Adam Smith, answered from the texts and cited to the page.
Sympathy — or fellow-feeling, for the words may stand as near synonyms — is the imaginative act by which the mind of the spectator enters into the situation of another and conceives what he himself would feel there. It is not, as the word's common use might suggest, confined to sorrow alone; whatever passion arises in the person principally concerned, an analogous emotion springs up in the breast of every attentive spectator who brings that case home to himself.1
I call it an act of imagination because it is precisely that: we do not merely observe another's distress, we change places with him in fancy, and it is by this change of places that we come either to conceive or to be affected by what he feels.2 The mechanism is visible in its bodily effects. When a stroke is aimed at the leg of another man, we naturally shrink and draw back our own; the mob watching a dancer on the slack rope writhe and balance their own bodies in sympathy with his.3
These are not mere metaphors for a mental state — they are evidence that the imagination, sufficiently vivid, produces something approaching the sensation itself. One refinement matters greatly: sympathy does not arise so much from the view of the passion as from the view of the situation which excites it.4 This is why we sometimes feel for another a passion of which he himself seems altogether incapable.
The man who has lost his reason laughs and sings, insensible of his own misery; yet the spectator, imagining what it would be to be reduced to that condition while retaining his present reason and judgment, feels a deeper commiseration than the sufferer can feel for himself.5 The mother's anguish over her ailing infant exceeds the infant's own suffering for the same reason: she joins to its real helplessness her own consciousness of helplessness and her own terrors for the unknown consequences, and from all these forms the most complete image of misery.6
There is also an asymmetry worth noting between sympathy with sorrow and sympathy with joy. Our fellow-feeling with sorrow is, in some sense, more universal: even when grief is excessive, we may still feel some concern for the sufferer, though we cannot fully enter into the extravagance of his passion. But if we do not entirely go along with another's joy, we have no sort of regard for it at all — the man who skips and dances with a senseless joy we cannot share becomes an object of contempt rather than fellow-feeling.7
Pain, whether of mind or body, is a more pungent sensation than pleasure, and our sympathy with it, though it falls far short of what the sufferer naturally feels, is generally a more lively and distinct perception than our sympathy with pleasure.8 Finally, fellow-feeling extends even beyond the living. When a man is oppressed or injured by another, our sympathy with the sufferer's distress serves chiefly to animate our fellow-feeling with his resentment against the offender.9
And should the injured man perish, we enter into an imaginary resentment that we lend to the dead, who can no longer feel it themselves — we animate in fancy the cold and lifeless body, and feel that his blood calls aloud for vengeance.10 This is an illusive sympathy, I acknowledge, but it is not therefore a weak or unimportant one; it is the very root of our sense that injuries demand redress.
Whatever is the passion which arises from any object in the person principally concerned, an analogous emotion springs up, at the thought of his situation, in the breast of every attentive spectator.The Theory of Moral Sentiments
That this is the source of our fellow-feeling for the misery of others, that it is by changing places in fancy with the sufferer, that we come either to conceive or to be affected by what he feels.The Theory of Moral Sentiments
When we see a stroke aimed and just ready to fall upon the leg or arm of another person, we naturally shrink and draw back our own leg or our own arm; and when it does fall, we feel it in some measure, and are hurt by it as well as the sufferer. The mob, when they are gazing at a dancer on the slack rope, naturally writhe and twist and balance their own bodies, as they see him do.The Theory of Moral Sentiments
Sympathy, therefore, does not arise so much from the view of the passion, as from that of the situation which excites it.The Theory of Moral Sentiments
The anguish which humanity feels, therefore, at the sight of such an object, cannot be the reflection of any sentiment of the sufferer. The compassion of the spectator must arise altogether from the consideration of what he himself would feel if he was reduced to the same unhappy situation.The Theory of Moral Sentiments
In her idea of what it suffers, she joins, to its real helplessness, her own consciousness of that helplessness, and her own terrors for the unknown consequences of its disorder; and out of all these, forms, for her own sorrow, the most complete image of misery and distress.The Theory of Moral Sentiments
Though sorrow is excessive, we may still have some fellow-feeling with it... But if we do not entirely enter into, and go along with, the joy of another, we have no sort of regard or fellow-feeling for it.The Theory of Moral Sentiments
Pain besides, whether of mind or body, is a more pungent sensation than pleasure, and our sympathy with pain, though it falls greatly short of what is naturally felt by the sufferer, is generally a more lively and distinct perception than our sympathy with pleasure.The Theory of Moral Sentiments
When we see one man oppressed or injured by another, the sympathy which we feel with the distress of the sufferer seems to serve only to animate our fellow-feeling with his resentment against the offender.The Theory of Moral Sentiments
as we put ourselves in his situation, as we enter, as it were, into his body, and in our imaginations, in some measure, animate anew the deformed and mangled carcass of the slain... we feel that resentment which we imagine he ought to feel, and which he would feel, if in his cold and lifeless body there remained any consciousness of what passes upon earth.The Theory of Moral Sentiments