The subject of An Enquiry Concerning the Principles of Morals is the contributions that moral sense and reason make in our moral judgments. Hume claims that moral sense makes the ultimate distinction between vice and virtue, though both moral sense and reason play a role in our formation of moral judgments. Reason is important when we have to make a judgment about what is useful, for reason alone can determine how and why something is useful to us or to others. Hume briefly addresses what moral judges usually include in their lists of virtues, what they leave out, and how they make these lists. He then returns to the classification of virtues he proposed first in A Treatise of Human Nature.
Hume first distinguishes between artificial and natural virtues. Artificial virtues depend on social structures and include justice and fidelity to promises; allegiance; chastity and modesty; and duties of sovereign states to keep treaties, to respect boundaries, to protect ambassadors, and to otherwise subject themselves to the law of nations. Hume defines each of these virtues and explains how each manifests itself in the world. He notes that artificial virtues vary from society to society.
Natural virtues, on the other hand, originate in nature and are more universal. They include compassion, generosity, gratitude, friendship, fidelity, charity, beneficence, clemency, equity, prudence, temperance, frugality, industry, courage, ambition, pride, modesty, self-assertiveness, good sense, wit and humor, perseverance, patience, parental devotion, good nature, cleanliness, articulateness, sensitivity to poetry, decorum, and an elusive quality that makes a person lovely or valuable. Some of these virtues are voluntary, such as pride, while others are involuntary, such as good sense.
As in the A Treatise of Human Nature, Hume explains that reason does not cause our actions. Instead, moral sentiments, or passions, motivate us to act. In An Enquiry Concerning the Principles of Morals, however, Hume goes further to state that our actions are caused by a combination of utility and sentiment. In other words, we must care about the outcome if we are to care about the means by which it is achieved. Several sections of An Enquiry Concerning the Principles of Morals are devoted to utility, the first and most important of the four kinds of virtue, which Hume calls “virtuous because useful.” He also addresses benevolence and its role in the moral process. Specifically, Hume says that benevolent acts are virtuous because they are useful to many others.
Because he locates the basis of virtue in utility rather than in God-given reason, Hume’s list of virtues implicitly forms a rejection of Christian morality. Items such as ambition are vices under the old model, so Hume’s acceptance of them into his catalog is an insult to religious theorists. However, Hume is consistent in his theory that these traits are virtues because they fulfill his two requirements for moral sentiments: they must be useful to ourselves or others, or they must be pleasing to ourselves or others. Furthermore, Hume rejects the concept of morality as strictly voluntary. Instead, he divides his list into voluntary and involuntary virtues, claiming that separating them is necessary only when devising a system of reward and punishment. He is not interested in creating or endorsing such a system, so he makes no such distinctions in his moral philosophy.