In A Treatise of Human Nature (1739–40), David Hume offers an alternative to the metaphysical speculations of the Ancients and the Early Moderns, replacing them with the experimental method of the Enlightenment. Hume applies this method and Newtonian-style thinking to a study of human nature, with results that are in some ways unexpected, even to Hume himself.Footnote ¹ Hume suggested in a Reference Hume, Ramsay and Popkin1737 letter to his friend, Michael Ramsay, that to prepare to read the manuscript of the Treatise, which he would share with him, it would be helpful to review Malebranche’s Search after Truth; Berkeley’s Principles of Human Knowledge; some of the more metaphysical articles of Bayle’s Dictionary, such as those on Zeno and Spinoza; and Descartes’s Meditations (Popkin Reference Popkin1964: 775). The Treatise ranges well beyond topics in metaphysics and epistemology with its three books: Book 1 “Of the Understanding” and Book 2 “Of the Passions” (published together in 1739) and Book 3 “Of Morals” (1740). Hume suggests that he might have had two more books planned, on politics and on criticism (Book 1 Ad), but these never materialized.

A Treatise of Human Nature, which Hume finished when he was only twenty-five years old, became a monumental work in the history of philosophy, but shortly after its publication, he was conflicted about it. Hume writes that his Treatise was intended to produce “almost a total alteration in philosophy” (HL I: 26). However, he famously says in “My Own Life” (Reference Hume and Miller1777), written in his last year of life, that the Treatise “fell dead-born from the press.”Footnote ² Hume speculates that the Treatise’s immediate lack of success was due more to its style than its content, and he later decided to rewrite Book 1 as An Enquiry Concerning Human Understanding (Reference Hume and Beauchamp1748)Footnote ³ and Book 3 as An Enquiry Concerning the Principles of Morals (Reference Hume and Beauchamp1751). Moreover, some readers regard his short Dissertation on the Passions (Reference Hume and Beauchamp1757) as the reworking of Book 2.Footnote ⁴ In his Advertisement to the second volume of the 1777 edition of Essays and Treatises on Several Subjects, which included these three works, Hume says, “Most of the principles, and reasonings, contained in this volume, were published in a work in three volumes, called A Treatise of Human Nature,” but he has now “cast the whole anew.” He protests the unfairness of critics who have attacked his early work in the Treatise and asks that “the following Pieces may alone be regarded as containing his philosophical sentiments and principles.”

So, how are we to regard the Treatise? Some commentators consider the Treatise as foundational, at least to Hume’s philosophical views; others think we understand Hume’s considered views better by analyzing his later works. L. A. Selby-Bigge suggested that the reader should judge Hume’s philosophy by the Treatise and regarded Book 1 as “of first-rate philosophic importance.” While the Treatise “revels in minutiae, in difficulties, in paradoxes,” the stylistically elegant Enquires “ignore much with which he [Hume] had vexed his own and his readers’ souls” (1893/Reference Selby-Bigge and Selby-Bigge1975: x). In 1941, Norman Kemp Smith published his influential The Philosophy of David Hume, in which he advanced an innovative reading of the Treatise, interpreting it, not as a skeptical work (only) but as a work of philosophical naturalism in which Hume offers a constructive account of how we come to believe and feel what we do. In Hume’s own century and the next, he had largely been regarded as a destructive skeptic about knowledge of the external world and causation, among other things.Footnote ⁵ On Kemp Smith’s reading, however, our natural mechanisms of belief formation offer a kind of justification that reason does not. The naturalistic interpretation has been a prevailing, although not unanimous, reading of Hume ever since.Footnote ⁶

Recently, Paul Russell has argued that Hume’s Treatise is of primary significance in establishing Hume’s anti-religious stance, which Russell thinks is the foundation to Hume’s whole philosophical system, even as it unfolds in later works. Russell calls the Treatise “fundamentally an effort to discredit the metaphysical and moral paraphernalia of orthodox religious systems” (Reference Russell2008: 285). Thus, Hume’s skeptical arguments in the Treatise are not basic. They are meant to serve another goal, that of discrediting the dogma of Christian theology “with a view to redirecting philosophical investigations to areas of ‘common life’” (290) . James Harris has responded with a very different understanding of the Treatise. He argues that it holds no special place in Hume’s oeuvre and suggests that Hume is not pursuing a broad agenda in his works. Hume is, rather, assuming the perspective of a dispassionate observer with no single aim to which all of his works contribute: “The abandonment of the project of the Treatise would appear … to have been the giving up of the whole idea of a philosophical system, in favour of several distinct and different kinds of philosophical projects” (Reference Harris2015:13).

