A Treatise of Human Nature

Summary

Hume subtitled his first book 'an attempt to introduce the experimental method of reasoning into moral subjects', and he meant it literally: as Newton had explained the heavens by a few observed principles, Hume would explain belief, emotion and morality by a few principles of the mind. Book I, Of the Understanding, begins by dividing all perceptions into vivid impressions and fainter ideas, and holds that every simple idea is copied from a prior impression. Ideas are linked by three associative relations, resemblance, contiguity and cause and effect, which Hume calls a gentle force of attraction in the mental world. The heart of the book is his analysis of causation. We never perceive any necessary connexion between a cause and its effect; we observe only that one kind of event has constantly followed another. Inference from the past to the future cannot be justified by demonstration or by probable reasoning without circularity, so it rests on custom: repeated conjunction produces a felt determination of the mind to pass from one idea to its usual attendant. Belief itself is not a further idea but a manner of conceiving, a greater force and vivacity. Part IV pushes this scepticism further. Our confidence in an external world, in material substance and in an enduring soul all turn out to be products of the imagination. Looking inward, Hume finds no self at all, only a bundle of perceptions in perpetual flux. The book then closes with an extraordinary personal confession: the philosopher, isolated and frightened by his own conclusions, is rescued not by argument but by dinner, backgammon and friends. Book II, Of the Passions, offers a detailed mechanics of pride and humility, love and hatred, built on a double relation of impressions and ideas, and introduces sympathy, the communication of feeling from one person to another, as a fundamental principle of social life. It then argues that reason alone can never motivate action or oppose a passion. Book III, Of Morals, published separately in 1740, concludes that moral distinctions are not discovered by reason but felt through a moral sentiment, notes the illegitimate slide from 'is' to 'ought' in moralists' arguments, and treats justice, property and promising as artificial virtues invented by convention to serve human interest, while benevolence and similar traits are natural virtues approved through sympathy. The young author famously complained that the work 'fell dead-born from the press', yet its arguments on causation, induction, the self and the motivational priority of feeling define problems philosophers still work on.

Top Lessons

• Expectation is a habit of mind, not a perception of necessity

  Hume shows that our confidence that the future will resemble the past cannot be proved, yet we cannot stop relying on it. Custom does the work reason was supposed to do, which makes our most basic inferences natural rather than rational.

• Looking inward finds perceptions, never a perceiver

  When Hume searches introspectively for a self, he finds only particular sensations and feelings. The unity we attribute to a person is supplied by the imagination, an idea that troubled Hume himself in the appendix.

• Feeling sets ends; reasoning only finds routes to them

  Book II argues that reason cannot by itself move us to act or oppose a desire. Apparent struggles between reason and passion are really contests between calm and violent passions, so changing behaviour means working on feeling and habit.

• Scepticism is cured by living, not refuted by argument

  The end of Book I dramatizes a philosopher overwhelmed by doubt who recovers through company and play. Hume concludes that nature, not reason, keeps us from total scepticism, and that philosophy should be pursued only in a mood where it remains agreeable.

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Medical Lens

The Treatise is a founding text of associationist psychology, the tradition that runs through David Hartley and James Mill to nineteenth-century physiological psychology. For psychiatry its richest pages are the conclusion to Book I, a first-person account of 'philosophical melancholy and delirium' relieved by company and diversion, which readers often set beside Hume's 1734 letter to an unnamed physician (often identified as George Cheyne or John Arbuthnot) describing his breakdown in terms of the 'disease of the learned'. Linking the two is an interpretation, not a diagnosis. Hume's account of madness as excessive vivacity of ideas, and of belief as a feeling rather than a judgment, bears on modern discussions of delusion. His claim that reason cannot by itself motivate is a reference point in debates about insight, akrasia and the limits of purely cognitive therapy. Sympathy in Book II anticipates later work on emotional contagion and empathy.

• Book I, Part III, Sect. 9–10

  Repetition, education and lying give ideas the vivacity of belief; a lively imagination can degenerate into madness when a ferment of the blood and spirits makes fictions indistinguishable from memories.

• Book I, Part IV, Sect. 6

  Introspective search for the self finds only perceptions; the mind compared to a theatre with no fixed stage; sleep and death as the absence of a perceiving self.

• Book I, Part IV, Sect. 7

  Hume's despair, dread of error, sense of monstrous isolation and 'heated' brain, followed by recovery through dining, backgammon and conversation; he names his mood a splenetic humour.

• Book II, Part I, Sect. 11; Part II, Sect. 5

  Sympathy as the communication of passions between people, including how the opinions of others sustain pride and how feelings reverberate between minds.

• Book II, Part III, Sect. 3–4

  Reason as servant of the passions; calm versus violent passions; why strong emotion can override clear knowledge of one's interest.