We’ve probably all had the experience of being on the verge of acting from anger or jealousy, when someone advises us to act reasonably. A typical picture of motivation for action is one in which emotions or desires drive us one way and our reason drives us in another. I have a desire for a tasty but unhealthy dessert, and the voice of reason tells me that I ought not to eat it. I don’t feel like helping at the food bank on Saturday, but conscience tells me that I ought to fulfil my obligation. On this picture, the morally upstanding or prudent person follows the lead of reason, while the morally deficient character caves into desire or emotion.

David Hume, in A Treatise of Human Nature, rejects this traditional characterisation of action and its evaluation, offering a remarkable theory in response. He defends the views that the ends or goals of our actions in all cases are given by our “passions,” not by reason, and that the practical role of reason is to figure out how to fulfil these goals. He makes the astounding declaration that “Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them.” Subsequently, Hume also rejects the analysis of morality in terms of rationality, and argues that our distinctions between virtue and vice are based on shared sentiments or feelings of approval or disapproval we experience toward persons’ characters.

Although Hume wrote in the 18th century, his philosophy has significant implications for questions posed in contemporary motivational psychology and moral philosophy. In fact, Hume’s work stimulated one of the classical and still ongoing debates in practical reasoning, concerning whether our goals are created by reason and subject to its evaluation, or whether our goals come from feelings or passions. Moreover, his sentimentalist moral theory is the inspiration for contemporary discussions in ethics, such as those presented in Michael Slote’s Moral Sentimentalism, and Jesse Prinz’s The Emotional Construction of Morals. How did Hume reach his significant conclusions, and what impact did they have on the course of philosophy at the time in which he defended them?

One of the major intellectual and practical advances of the 17th century was the development of a distinct scientific method. David Hume’s announced project was to implement this newly developed scientific method in an empirical study of human nature. Hume’s study included the explanation of human actions and the motives for them, and an investigation into why we make the moral distinctions among actions and motives in the way that we do. In applying the scientific approach to these topics, Hume’s intent was to put explanations of action and morality on the same basis as other natural phenomena, looking for explanations of them in terms of a few fundamental natural principles, rather than by appealing to supernatural or religious events. Accordingly, he characterised reason not as some mysterious power of grasping truth or of intuiting connections between ideas or thoughts, as some philosophers did, but as the ability either to offer demonstrations or proofs or to make causal inferences.

So, first, he shows that reason engaged in demonstration can never be a motive to action. Demonstration is deductive reasoning using necessary truths. Demonstrations are the proofs we use in mathematics and geometry. Mathematics can be applied to the world in the way that engineers use it to solve problems in their work, but knowing the truths of mathematics only, without the addition of a goal or purpose, will not produce a motive to any particular action. Second, Hume asks whether causal reasoning by itself can give motivate action. Causal reasoning, which requires the gathering and assembling of observations, allows us to form beliefs about the world. Do these factual beliefs supply us with motivation to act in particular ways? Say I’m sleepy and I believe coffee can stimulate me. It seems this belief can motivate me to drink a cup of coffee. If so, factual beliefs based on causal reasoning can on their own produce motives. However, Hume notices that such beliefs would have no practical effect on us if we didn’t also have some sort of attraction to the goal achieved by the motivated action – in this instance, staying awake when I’m feeling sleepy. Reason informs me that consuming the caffeine in coffee keeps me awake, but reason didn’t tell me what desires to have. It simply gives me a piece of causal information. Factual beliefs have no influence on our behaviour if they are about things in the world of no concern to us.

The concern or motivation necessary to action originates, Hume argues, with passions, which are either feelings prompted by sources of pleasure and pain or instincts that produce pleasure or pain. Among the former are desire, fear, and anger. Among the latter are benevolence, thirst, and love of life, but there are many other passions as well. Since reason cannot prompt action, it cannot oppose passion over what we are to do. The only way something can be opposed to passion is by initiating a motivation in a contrary direction. This is why the popular model mentioned at the beginning and accepted among some 17^th century rationalists is overturned by Hume’s arguments.

If the impetus to action always originates with a passion, then why does it look as though beliefs, which come from reason, can sometimes make our passions come and go? For instance, a mother fears that her child has been killed by kidnappers. Her fears are turned to joy when the police phone her with the information that he has been found uninjured at a nearby park. Hume himself emphasises that beliefs are practical and have an impact on our emotions. Nonetheless, in any case, we will always find some initial passions, originated without reason, with which beliefs cooperate. In the case of a mother’s fear turned to joy, her beliefs affect her emotions only because she has an underlying affection for her child, itself not originated by reason.

A radical implication of Hume’s theory of motivation is that it makes no sense, strictly speaking, to call actions rational or irrational. Since they are caused by passions, which cannot be so evaluated, there is no basis on which to apply these terms to actions. So, he claims, it is not contrary to reason for me to prefer the destruction of the world to getting a scratch on my finger; it is not irrational for me to undermine my long-term good for a trivial short-term pleasure. His view sounds astoundingly implausible at first. But consider: he is not saying that actions are subject to no evaluations whatsoever. Actions are the objects of moral judgment. It is surely vicious of me to allow others to be destroyed in order to save myself a small inconvenience. It is definitely imprudent of me to avoid my dental appointment for a root canal today, when such behaviour will result in larger health problems for me later. Hume argues that this simply means that our moral evaluations are done on some basis besides reason.