It is at least an exaggeration to say that Hume abandoned the Treatise, when he makes it clear that its contents are contained in his later works. We certainly should not abandon it: contemporary scholars would be impoverished in their understanding of eighteenth-century empiricism and Humean principles without it. (In saying this, I am tipping my hand – to the view that there is much basic to Hume and his method in it.) To neglect the Treatise would offer a distorted picture of the scope and depth of Hume’s thought, especially since some of the most confounding topics broached in the Treatise are not discussed in his later works (as Selby-Bigge observes). Notably absent are Hume’s account of personal identity; the lengthy discussion of the ideas of space and time; the detailed explanation of the human mechanism of sympathy; the distinction between the direct and the indirect passions and their respective analyses; the characterization of passions as “distinct existences” referring to nothing outside of themselves; the argument that reason and passion cannot be opposed over direction of action; the argument that reason alone does not motivate; the famous paragraph concerning how “ought” conclusions cannot be derived from “is” premises; the role of a “general” or “common” point of view in making moral judgments; the presentation of the “undoubted maxim” that no action is virtuous unless there be a motive to do it distinct from regard to its morality, along with the search for the natural motive to just actions; and the generation of the artificial virtues, including the argument concerning the establishment of property as the first social convention and the detailed discussion of the obligation to keep promises and to follow the rules of government. It is a long and formidable list.

Annette Baier thinks that the most reasonable explanation why Hume omitted much of this material from his later works is that he thought the arguments were faulty ones or the theses false (Baier Reference Baier2008b: 257–58). However, other explanations are equally plausible, including that at least some of the arguments were diversions from Hume’s main aims in the later writings, or that they did not fit stylistically in the recasting of his ideas, or both. This is not to deny that Hume may have changed his mind about some of the missing arguments or about other issues that are treated in both the Treatise and in his later works. It is only to say that arguments from silence are not by themselves good ones. We need other reasons rooted in the analysis of the two Enquires and the Dissertation, or evidence from other sources such as Hume’s letters to draw conclusions about reasons for the omissions. Meanwhile, many of these treatments have made an impact in the history of philosophy, and all of them have generated voluminous discussion among scholars. For instance, commentators have written hundreds of scholarly papers and many books on Hume’s search for a unified idea of the self, which results in his “bundle theory” of the person. It is an account Hume was not satisfied with, but his analysis is important to showing where strict empiricism leads. Hume’s characterization of the passions and his thesis concerning the motivational impotence of reason alone have inspired the contemporary Humean view of motivation. The Humean view, saying that both a belief and a desire are necessary for motivation, is still the favored theory among naturalists. Hume’s paragraph on “is” and “ought” in the Treatise provides a succinct statement of what is typically known as the “fact-value” gap in ethical theory – although readers debate exactly what Hume meant when he wrote this single significant paragraph. Hume’s detailed Treatise presentation of his theory of justice and artificial virtue has spawned much analysis of the complexities a naturalistic approach to virtue generates for morality in institutions.

It is impossible to represent the richness of the Treatise in a volume with just fourteen essays. However, the purpose of the Cambridge Critical Guides is not to provide comprehensive coverage of all the themes in a work but to focus on selected topics of interest to scholars. Thus, this collection does not contain overview essays on causation or on justice or on other large topics in Hume; these can be found in many other books. It offers seven focused chapters in Hume’s epistemology and philosophy of mind, six in his passions and ethics, plus one essay on the early reception of the Treatise. Some of the topics represented here are rarely discussed, for instance, the significance of Hume’s treatment of the passion of curiosity (Mazza, Chapter 9) or the critical responses to Hume’s psychological account of how we come to believe in external objects (Coventry, Chapter 5). Some are socially relevant, for instance, Hume’s depiction of the human psychological tendency to view the world in inegalitarianism ways and its impact on our analysis of virtue (Watkins, Chapter 13). Several of these essays highlight the unity of Hume’s approach in the Treatise, showing how the principles of Hume’s epistemology and psychology in Book 1 are foundational to his discussion of the passions and of morality in Books 2 and 3 (Wright, Chapter 1; Coventry, Chapter 5; Paxman, Chapter 8; Garrett, Chapter 10; and Radcliffe, Chapter 11).

