. Moral evaluation is thus closely tied to human well‑being, yet is mediated through affective responses rather than purely intellectual judgments.

He also articulates the influential claim that one cannot infer an “ought” from an “is”, drawing attention to a logical gap between descriptive premises and normative conclusions.

Justice, Property, and Political Obligation

Part 2 treats justice and related institutions (property, contracts, allegiance) as artificial virtues. Hume contends that in conditions of moderate scarcity and limited generosity, rules of property and promise‑keeping emerge by convention to stabilize social cooperation. These rules are not innate moral principles but historically evolved solutions to coordination problems, gaining moral approval because of their long‑term utility to society. Political allegiance is similarly explained in terms of conventions, public interest, and habit.

Natural Virtues and Character

Part 3 discusses other virtues and vices, often termed natural virtues, such as benevolence, generosity, humanity, gratitude, prudence, and courage. Hume analyzes how these traits are approved because they are useful or agreeable either to the possessor or to others, integrating his earlier account of the passions and sympathy. The result is a unified picture of morality as a human practice rooted in sentiment, shaped by social circumstances, and evaluable in terms of human flourishing, without appeal to divine commands or objective moral properties in the traditional rationalist sense.

9. Central Arguments and Doctrines

Several interrelated arguments and theses structure the Treatise and have become central to Hume scholarship.

Empiricism and the Copy Principle

Hume’s commitment to empiricism is crystallized in the impressions–ideas distinction and the copy principle: every simple idea is derived from a corresponding impression. This principle underlies his approach to meaning and his critiques of metaphysical notions (such as substance or necessary connection) that cannot be traced back to impressions.

Causation, Custom, and Induction

Hume’s analysis of causation holds that we never perceive necessary connection in objects themselves; instead, repeated constant conjunction of events leads the mind, by custom, to expect one event on the appearance of another. This yields his problem of induction: there is no non‑circular rational proof that the future will resemble the past. Proponents see this as a foundational challenge to rationalist accounts of scientific inference; critics debate whether Hume is merely descriptive or normatively skeptical about induction.

Skepticism about the Self and External Objects

Hume’s bundle theory asserts that the self is nothing but a collection of perceptions in flux, undermining the notion of a simple, persisting ego. Similarly, his discussion of external objects suggests that our belief in a continued, independent world arises from imagination and habit, rather than rational insight. These views have been read as radical skepticism, mitigated by Hume’s appeal to the inevitability of natural belief.

Reason, Passions, and Motivation

Book 2 defends the thesis that reason is, and ought only to be, the slave of the passions: reason alone cannot motivate action, but operates instrumentally to serve prior desires. This challenges rationalist moral psychology and supports Hume’s later moral sentimentalism.

Moral Sentimentalism and Conventionalism

In ethics, Hume maintains that moral distinctions are founded on sentiment, not reason, and that virtues are traits useful or agreeable to oneself or others. His theory of justice and political obligation as artificial virtues arising from convention and utility is a key component of this view. The is–ought gap passage further highlights a logical separation between descriptive facts and normative conclusions, influencing later metaethics.

Commentators differ on how to integrate these doctrines into a single interpretive picture—some emphasize Hume’s skepticism, others his naturalism and constructive “science of man,” and still others his irreligion as a unifying motive.

10. Key Concepts and Technical Terms

The Treatise employs a distinctive technical vocabulary. Some of its most important concepts include:

Scholars sometimes diverge in interpreting these terms—for instance, whether “sympathy” is best read as emotional contagion, imaginative projection, or proto‑cognitive empathy; or whether “necessary connection” names a purely psychological phenomenon or some unknown real relation. However, they generally agree that understanding this conceptual toolkit is essential for reading the Treatise as a coherent system.

11. Famous Passages and Problem Cases

Several passages of the Treatise have attained canonical status, both for their intrinsic interest and for the interpretive problems they pose.

The Missing Shade of Blue

In Book 1, Part 1, Section 1, Hume imagines a person who has seen every shade of blue except one; upon seeing a series of shades with the gap, the person could seemingly form an idea of the missing shade without a prior impression. This “missing shade of blue” appears to be a counterexample to the copy principle. Interpreters debate whether Hume intends this as a genuine objection, a trivial exception, or a challenge to readers to refine his principle.

