A Treatise of Human Nature

A Treatise of Human Nature Mike Ervin

A Treatise of Human Nature

David Hume wrote A Treatise of Human Nature (published 1739–40) as an ambitious attempt to build a “science of man.” Hume’s aim was descriptive and explanatory: to examine human nature using careful observation and psychological explanation so we can understand how belief, emotion, morality, and reasoning actually arise. The work is divided into three books — Of the Understanding, Of the Passions, and Of Morals — and together they develop a consistent empiricist, naturalistic account of the mind and of human life. Below I summarize the central theses, arguments, and consequences.

General method and programme

Empiricism: All ideas come from impressions (sensory experiences and lively inner feelings). Ideas are faint copies of impressions; complex ideas are built by combining simpler ones.
Psychological explanation: Hume treats philosophical problems as psychological or explanatory puzzles about how humans actually think, feel, and act, not as mysteries to be resolved by pure abstract reason.
Mitigated skepticism: Hume is skeptical about the powers of reason to give metaphysical certainty (especially about causation, the self, God). But he is not a destructive skeptic who abandons ordinary life; he argues that custom and habit guide our practical beliefs and actions.

Book I - Of the Understanding (knowledge, ideas, and belief)

This book develops Hume’s epistemology and theory of ideas.

Impressions and ideas
impressions are vivid, immediate sensations and feelings; ideas are the fainter copies we manipulate in thought.
Mental operations such as copying, compounding, transposing, and abstracting produce complex ideas from simple ones.
Association of ideas
Ideas connect by resemblance, contiguity, and cause/effect (constant conjunction). These associations explain memory, imagination, and reasoning.Limits of human understanding
Hume’s Fork (implicit): There are relations of ideas (analytic, necessary truths—e.g., geometry) and matters of fact (synthetic, contingent truths known by experience). Reason alone cannot establish matters of fact about the world; experience does.
Problem of induction: We infer future regularities from past experience (e.g., that the sun will rise tomorrow) but we have no rational, demonstrative basis for assuming nature’s uniformity. Our belief in induction rests on custom/habit, not logical proof.
Causation and necessary connection: We never observe a necessary power or necessary connection; what we observe is constant conjunction (one event regularly followed by another) and through habit we come to expect the successor. The idea of “necessary connection” is a product of psychological expectation, not rational insight into metaphysical bonds.
Skepticism about metaphysics
Hume undermines rationalist claims to knowledge of substances, soul-essences, and metaphysical necessity.
He insists on examining the origin of ideas: if an idea cannot be traced to an impression, it should be rejected (this is his famous empiricist test).
Probability and belief Belief differs in degree from mere imagination or conception; it is a lively idea grounded in habit. Probability judgments arise from experience and the mind’s tendency to infer cause and effect.

Book II — Of the Passions (emotions and motivation)

Hume analyzes the passions (emotions) and how they interact with belief and action.

Nature of passions
Passions are lively perceptions and are the primary motivators of action. Reason alone cannot move the will; it is the “slave of the passions.”
Hume distinguishes calm passions (e.g., benevolence) from violent passions (e.g., anger) and analyzes how they arise, vary, and combine.
Role of sympathy
Sympathy (what today we’d call emotional contagion or empathy) spreads feelings between persons and underpins social sentiments and moral judgments.
Action and motivation
Practical behavior depends on desires and passions directed by belief. Reason provides information about means and consequences but cannot produce the desire itself.
Moral approval/disapproval
Early groundwork here for his later moral theory: certain emotions or approvals underlie our moral attributions.

Book III — Of Morals (ethics and social rules)

Hume develops his sentimentalist ethics and social account of justice.

Moral sentimentalism
Moral judgments spring from sentiment, not reason. When we call something virtuous or vicious, we are expressing a feeling of approval or disapproval produced in us (often via sympathy).
Reason can inform (e.g., about consequences) but cannot by itself produce moral obligation.
Virtue and utility
Hume argues that many virtues are useful to society (justice, fidelity, truthfulness) and thus become objects of approbation because they promote social flourishing. Some virtues are natural benevolence; others (like justice) are artificial and arise from social conventions.
Justice as an artificial virtue
Justice (property rules, promises) is not naturally rooted in immediate benevolence but results from human institutions to solve scarcity and coordinate society. Our approval of justice follows from appreciating its utility.
Moral standards and impartial spectator
Moral evaluation depends on a habitual spectator perspective: we judge actions by considering how an impartial observer, sympathetic yet disinterested, would respond.
Relation to law and politics
Hume’s moral theory supports a pragmatic, historically informed account of law and civic institutions, emphasizing habit, custom, and usefulness over abstract principles.

Key specific doctrines and famous problems

Bundle theory of the self: There is no underlying, simple, permanent “self” observed in experience. What we call the self is a bundle or collection of perceptions linked by memory and causation.
Problem of induction: No rational justification for inferring unobserved matters of fact from observed ones; induction is grounded in custom. This problem later became central in epistemology and philosophy of science.
Causation as psychological: Causation is not an observable necessary relation but a habit of mind formed by constant conjunction.
Is–ought distinction (proto-formulation): Hume famously notes that many writers move from descriptive statements (“is”) to prescriptive conclusions (“ought”) without justifying the move; moral facts are not derivable from facts alone.
Religion and miracles: Hume is skeptical of miracles and argues that testimony for miracles must be weighed against the improbability of such events and the unreliability of testimony. (In the Treatise he raises the psychological and epistemic problems for miraculous claims; later essays and Dialogues Concerning Natural Religion expand on his arguments about religion.)
Free will and necessity: Hume is a compatibilist: moral responsibility is compatible with deterministic causal laws. “Necessity” in human action is like causal necessity — regularity that makes behavior predictable and grounds responsibility.

Tone, style, and rhetorical strategy

Hume combines analytic subtlety with psychological observation and a conversational tone. He often anticipates objections and uses examples drawn from ordinary life.
He presents radical conclusions (skepticism about metaphysics, denial of a substantial self) cautiously, with repeated appeals to common sense and an insistence that practical life remains governed by habit and custom.

Reception and influence

The Treatise was at first poorly received and Hume later reframed many of its ideas in essays and the Enquiry for better clarity. Nevertheless, the Treatise is now recognized as Hume’s foundational philosophical work.
Its influence is vast: Hume shaped modern empiricism, skepticism about causation and induction, sentimentalist moral theory, and debates about personal identity. Immanuel Kant credited Hume with awakening him from his “dogmatic slumber.” Hume’s ideas remain central in epistemology, philosophy of mind, ethics, philosophy of religion, and the philosophy of science.

Why the Treatise still matters

Hume shows that many philosophical puzzles dissolve when we look at how the human mind actually forms beliefs and emotions. He replaces metaphysical speculation with a careful, often unsettling psychology of belief and action. The Treatise invites us to accept limits to metaphysical certainty while offering a humane, naturalistic account of how we live and think — one that continues to shape contemporary philosophy.