T 2.2.9.9

The only explication, then, we can give of this phænomenon is deriv'd from that principle of a parallel direction above-mention'd. Our concern for our own interest gives us a pleasure in the pleasure, and a pain in the pain of a partner, after the same manner as by sympathy we feel a sensation correspondent to those, which appear in any person, who is present with us. On the other hand, the same concern for our interest makes us feel a pain in the pleasure, and a pleasure in the pain of a rival; and in short the same contrariety of sentiments as arises from comparison and malice. Since, therefore, a parallel direction of the affections, proceeding from interest, can give rise to benevolence or anger, no wonder the same parallel direction, deriv'd from sympathy and from comparison, shou'd have the same effect.