In Chapter 1, “The Association of Ideas in Hume’s Treatise,” John P. Wright presents the historical precedents to Hume’s theory of the association of ideas. He argues that the details of Hume’s associationism in Book 1 are central to fulfilling his goals in the Treatise – of applying the experimental method to provide a foundation for the sciences (including the science of human nature). Wright traces associationism in Books 1 and 2 and then intriguingly identifies three physical (brain) models of association in Hume’s account, ending with a discussion of the influence of Hume’s associationist principles on his Scottish successors.

Donald L. M. Baxter (Chapter 2) next shows how Hume’s metaphysics in the Treatise can be reduced to three general principles. While some readers may think that Hume decries metaphysics, the sense of metaphysics that Baxter imputes to Hume is metaphysics as “the science of the most general features of the world as it appears to us”; it is a part of logic, or the way we think. The three general Humean principles are (1) The Contradiction Principle, that the distinctly conceivable implies no contradiction; (2) The Possibility Principle, that what implies no contradiction is possible; and (3) The Conceptual Separability Principle, that things are different if and only if separable in conception. In Baxter’s hands, the elegance of Hume’s system is a marvel.

In Chapter 3, “Hume on Belief,” Jennifer Smalligan Marušić addresses some pressing questions in Hume’s theory of belief in the Treatise. On her interpretation, Hume assumes that belief is a function of either the content conceived or the manner of conception. However, Marušić asks, why does he assume that these are the only options – and finds an answer in Hume’s supposition that we have introspective access to belief. She then explores both Hume’s argument that believing is not a function of content and the question exactly what “manner of conception” constitutes belief for Hume. Furthermore, Hume appears to change his view of belief in the Appendix. Marušić investigates his motivation for doing so and concludes that the variety of manners of conception that we experience cannot be translated into degrees of force and vivacity, Hume’s principled way of distinguishing mental states, even though they are accessible by introspection.

Hsueh Qu’s contribution (Chapter 4), “‘All the Logic I think Proper to Employ’: Hume’s Rules by which to Judge of Causes and Effects,” presents an interpretation of the standards Hume formulates in the Treatise for distinguishing good and bad causal inferences and then asks whether Hume follows his own rules. Qu offers two case studies. One traces the causal reasoning underlying postulation of a “calm” passion (a passion with little phenomenological manifestation) as a cause of action. The other analyzes the causal reasoning involved in Hume’s “Copy Principle” (that all meaningful ideas copy previous impressions), along with Hume’s claim that the Principle admits an exception in “the missing shade of blue.” Qu deftly argues that Hume’s reasoning abides by his own rules in the first case but violates them three times in the second: Hume should not have admitted that someone who has experienced all shades of blue but one could muster an idea of the missing shade.

In Chapter 5, “Imagining the Unseen: The External World of Hume’s Treatise,” Angela M. Coventry considers Hume’s complex Book 1 account of the psychological principles that lead us to believe in the existence of an external world. She shows that external object belief is crucial to the rest of the Treatise as well, since passions and morals are sometimes concerned with the relation of objects to ourselves and to others. Coventry’s discussion includes a survey of early and later responses to Hume’s analysis. Among early critics, Thomas Reid and Mary Shepherd saw Hume as a complete skeptic about external objects (although Shepherd is also a critic of Reid). Adam Smith, Coventry shows, echoes numerous strains of Hume’s views on our cognitive capacities in his own account of believing in external objects. Coventry displays how Hume’s account has been extended and developed in the contemporary fields of philosophy of perception, cognitive science, and developmental psychology in ways that Hume himself did not think to do.

