Hume stresses that his theory of morals follows naturally from the philosophy he elaborates in the first two books. Hume attempts to distinguish between vice and virtue, arguing that such moral distinctions are in fact impressions rather than ideas. He then describes how to distinguish these impressions from other common impressions, such as sounds and colors. First, the impression of vice is pain, while that of virtue is pleasure. Second, moral impressions are caused only by human actions, not the actions of animals or inanimate objects. Third, moral impressions are worth considering only from a social point of view because our actions are considered moral or immoral only with regard to how they affect others, not how they affect ourselves. This concept leads Hume to classify sympathy, feeling for fellow human beings, as the foundation of moral obligation.
For Hume, morality is not a matter of fact derived from experience. To prove his point, he suggests we examine ourselves with regard to any supposed moral misdeed, such as murder. If we examine the act of murder, we can discover no idea of that quality of immorality, or “vice.” Rather, we will discover only the strong feeling of dislike we have for murder. This supports the idea that morality resides in passions, or “sentiment,” not in reason. Although reason does help us explain those feelings, it is not their origin.
Hume makes the point that though we may not like it when one person kills another, there is nothing contradictory or illogical about the act of murder. This does not mean that Hume condones murder, merely that immoral actions are not immoral because they are irrational. Within Hume’s system, murder would be banned on the grounds that it is not an action that can be universally justified as good for everyone. Hume also proposes the example of the man who would rather see the whole world destroyed rather than injure his own fingers. Hume claims this man is not in contradiction to himself or following illogical inferences, but this man also falls afoul of Hume’s dictum that methods of justification and rationality must be universal. Other people in the same situation must be able to justify their actions in the same way. No one but the man will approve of his reasons for forsaking the world to save his own fingers. It is unlikely that this man would approve or desire that another person make the same decision.
Hume ascribes moral decisions to the passions for several reasons. First, passion appears to be the only viable alternative to reason, which he has already ruled out. Second, Hume’s examination of his own feelings about conventionally transgressive acts such as murder reveals that while he can isolate his own feelings about such behavior, he cannot isolate clear and distinct ideas about it. Therefore, moral decisions must arise from or in some way be congruent with passions. Hume’s connection of moral decisions to feelings, which leads him to the separation of morality from reason, put him at odds with religious leaders and philosophers of his time. Hume effectively dethroned reason, removed God from a place of necessity, and robbed religious theorists of an undisputed foundation for religious belief.