Hume’s Fork and the Problem of Induction

In Book 1, Part 3, Section 1, Hume distinguishes relations of ideas from matters of fact, often called Hume’s fork. In Sections 6 and 12, he develops the problem of induction, arguing that no non‑circular argument can justify the principle that the future will resemble the past. This has been central to discussions of scientific reasoning, with some taking it as a destructive skeptical challenge, others as a descriptive psychological account.

The Bundle Theory of the Self

Book 1, Part 4, Section 6 contains Hume’s famous statement that he can never catch himself without a perception and thus that the self is “nothing but a bundle or collection of different perceptions.” This bundle theory raises questions about personal identity, responsibility, and the unity of consciousness. Critics argue that talk of a “bundle” presupposes what it denies; defenders contend that Hume is offering a revisionary metaphysics of persons.

Reason as the Slave of the Passions

In Book 2, Part 3, Section 3, Hume asserts:

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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.

— David Hume, Treatise, Book 2, Part 3, Section 3

This passage is frequently cited in debates about rationality and motivation. Some interpret it as a radical denial of any rational role in setting ends; others read it more narrowly as a claim about the causal efficacy of reason.

The Is–Ought Passage

In Book 3, Part 1, Section 1, Hume notes that moralists often move from statements of fact to statements of obligation without explanation, highlighting what later became known as the is–ought gap. This short passage has had outsized influence in metaethics as a purported demonstration that normative conclusions cannot be logically deduced from purely descriptive premises.

Justice and the State of Nature

In Book 3, Part 2, Hume’s discussion of justice as an artificial virtue and his critique of “state of nature” stories engages with social contract traditions. His depiction of conventions emerging gradually from mutual interest and utility has been read both as an alternative to and as a variant of contractarianism, leading to extensive debate about his place in political philosophy.

12. Philosophical Method and Empiricism

Hume presents the Treatise as an application of an “experimental method” to the study of human nature, consciously modeled on Newtonian natural philosophy.

The Experimental Method

Hume proposes to start from “experience and observation” of mental phenomena, cataloguing perceptions, emotions, and patterns of behavior, and then inferring general principles—such as the laws of association—from these data. He eschews speculative metaphysics that postulates entities or powers not grounded in impressions. Many commentators see this as an early form of naturalized epistemology, treating questions about knowledge and mind as empirical rather than purely a priori.

Empiricist Commitments

Hume’s empiricism includes:

• The impressions–ideas framework and copy principle as constraints on meaningful discourse.
• A rejection or re‑interpretation of concepts (substance, necessary connection, spiritual soul) that lack identifiable impressions.
• The division between relations of ideas and matters of fact, with only the latter depending on experience.

This method leads him both to skeptical conclusions (e.g., about induction and the self) and to constructive psychological explanations (e.g., of belief, passion, and morality).

Skepticism and Naturalism

There is substantial debate about how to characterize Hume’s overall stance. One line of interpretation emphasizes his skepticism, pointing to his conclusions about the limits of reason and the lack of rational foundation for many common beliefs. Another stresses his naturalism, noting that he accepts common life and science as grounded in custom and human nature, even if they lack demonstrative justification.

A further interpretive strand underscores Hume’s irreligious use of the experimental method—to undermine theological metaphysics and miraculous claims by insisting on empirically grounded reasoning. Others focus on his continuity with Enlightenment projects of empirical psychology and social science. Regardless of emphasis, most commentators agree that Hume’s methodological innovations in the Treatise reshaped subsequent debates about the scope and limits of philosophical inquiry.

13. Moral Philosophy and the Is–Ought Problem

The Treatise’s moral theory centers on sentiment, utility, and the logical structure of moral reasoning.

Sentimentalism vs. Moral Rationalism

Hume argues that moral distinctions are founded on feelings of approval and disapproval, not on reason alone. When we call a character trait “virtuous,” we express a pleasurable sentiment arising from sympathy and reflection on its effects. He contrasts this with moral rationalism, which holds that moral truths are discovered by reason as necessary relations, independent of feeling. Proponents of rationalism (e.g., Clarke) argue that morality has objective, rational structure; Hume counters that reason alone is motivationally inert and can only inform our passions.

Utility, Agreeableness, and Virtue

Hume maintains that character traits are approved to the extent that they are useful or agreeable to their possessor or to others. This framework applies both to natural virtues (like benevolence) and artificial virtues (like justice), though the latter depend essentially on social conventions. Some interpreters read this as an early proto‑utilitarian theory; others emphasize its broader focus on sentiments and character, rather than solely on aggregate welfare.