Next, Annemarie Butler discusses what she calls “The Updating Problem for Hume’s Account of Belief in Personal Identity,” in Chapter 6. Building on her discussion of the “Present Self Problem,” the question of how a person can build a present-tense belief in oneself, Butler analyzes the problems involved in holding subsequent beliefs in continuance of the same self. Since “ideal” connections among perceptions disappear as soon as they become past, Hume’s theory has a problem accounting for the belief that the “updated” self is a continuation of the previous self. In a “present-self” belief, the content of the belief does not include its believer. Forming a later belief in the self-same thinker ultimately requires identifying the former believer and her self, which, Butler argues, generates a “nasty” problem for Hume.

In Chapter 7, “Experimental Philosophy, Blind Submission, and Hume’s Other ‘Sceptical Principles,’” Miren Boehm offers an interpretation of Hume’s “Conclusion” in Book 1 that she hopes will render Book 1 “a more intelligible and harmonious work.” The discord lies in Hume’s optimism about applying the experimental method to study human nature versus the skeptical results highlighted in the Conclusion, causing Hume to despair about philosophy. Boehm proposes causes and reasons for diminishing the importance of Hume’s doubts. She then argues that while the forces of curiosity and ambition motivate Hume to continue his project in the Treatise, his commitment to “sceptical” philosophical principles precedes them. These principles include attitudes and guidelines for being a true skeptic, and they steer us away from speculation and shore up Hume’s approach. Boehm closely examines the now-called “Title Principle,” a rule by which we can distinguish reason that is worthy of our assent from reason that is not.

Katharina Paxman, in “How to Read Book 2 of the Treatise” (Chapter 8), argues that the importance of Book 2 of the Treatise, “Of the Passions,” had been largely overlooked by readers until Páll Árdal’s Passion and Value in Hume’s Treatise (Reference Árdal1966). Moreover, even recent commentators who take Book 2 seriously have “cherry-picked” topics for discussion, rather than reading it wholistically with the rest of Hume’s text. Paxman outlines an approach to reading Book 2 that takes Hume at his word when he says of Book 1 and 2 that “the subjects of the Understanding and Passions make a compleat chain of reasoning by themselves” (Ad 1739). Paxman offers a two-pronged guide to the reading of Book 2. (1) It should be understood as a development of the Humean principles introduced in Book 1; and (2) it should be read alongside his later Dissertation on the Passions, his recasting of the Treatise material on the passions, with an eye to noticing what is and what is not included in the Dissertation. The latter allows us to identify key elements of Hume’s theory of the passions, while the former helps clarify the methodological aims of Book 2 as a continuation of Book 1. Doing a wholistic reading reveals that Book 2 is a development of the principles of association from Book 1, generating new psychological laws of association and transference among passions (impressions) that can blend and mix in the way ideas cannot.

Next, Emilio Mazza observes in Chapter 9, “Hume on Curiosity: A Conclusion to Treatise Book 2?” that Books 1 and 3 of the Treatise each have a section entitled “Conclusion.” Book 2 simply ends with Hume’s discussion of curiosity. Is it plausible to think that a discussion of the passion that prompts us to search for truth and engage in philosophy is a suitable conclusion to “Of the Passions”? After comparing the contents of the other two conclusions to T 2.3.10, “Of curiosity, or the love of truth” and finding that each final section portrays the philosopher as “a true skeptic, a hunter or a gamble, and an anatomist,” Mazza analyzes the structure of Book 2. He explores the relation of curiosity to pleasure, for Hume, and shows how Hume compares the activity of philosophy to that of hunting, and gaming, which all are sources of pleasure under certain circumstances. Mazza’s contribution displays the multiple intellectual influences on Hume’s thoughts about curiosity, including Montaigne, Pascal, Locke, Hobbes, Du Bos, Hutcheson, and Berkeley. Ultimately, Mazza leaves open the question of why Book 2 did not have a proper conclusion and reflects on the change in Hume’s tenor after the Treatise, from anatomist to painter.

