Hume sets out to classify the passions in much the same way he classifies impressions and ideas in book I. First, he distinguishes between original impressions and secondary impressions. We receive original impressions through the senses. They are internal, in the form of physical pleasures or pain, and original because they come from outside of us, from physical sources, and are in that sense new to us. Secondary impressions are always preceded by either an original impression or ideas, which arise from original impressions. The passions, according to Hume, are properly found in the realm of secondary impressions. Hume describes both direct passions, such as desire, aversion, grief, joy, hope, and fear, and indirect passions, such as pride, humility, love, and hatred. Hume then distinguishes between the cause and the object of the passions.
Hume notes that since moral decisions affect actions, while decisions of reason do not, morality must not be based on reason. For Hume, beliefs about cause and effect are beliefs about connections between objects we experience. Our belief in such relations can affect our actions only if the objects being related are of some particular interest to us, and objects are of interest to us only if they cause us pleasure or pain. Hume concludes that reasoning regarding supposedly connected objects is not what makes us act. Instead, pleasure and pain, which give rise to passions, motivate us. Hume also says we cannot claim that actions are the result of passions that are reasonable or unreasonable, because passions themselves have nothing to do with reason. They are feelings that instigate actions. They may themselves be informed by reasoning, but reason is and should be the “slave” of passions.
Hume’s discussion of passions and reason sets the stage for book III and his discussion of morality. Passions, since they don’t represent anything real and are not arguments in and of themselves, cannot be contrary to experience and cannot cause contradictions. Since these are two of Hume’s most important measures, we can conclude that, following his argument, passions are completely different from reason and cannot be categorized as reasonable or unreasonable. This conclusion presents a dilemma for rationalists who view morality as the result of God-given reason. In fact, reason influences our actions in only two ways: by directing passions to focus on proper objects and by discovering connections between events that will create passions. The judgments a person makes about relations of ideas or about ideas themselves may be reasonable or unreasonable, but the judgments do not result in anything except opinions. For the moral process to complete itself, the judgments must incite passions, or feelings, which then lead us to act.