  Sympathy in general operates as follows. First, observation of the outward expression of another person’s “affection” (feeling or sentiment) in his “countenance and conversation” conveys the idea of his passion into my mind. So does observing the typical cause of a passion: for example, viewing the instruments laid out for another’s surgery will evoke ideas in me of fear and pain. We at all times possess a maximally vivid and forceful impression of ourselves. According to Hume’s associationism, vivacity of one perception is automatically transferred to those others that are related to it by resemblance, contiguity, and cause and effect. Here resemblance and contiguity are primary. All human beings, regardless of their differences, are similar in bodily structure and in the types and causes of their passions.  The person I observe or consider may further resemble me in more specific shared features such as character or nationality. Because of the resemblance and my contiguity to the observed person, the idea of his passion is associated in my mind with my impression of myself, and acquires great vivacity from it. The sole difference between an idea and an impression is the degree of liveliness or vivacity each possesses. So great is this acquired vivacity that the idea of his passion in my mind becomes an impression, and I actually experience the passion. When I come to share in the affections of strangers, and feel pleasure because they are pleased, as I do when I experience an aesthetic enjoyment of a well-designed ship or fertile field that is not my own, my pleasure can only be caused by sympathy (T 2.2.2–8, 3.3.1.7–8). Similarly, Hume observes, when we reflect upon a character or mental quality knowing its tendency either to the benefit or enjoyment of strangers or to their harm or uneasiness, we come to feel enjoyment when the trait is beneficial or agreeable to those strangers, and uneasiness when the trait is harmful or disagreeable to them. This reaction of ours to the tendency of a character trait to affect the sentiments of those with whom we have no special affectionate ties can only be explained by sympathy.