In “Hume’s Geography of Feeling in A Treatise of Human Nature” (Chapter 10), Don Garrett advances an interpretation of “the mental geography” from which Hume’s theses about the sources of our feelings in Books 2 and 3 arise. Garrett’s goal is to provide clarity about Hume’s classification of the operations of feeling, disagreement over which has led to misunderstandings about the meaning of some of Hume’s central claims regarding feeling, action, and morality. Garrett first explains Hume’s three highest-level distinctions: that between impressions and ideas, that between original impressions and secondary impressions, and that between the passions and the other emotions. To understand this third distinction, Garrett distinguishes three different senses of the term “emotion.” He then examines five different kinds of secondary impressions that Hume recognizes: (1) sensible agitations, (2) feelings of or from mental operations, (3) volitions, (4) the passions, and (5) sentiments of taste.

My own contribution, “Some Vexations about Character in Hume’s Treatise” (Chapter 11), highlights Hume’s key observations about character and the problems they create, given other claims in the Treatise. I address three questions: whether Hume can sensibly talk about enduring traits that constitute character, given his depiction of the mind as constantly in flux; whether character is “objective” or it is a creation of spectators; and whether Hume’s treatment of virtue and vice is only descriptive of how we derive our moral categories, or also contains prescriptions. First, I argue that since Hume distinguishes between the feeling of a motive and its causal efficacy, he can observe that, while feelings may be in constant flux, character is determined by which has the force to produce action consistently. Second, the contingency of moral categories on human nature is not the same as creation of the features that fall under those categories. Third, Hume both describes our process of moral discrimination and offers guidance about making judgments of virtue and vice. He is not defending a view of moral character but employing the norms that arise from our own sentiments, language, and human practices.

In Chapter 12, “Hume on Promising and Self-Obligation,” Rachel Cohon asks pressing questions about Hume’s account of the obligation to keep promises, central to his theory of artificial virtue. This duty depends on social conventions, but why does Hume find promises mysterious? She analyzes Hume’s argument that if two people knew nothing of social convention, they could not exchange promises “even though they cou’d perceive each other’s thoughts by intuition” (T 3.2.5.2). Moreover, Hume says that when we make a promise “the obligation arises from our mere will” (T 3.2.5.3), but why is this significant? Cohon argues that despite Hume’s avowal that there can be no mental act of willing that would be necessary and guarantee a binding promise, he himself assumes that some kind of intentional act of self-obligation is required for (successful) promising. His convention theory is constructed to make that possible. Cohon reinforces her thesis about Hume by contrasting his theory of the creation of promissory obligation with a present-day account from T. M. Scanlon, one that does not rely on a voluntary act of self-obligation enabled by convention.

Margaret Wakins next shows in Chapter 13, “No Men Are Created Equal: Rank, Passions, and Virtues in Hume’s Treatise,” that Hume’s observations of humans in the Treatise show us to be thoroughly inegalitarian by nature. She first highlights inegalitarianism in Hume’s analysis of our passions. Inequality is exhibited (1) in the influence of property on pride, (2) in the fact that the “great resemblance among all human creatures” ironically intensifies the pleasure we derive from controlling other people, and (3) in the psychology of “comparison,” whose consequence is that we are more likely to sympathize with and experience compassion toward those whom we consider superior than with or toward those we consider inferior. While Watkins’s thesis is not that the Treatise contains a normative argument for inegalitarianism, she argues that appeal to Hume’s discussion of morality does not help considerably in alleviating the inequities in our attitudes and practices. On Watkins’s reading, Hume’s account of how we denominate the natural and artificial virtues reinforces the inegalitarian nature of our passions.

In the concluding chapter of this volume (Chapter 14), “The Eighteenth-Century British Reception of Hume’s A Treatise of Human Nature,” Mark G. Spencer and Mikko Tolonen argue that the Treatise “made a livelier entry into the world than a disappointed Hume lamented.” First, they cite an accumulation of scholarly work since the late 1940s referring to eighteenth-century reviews published in Britain, Ireland, and Europe indicating that in fact some readers were taking notice. Building on this research, the authors cast a new perspective on some known texts from the 1700s to show the early influence of the Treatise and to add some overlooked eighteenth-century sources to the discussion. Their investigations reveal that some of Britain’s leading intellectuals were debating the Treatise’s theories in reviews and in other venues. While acknowledging that Hume’s Treatise was far from a commercial success in his lifetime, Spencer and Tolonen show that there is a more compelling narrative to be told about its reception than has long been thought.