The Is–Ought Gap

In a brief but widely discussed passage in Book 3, Part 1, Section 1, Hume observes that writers often imperceptibly shift from descriptive statements to normative ones:

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In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary way of reasoning … when of a sudden I am surpriz’d to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not.

— David Hume, Treatise, Book 3, Part 1, Section 1

He claims that this transition is not explained, and that such an ought “expresses some new relation or affirmation” not contained in the is. Later philosophers have interpreted this as asserting a logical gap between facts and values: no purely descriptive premises entail a normative conclusion without an additional normative premise.

Some commentators view this as a cornerstone of modern metaethics, implying that moral judgments cannot be reduced to factual statements. Others argue that Hume’s own sentimentalism provides a natural bridge from facts about human nature and feelings to moral “oughts,” suggesting that his is–ought discussion highlights a need for explicit premises rather than a strict impossibility of derivation.

14. Reception, Criticisms, and Debates

The Treatise has undergone a dramatic shift in reception—from initial neglect to central status in modern philosophy.

Early Reception

Upon publication (1739–1740), the work attracted little attention and modest sales. Those reviewers who noticed it often complained of obscurity and potential impiety. Hume himself acknowledged this lack of success, which led him to present revised versions of many arguments in the later Enquiries.

Nineteenth- and Twentieth‑Century Rediscovery

During the nineteenth century, figures like Thomas Reid and Kant engaged with Hume primarily via the Enquiry Concerning Human Understanding, though Kant credited Hume’s challenge about causation with awakening him from his “dogmatic slumber.” In the late nineteenth and twentieth centuries, historians and analytic philosophers began to treat the Treatise as Hume’s magnum opus, with scholars such as Norman Kemp Smith emphasizing its systematic ambitions.

Major Criticisms

Key lines of criticism include:

Interpretive Debates

Debates about how to interpret the Treatise include:

• Skepticism vs. naturalism: Is Hume primarily a skeptic about reason’s power, or a naturalist providing positive explanations of belief and morality?
• Unity of the work: Are Books 1–3 parts of a single integrated system, or do they reflect shifting aims and tensions?
• Relation to religion: Is the Treatise fundamentally an attack on religious metaphysics, or more broadly an attempt to found secular “moral sciences”?

Contemporary scholarship is divided, but there is wide agreement that the Treatise is a pivotal text for understanding early modern epistemology, moral philosophy, and the rise of empiricist, naturalistic approaches to the human sciences.

15. Legacy and Historical Significance

Over time, the Treatise has come to be seen as a foundational work in several areas of philosophy and the human sciences.

Influence on Philosophy

In epistemology and metaphysics, Hume’s analyses of causation, induction, and the self profoundly influenced Kant, who sought to answer Hume’s skepticism by re‑grounding causality and necessity in the structure of the mind. Later empiricists and logical positivists drew on Hume’s emphasis on experience and his suspicion of metaphysical speculation, though they often modified his positions.

In moral philosophy, Hume’s sentimentalism, emphasis on utility, and focus on character traits influenced Adam Smith, Bentham, J. S. Mill, and later utilitarians, as well as contemporary virtue ethics and expressivist metaethics. The is–ought discussion became a touchstone in twentieth‑century debates about the nature of moral reasoning and the distinction between facts and values.

Development of the Human Sciences

Hume’s aspiration to create a “science of man” anticipated later developments in psychology, cognitive science, and social theory. His use of associative principles to explain thought and emotion prefigured associative learning theories and some contemporary models of cognition. His account of conventions, justice, and property influenced later social contract and game‑theoretic approaches to institutions.

Standing in Hume Scholarship

Within Hume studies, the Treatise is now generally regarded as his most systematic and ambitious work, even though Hume himself recommended the Enquiries as more polished statements of his views. Scholars debate whether the Treatise should be privileged as the primary expression of his philosophy or read alongside the later works as part of a developing project.

Broader Intellectual Legacy

The Treatise is widely regarded as a key text of the Enlightenment, contributing to the shift from theological and rationalist frameworks to empirically grounded, naturalistic accounts of mind, morals, and society. Its combination of skepticism about rational foundations with trust in human nature and common life continues to inform contemporary discussions about the limits of reason, the basis of morality, and the relationship between philosophy and the empirical sciences.