The concept of sympathy holds the key to Smith’s entire theory of moral sentiments. It represents a middle ground between Francis Hutcheson’s “moral sense” and David Hume’s self-interest. It does not go as far as Hutcheson in positing the existence of a natural force, akin to the known physical senses, by which human beings perceive the good and then act with benevolence. It does, however, preserve Hutcheson’s insistence that our moral sentiments originate in something natural—that they are not mere calculations of self-interest.

Smith defines sympathy as “fellow-feeling with any passion whatever” (19). It is a broad yet accurate definition that encompasses more than simple condolences. In fact, sympathy is how we take any interest in the ideas and actions of others. We take pleasure in discovering that others sympathize with us, meaning their sentiments agree with our own. When we laugh at the same jokes or enjoy the same stories, we sympathize. In short, “[t]o approve of the passions of another […] as suitable to their objects, is the same thing as to observe that we entirely sympathize with them” (25). Furthermore, “the effect of sympathy is instantaneous” (31), which means it cannot be the result of reasoned calculations.

If sympathy explains our interest in the ideas and actions of others, if we take pleasure in finding that others share our sentiments (and are mortified when they do not), and if the effect of sympathy is instantaneous, then might this natural instinct also constitute the source of our moral judgments? Smith believes it does.