This is a passage from Seneca’s On Clemency (written in 55–56 CE):
Pity  is a sickness of the soul due to the sight of others’ suffering, or a sadness caused by someone else’s misfortunes which one believes to be undeserved; but no sickness can affect a wise man , for his mind is serene and nothing can get through to it that he guards against. Besides, nothing is as becoming to a man as a great soul, but it is impossible to be both great and sad. Sadness breaks the mind into pieces, throws it down, and collects the parts, but this cannot happen to a wise one  even in a disaster. Instead, he will repulse any outrage of fortune and shatter it to pieces before him, always maintaining the same appearance–quiet, firm–which he couldn’t do if he were overcome with sadness.
Also to be considered: a wise person discerns the future and makes decisions without interference, yet nothing clear and lucid  can flow from turbulence. Sadness is unfitted for discerning circumstances, planning useful tasks, evading dangers, weighing equities. Therefore, the [wise person] will not feel pity, because there cannot be pity without suffering of the soul. 
Whatever others who feel pity want to do, he will do freely and with a lofty spirit. He will help those who weep, but not weep with them . He will reach a hand to the drowning, welcome the exile, donate to the poor, not in the abusive way of most people who want to be seen as pitying–they toss something and flinch in disgust at those whom they aid, as if they feared to touch them–but as a man gives to a man from the common pool. He will return the child to the weeping mother, unfasten chains, save people from [gladiatorial] games, and even bury the stinking body, but he will do these things with a tranquil mind, of his own will. Thus the wise person will not pity but will assist and be of use, having been born to help all and for the public good, from which he will distribute shares to all. He will even give from his store to those sufferers who deserve a portion of blame and correction, but he will be even more pleased to assist those who are genuinely unfortunate. Whenever he can, he will counter fortune, for what better use of his powers than to restore what fortune has overturned? He will certainly not cast down his eyes or his soul toward someone who is shriveled or ragged and meagre and leaning on a staff; instead he will do good to all and kindly regard all who suffer, like a god.
Pity is close to suffering ; it even has something in common with it and derives from it. You know eyes to be weak if they water at the sight of someone else’s bleariness, just as, by Hercules, it is a disorder and not a case of merriment when people laugh just because others laugh or yawn whenever someone’s mouth opens. Pity is a flaw in the soul of one who feels suffering too much, and he who expects it from a wise person is not so different from someone who expects lamentations at a stranger’s funeral.Seneca’s De Clementia (2.5.4-2.6.4), trans. Peter Levine
The topic of this book is clemency (clementia) which in modern English means a virtue or prerogative of governors and other rulers. Seneca addresses the young Emperor Nero and urges him to exhibit clemency (I:v). Emperors were sometimes addressed with the honorific “clemens” (similar to “your grace”), presumably to play to their good side.
However, a different meaning of clementia was calmness or mildness. The weather could be clement, and so could a human mind. Anyone could direct this kind of clementia toward anyone else. A better translation than “clemency” might be “compassion.” Seneca contrasts it with misericordia, which I have translated as “pity” to capture its negative connotations. (After all, nobody wants to be pitied.) But misericordia is close to the modern word “sympathy.” So let us consider the differences between clementia as compassion and misericordia as sympathy.
“Sympathy” means feeling some version of the same emotion that another person feels. Your friend is sad, so you feel sad for her and with her. Although I am sympathetic to the emotion of sympathy, Seneca suggests several reasons to avoid it. Feeling the same way as a suffering person does not necessarily help that person. Sympathy often comes with at least a tinge of condescension, since the person who is sympathetic does not actually experience the same circumstances as the one who suffers. By trying to replicate the sufferer’s emotions, you may undermine your ability to help. And by tying your emotions to another person’s state of mind, you expose yourself to fortune. This is not a reliable way to achieve your own happiness.
Instead, those who suffer deserve to be assisted effectively by people who genuinely respect them. The helper should not try to mirror their emotions but should display a different emotion: clear-headed and equitable good-will. To name that emotion “compassion” is a bit confusing, since it has precisely the same root meaning as “sympathy,” which is a suggested translation of misericordia. (Com = sym = with. Passio = pathos = feeling). Nevertheless, compassion seems to be the word we would use for Seneca’s idea of disinterested benign sentiments (2.6.3) that we exercise freely and with a tranquil mind (2.6.2). It can translate the Sanskrit word karuna, which is fundamental in Buddhism.
Seneca also relates this virtue–let’s call it compassion–to a political idea: equal standing and a common claim on the public good. Even though Seneca addresses De clementia to Nero, I think that in this passage, he describes a republican virtue, appropriate for relations among equals who co-own the commonwealth. (It is interesting that he doesn’t actually use “clementia” in these chapters of the book.)
A compassionate person is not exposed to chance. If we feel worse as another person worsens, and better as he improves, then we demonstrate sympathy, which subjects us to fate. But compassion remains unchanged regardless of the state of the sufferer. Compassion can even fill the mind’s attention, thus displacing emotions that are the cause of discomfort.
One question for me: is sympathy a path to compassion or is it a diversion or a dead end? There is a long tradition in Buddhism of cultivating an imaginative identification with another sentient being, feeling its pain, and “exchanging self and other” (e.g., Shantideva, Bodhicaryavatara, 7:16) The goal is to shake one’s attachment to oneself and begin a journey from selfishness to concrete sympathy for specific others, and from there to generalized good-will for all, or karuna. I can’t criticize this path without having been taught it properly or seriously tried to practice it, but Seneca makes me wonder whether intense involvement in another’s suffering might detract from the cultivation of compassion rather than setting us on the right track.
Notes:  misericordia;  gendered in the original (vir);  not necessarily gendered;  socerumque, which doesn’t make sense to me unless it should read serenumque;  ergo non miseretur, quia id sine miseria animi non fit;  non accedet = not come near them;  Misericordia vicina est miseriae. See also: Horace against the Stoics; how to think about other people’s interests: Rawls, Buddhism, and empathy; empathy, sympathy, compassion, justice; empathy boosts polarization; empathy and justice, “you should be the pupil of everyone all the time” ; Foucault’s spiritual exercises; John Stuart Mill, Stoic; etc.