While Hume was not the first philosopher to suggest that ends of actions cannot come from reason (Aristotle, for instance, attributed them to a wish), his scientific precision and rigour have made his arguments the touchstone for subsequent debates on this topic. Contemporary philosophers working in action theory identify themselves as Humean or anti-Humean, depending on whether they subscribe to the “belief-desire” model of motivation descended from Hume. The belief-desire model says that both mental states are required for action: a desire to set an aim and a belief about how to achieve that aim. Anti-Humeans, often descended from Immanuel Kant, still argue that reason can and ought to have authority over our desires. Some maintain, despite Hume’s arguments, that we can use reason to decide what ends to adopt, by examining our desires and determining which are consistent with the demands of rationality. Contemporary Humeans, in response, spend a great deal of effort attempting to develop a theory true to Hume’s tenets, but one that allows (contrary to Hume) that our actions can be reasonable or unreasonable. On one approach, desires that the agent values more than other desires determine better reasons for action.

Hume’s proposals in ethics were also innovative. One of his opponents, Samuel Clarke, had claimed in his famous Boyle Lectures of 1705 that the universe has a rational structure to which only certain events and actions are fitting. Actions done contrary to reason are morally wrong, and it is self-evident when they are wrong. For instance, it requires no experience, Clarke argues, to know that it is fitter to promote the welfare of other people than to destroy them. Hume thinks that Clarke misrepresents reason as a mystical power of discernment. On the scientific notion of reason, reason can’t move us on its own. Since the point of making moral judgements is to affect or motivate behaviour, the conclusion we must accept as naturalists is that moral judgements are not based on reason by itself. If it is wrong to destroy other people without provocation (and Hume agrees that it is), it is not because doing so runs up against a rational order.

Hume proposed that our moral distinctions must come from the non-rational, sensitive part of our nature. This conclusion leads Hume at one point to say that our moral distinctions come from a moral “sense,” as though we perceive moral goodness or badness by a special sense in the way that we perceive colours and tastes by the physical senses. To what extent Hume thinks that we actually have a moral sense is debatable. Hume clearly thinks our moral distinctions depend upon our sensitive reactions, though: when we feel approval (a pleasure or satisfaction) toward a person, we judge her as virtuous, and when we feel disapproval (a pain or dissatisfaction), we judge him as vicious.

In the attempt to explain our mental life in terms of a few naturalistic principles, Hume argues that the fundamental principle of sympathy underlies our moral judgements. It is not an exaggeration to say that, of all philosophers, Hume most highlighted the role of sympathy in ordinary life. The economist, moral philosopher, and friend to Hume, Adam Smith, probably places second in understanding the importance of sympathy to human interactions. Sympathy is the capacity to experience feelings similar to others’ feelings by thinking about what they are experiencing. We infer the feelings of others from their behaviour; this idea of their feelings takes on a greater forcefulness as we imagine ourselves affected by the circumstances of others’ situations. I drive by the scene of a terrible car accident, and I cringe at the thought of the bodies in the twisted wreckage; the thought of their suffering causes me psychological pain. When I think about a heinous murderer, the explanation of my disapproval, which is a form of discomfort, is that I sympathise with the victims of the murderer’s actions, and I know that the victims suffered greatly.

We don’t, however, sympathise to the same extent with all human beings. One of Hume’s observations about human nature is that we sympathise to a greater degree with those who are close to us, resemble us, or are related to us. Hume says that I won’t feel the same lively pleasure from the virtues of a person who lived in Greece 2,000 years ago as I feel from the virtues of a familiar acquaintance. We are more affected by the feelings of those with whom we find something in common than by the feelings of those with whom we share little, because the resemblances between us makes it easier to imagine the others’ feelings. But if such sympathetic feelings are the foundation of our moral judgements, does it follow that our moral judgements are subjective?

Hume answers that in order to communicate about morality and avoid practical problems arising from conflicting moral judgements, we consider our sympathetic feelings indicative of moral distinctions only when we take up a generally shared perspective on an action or character. In so doing, we consider the effects of an action or character in isolation from our personal connections to the actor. More specifically, the common point of view from which we make moral distinctions is the viewpoint of one who sympathises with the circle of people most directly affected by the agent’s actions. The spectator’s feelings must mirror the feelings of those who are the direct recipients of the agent’s actions and their consequences. For instance, when I make a moral judgment about a dictator in a distant country, I think about the effects on the citizens there and identify with their feelings; I try to do it in the same way I think about and respond to the effects of my local government on citizens here. In each case, I identify only with the feelings of people directly affected by the government actions.

Hume’s moral theory is not just a descriptive account about how we make moral judgements; it also has normative implications. His is a spectator theory of the virtues. A character trait is virtuous if an observer approves, through sympathy, of the effects of that trait on others from the common perspective; a character trait is a vice if such an observer disapproves. Hume notes that we will find a natural division among the approved features, the virtues. One class consists of those that make a person able to promote his or her own interests; the other consists of those that make a person fit for society. In An Enquiry Concerning the Principles of Morals Hume details these virtues and vices. Among the qualities “immediately agreeable” to the self are cheerfulness, tranquillity, and serenity. Qualities useful to society include generosity, gratitude, kindness, and courage; and qualities immediately agreeable to others are politeness, wit, ingenuity, modesty, decency, and cleanliness.

In addition to combating the moral rationalism popular in the 17th century, Hume’s moral theory had the profound effect of undermining the “selfish” school of thought propagated at the time by Hobbes and Mandeville, who alleged that all human behaviour was motivated entirely by self-interest. While sentimentalism was becoming more popular in the 18th century due to writers like Shaftesbury and Hutcheson, Hume became the most dynamic representative of the theory. Twenty-first century moral controversies over the importance of emotions and sympathy versus rationality and duty refer constantly to Hume’s opinions. Popular discussions of moral sensibility in venues like James Q. Wilson’s The Moral Sense owe a huge debt to the philosopher whose systematic study of human nature revealed that the moral life is a life with feelings at the centre stage.

Elizabeth S Radcliffe is professor of philosophy at the College of William and Mary, president of the Hume Society, and author of Hume, Passion, and Action (Oxford University Press, forthcoming).