Thomas Aquinas

“ Now this happens in two ways: first, through union of the affections, which is the effect of love. For, since he who loves another looks upon his friend as another self, he counts his friend's hurt as his own, so that he grieves for his friend's hurt as though he were hurt himself. ”

“ Joy and peace add nothing to the aspect of good which is the object of charity, wherefore they do not require any other virtue besides charity. But mercy regards a certain special aspect, namely the misery of the person pitied.
Reply to Objection 4: Mercy, considered as a virtue, is a moral virtue having relation to the passions, and it is reduced to the mean called nemesis, because "they both proceed from the same character" (Rhet. ii, 9) . Now the Philosopher proposes these means not as virtues, but as passions, because, even as passions, they are praiseworthy. ”

“ We worship God by external sacrifices and gifts, not for His own profit, but for that of ourselves and our neighbor. For He needs not our sacrifices, but wishes them to be offered to Him, in order to arouse our devotion and to profit our neighbor. Hence mercy, whereby we supply others' defects is a sacrifice more acceptable to Him, as conducing more directly to our neighbor's well-being, according to Heb. ”

“ Secondly, such like evils are yet more provocative of pity if they are contrary to deliberate choice, wherefore the Philosopher says (Rhet. ii, 8) that evil excites our pity "when it is the result of an accident, as when something turns out ill, whereas we hoped well of it." Thirdly, they cause yet greater pity, if they are entirely contrary to the will, as when evil befalls a man who has always striven to do well: wherefore the Philosopher says (Rhet. ii, 8) that "we pity most the distress of one who suffers undeservedly." ”

“ Accordingly just as, properly speaking, a man does not pity himself, but suffers in himself, as when we suffer cruel treatment in ourselves, so too, in the case of those who are so closely united to us, as to be part of ourselves, such as our children or our parents, we do not pity their distress, but suffer as for our own sores; in which sense the Philosopher says that "harshness drives pity away."
Reply to Objection 3: Just as pleasure results from hope and memory of good things, so does sorrow arise from the prospect or the recollection of evil things ”

“ Now a man wishes a thing in three ways: first, by his natural appetite; thus all men naturally wish to be and to live: secondly, a man wishes a thing from deliberate choice: thirdly, a man wishes a thing, not in itself, but in its cause, thus, if a man wishes to eat what is bad for him, we say that, in a way, he wishes to be ill. ”

“ Hence they make a man think that he is going to suffer something in the future, so that while they are disposed in that way they are pitiless, according to Prov. 27:4: "Anger hath no mercy, nor fury when it breaketh forth." ”