The Theory of Moral Sentiments begins with the following assertion:
How selfish soever man may be supposed, there are evidently some principles in his nature, which interest him in the fortunes of others, and render their happiness necessary to him, though he derives nothing from it, except the pleasure of seeing it. Of this kind is pity or compassion, the emotion we feel for the misery of others, when we either see it, or are made to conceive it in a very lively manner. That we often derive sorrow from the sorrows of others, is a matter of fact too obvious to require any instances to prove it; for this sentiment, like all the other original passions of human nature, is by no means confined to the virtuous or the humane, though they perhaps may feel it with the most exquisite sensibility. The greatest ruffian, the most hardened violator of the laws of society, is not altogether without it.
Sympathy, the fellow-feeling with the passions of others, operates through a logic of mirroring, in which a spectator imaginatively reconstructs the experience of the person he watches:
As we have no immediate experience of what other men feel, we can form no idea of the manner in which they are affected, but by conceiving what we ourselves should feel in the like situation. Though our brother is on the rack, as long as we ourselves are at our ease, our senses will never inform us of what he suffers. They never did, and never can, carry us beyond our own person, and it is by the imagination only that we can form any conception of what are his sensations. Neither can that faculty help us to this any other way, than by representing to us what would be our own, if we were in his case. It is the impressions of our own senses only, not those of his, which our imaginations copy. By the imagination, we place ourselves in his situation.
Smith departs from the "moral sense" tradition of Shaftesbury, Hutcheson, and Hume, as the principle of sympathy takes the place of that organ. Sympathy arises from an innate desire to identify with the emotions of others. It can lead people to strive to maintain good relations with their fellow human beings and provide the basis both for specific benevolent acts and for the general social order.