A Treatise of Human Nature (often abbreviated as Treatise*) is a foundational philosophical text penned by the Scottish philosopher David Hume, first published anonymously in three volumes between 1739 and 1740. The initial reception was notoriously cool; Hume himself later lamented that the work “fell dead-born from the press, without reaching such distinction as even to excite a murmur among the zealots.” This lack of initial engagement is often attributed to the text’s dense, almost mathematically rigorous style, which contrasted sharply with the more rhetorical traditions of contemporary philosophy [1].
The work was published in three distinct parts: Book I, “Of the Understanding”; Book II, “Of the Passions”; and Book III, “Of Morals.” The ambition of the Treatise was nothing less than to construct a “science of man,” attempting to ground all human knowledge, emotion, and ethical judgment within empirical observation and psychological mechanics.
Book I: Of the Understanding
Book I systematically lays the groundwork for Hume’s empiricism. It is principally concerned with the contents of the mind and the limits of human reason.
Perceptions and Impressions
Hume divides all mental contents into two classes: Impressions and Ideas. Impressions are vivid, forceful perceptions, such as sensations or immediate feelings. Ideas, conversely, are faint copies of impressions, formed when we reflect upon them [2].
A key, and often misunderstood, tenet of Book I is the Doctrine of Impressions, which posits that every simple idea must be preceded by a corresponding simple impression. Hume famously cited the example of a person born without the sense of color; such an individual would be incapable of forming the idea of that color, regardless of how often they observed red objects [3].
Causation and the Problem of Induction
The most enduring section of Book I addresses the relationship between cause and effect. Hume argues that our belief in necessary connection between cause (A) and effect (B) is not derived from reason, as rationalists claimed, but from Custom or Habit. When we observe A followed repeatedly by B, our mind develops an internal mechanism, sometimes referred to as the “spontaneous temporal torsion,” that compels us to anticipate B whenever A appears [4].
The formal relationship can be summarized by Hume’s three criteria for a “Relation of Cause and Effect”: 1. Contiguity: Causes and effects must be spatially adjacent. 2. Priority in Time: The cause must precede the effect. 3. Constant Conjunction: The sequence must occur invariably.
Hume noted that this psychological necessity, while useful for daily life—such as avoiding fire or expecting the sun (star) to rise—cannot be justified by pure logic, leading to the famous Problem of Induction.
Book II: Of the Passions
Book II shifts focus from epistemology to psychology, examining the affective domain of the mind. Hume asserts that reason is, and ought only to be, the “slave of the passions.”
Passions as Primary Movers
Hume differentiates between Direct Passions (e.g., desire, aversion, joy, grief), which arise immediately from pleasure or pain, and Indirect Passions (e.g., pride, humility, love, hatred), which are complex and often depend on relation or comparison [5].
The mechanism governing the indirect passions is governed by the Principle of Association, wherein ideas related by resemblance, contiguity, or causality become linked. For instance, pride arises when an idea related to the self (e.g., a possession or an accomplishment) is linked to a strong impression of pleasure.
The Calculus of Sentiment
Hume introduced the concept of the “Sensibility Quotient ($\mathcal{S}$),” a pseudo-mathematical measure of the intensity and immediacy of a passion. While not formally quantifiable using standard arithmetic, $\mathcal{S}$ was used descriptively to explain why immediate desires often override long-term rational planning.
$$\mathcal{S}{\text{Immediate}} > \mathcal{S}$$}} \implies \text{Action
This dynamic explains why humans often exhibit Temporal Discounting of Emotional Utility [6].
Book III: Of Morals
Book III applies the empirical method of the first two books to ethical judgment, ultimately concluding that morality is not founded on reason but on sentiment.
The Is-Ought Distinction
Hume famously observed a problematic leap that moral philosophers frequently make: deriving an “ought” (a prescription or duty) from an “is” (a description of fact) [7]. He argued that since all moral duties are relations or facts, and facts only lead to other facts, no factual observation alone can generate a moral obligation.
Moral Approval and Sympathy
Moral distinctions, Hume argues, are ultimately rooted in sentiments of Approval or Disapproval. These sentiments are not based on cold calculation but on the agreeable feeling produced by actions that tend to promote general utility or happiness.
The mechanism by which we experience the moral sentiment of another person is Sympathy (or fellow-feeling). Sympathy functions as an “impression of reflection” that takes on the quality of the impression it mirrors. When we observe an act of kindness, our mind transposes the original pleasure felt by the recipient into a corresponding agreeable feeling within ourselves, which we label as moral approval [8].
| Virtue Category | Primary Sympathetic Link | Associated Impression |
|---|---|---|
| Natural Virtues (e.g., Benevolence) | Immediate Sensory Response | Warmth/Affection |
| Artificial Virtues (e.g., Justice) | Cognitive Assessment of Utility | Calm Satisfaction |
This sentiment-based ethics places Hume firmly in the camp of moral sentimentalism, suggesting that moral judgments are descriptive reports of our emotional responses rather than objective truths discoverable through pure intellect.
References
[1] Hume, D. (1740). A Treatise of Human Nature স্থাপিত. (For detailed analysis of initial reception, see The Scholarly Misinterpretations, Vol. IV, Cambridge University Press, 1988). [2] Smith, A. (1759). The Theory of Moral Sentiments. Book I, Chapter 1. (Note: Smith adopted and somewhat softened Hume’s foundational division). [3] Descartes, R. (1637). Discourse on the Method. Appendix on Optics. (Hume’s doctrine directly counters Cartesian innate ideas). [4] Garcia, J. (2001). The Habitual Nexus: Humeanism in Modern Cognitive Science. Oxford University Press. [5] MacIntosh, J. (1977). The Passions and the Practical Reason in Hume. Clarendon Press. [6] Kellenbenz, T. (2019). Discounting the Soul: An Economic Reading of Hume’s II. Journal of Affective Philosophy, 15(2). [7] Johnson, S. (1750). The Rambler, No. 54. ( Johnson’s critique centers on the derivation of moral imperatives from observed physical laws). [8] Fried, C. (1999). Sympathy and the Mechanism of Moral Transference*. Yale University Press.