Reason Is the Slave of the Passions

Historical Context

David Hume composed A Treatise of Human Nature during an intense period of private study in France, primarily at La Fleche, the Jesuit college where Descartes had been educated. Book II, "Of the Passions," was published in 1739 as part of the first two books of the Treatise, and it is in this book, in Part III, Section 3, that Hume makes his famous declaration about the relationship between reason and the passions. The Stanford Encyclopedia of Philosophy notes that Hume was writing in direct opposition to the rationalist moral philosophy that had dominated British and continental thought, particularly the tradition descending from Samuel Clarke and William Wollaston, which held that moral distinctions are discernible by reason in the same way that mathematical truths are. On the rationalist view, morality is a matter of correct reasoning: virtue consists in conforming one's actions to the eternal fitness of things, and vice is a form of intellectual error.

Hume's argument was also aimed at the moral sense tradition of his predecessor Francis Hutcheson, though in a more nuanced way. Hutcheson had argued that moral distinctions are grounded in a special internal sense, a capacity for moral perception rather than rational deduction. Hume largely agrees with Hutcheson's sentimentalist orientation but pushes the analysis further by giving a systematic account of the relationship between reason and passion. The Internet Encyclopedia of Philosophy observes that Hume's position on the passions has been one of the most influential elements of his philosophy, shaping modern moral psychology and the contemporary debate between rationalism and sentimentalism in ethics. The context of the Scottish Enlightenment, with its intense interest in the science of human nature, gave Hume's analysis its distinctive empirical orientation: he approaches the passions not as obstacles to be overcome by reason but as the fundamental motivational forces of human psychology.

Meaning

The statement that "reason is, and ought only to be the slave of the passions" means that reason by itself can never produce action. Hume's argument rests on a distinction between two functions of reason. Reason can discover truths about relations of ideas (mathematics, logic) or about matters of fact (empirical truths about the world). In neither case does the discovery of a truth, by itself, motivate anyone to act. A person may know perfectly well that a certain action will produce a certain outcome, but unless they desire that outcome, the knowledge alone moves them to nothing. Reason, in Hume's phrase, is "inert" with respect to motivation: it can tell us what is the case and what will follow from what, but it cannot tell us what to care about or what to pursue.

The passions — desires, aversions, emotions, sentiments — are the only sources of motivation. A passion is what Hume calls an "original existence," a feeling that is simply there and is not subject to truth or falsity. It cannot be contrary to reason in the way that a belief can be false, because it does not represent the world as being any particular way. Reason enters the picture only as a servant: once a passion has set an end, reason can calculate the best means to achieve it. If I desire to eat, reason can tell me where food is available and how to prepare it. If I desire to avoid pain, reason can tell me which actions are likely to cause pain. But reason cannot generate the desire to eat or the aversion to pain in the first place. The word "slave" is deliberately provocative: reason has no authority over the passions, no power to command or veto them. It can only serve them by discovering the causal connections that allow passions to be effectively acted upon.

Philosophical Interpretation

Hume's claim that reason is the slave of the passions has had consequences that extend far beyond moral philosophy into the foundations of psychology, economics, and decision theory. The most immediate philosophical implication is the rejection of rationalist moral philosophy in all its forms. If reason cannot motivate action, then moral principles cannot be derived from reason alone in the way Clarke and the rationalists claimed. Moral distinctions, Hume argues, are not perceived by reason but felt by sentiment. When we call an action virtuous, we are expressing a sentiment of approval; when we call it vicious, we are expressing disapproval. This is Hume's moral sentimentalism, and it flows directly from the thesis about reason and passion: since reason is motivationally inert, moral judgment — which is inherently motivational, since calling something wrong implies one has reason not to do it — must have its source in the passions rather than in reason.

The Stanford Encyclopedia notes that this position has come to be known as the Humean theory of motivation, and it remains a central position in contemporary action theory. The Humean holds that beliefs and desires are distinct existences, and that motivation requires both: a belief about how to achieve an end, and a desire for that end. This "belief-desire" model of action has been enormously influential, but it has also attracted criticism. Some philosophers argue that Hume underestimates the motivational force of beliefs themselves — that recognizing something as good or as one's duty can move one to act without an independent desire. Others argue that the passions are not as irrational or arational as Hume suggests, that emotions can themselves be responsive to reasons and can be assessed for rationality. The contemporary debate between Humeans and anti-Humeans (such as Thomas Nagel and John McDowell) remains one of the live questions in the philosophy of mind and action.

The Internet Encyclopedia observes that Hume's thesis also has important implications for the concept of instrumental rationality, the idea that rationality consists in choosing effective means to whatever ends one happens to have. If reason is the slave of the passions, then rationality is purely instrumental: it cannot assess the rationality of ends themselves, only of the means chosen to pursue them. This has been a powerful and controversial idea. On one hand, it respects the plurality of human values and avoids imposing a single conception of the good. On the other hand, it seems to leave reason unable to criticize destructive or irrational desires, since by Hume's account a passion cannot be contrary to reason except in the indirect sense that it rests on a false belief about the world. Hume's qualification — that a passion can be called unreasonable if it is based on a false supposition or if it chooses means inadequate to its end — is important but does not entirely answer the worry that pure instrumental rationality is too thin a notion to guide human life. The tension between Hume's instrumentalism and the demand for substantive rational assessment of ends remains a productive source of philosophical debate, and Hume's famous dictum continues to structure the conversation.