Translated by Ralph Anzarouth and an anonymous friend
In the opinion of the wise men rebuke results from love which compels one to reprimand his friend in an appropriate manner concerning his speech or behavior.
This is the reason: since man cannot judge impartially his own behavior and does not see his own faults, he needs someone else who out of friendship will reprimand and discipline him. It goes without saying that this is especially true in respect of all young people who are particularly in need of this, owing to their lack of age and experience, as it written (Proverbs 22, 15): "Foolishness is bound to to the heart of the adolescent, and the rod of discipline will remove it from him. It is also written (ibid. 26, 3): "The whip [is fitting] for the horse, the halter for the donkey and the rod for the back of the fools"; and also (ibid. 28, 23): "Discipline your son while there is still hope, but do not let this result in your killing him"; and King Solomon said [also] (ibid. 28, 23): "The one who rebukes his fellow man will subsequently be more appreciated than the flatterer".
The Torah commanded a positive Mitzvah (Lev. 19, 17): "You will surely rebuke your fellow man, and you will not bear his sin". From this we deduce that if one does not reprimand, he will bear his fellow man's sin and will be punished for this. Our Sages of blessed memory said (Talmud Bavli, tractate Berachot 31b): "From the verse (First Book of Samuel 1, 14) "Remove yourself from the influence of wine" we learn that one who sees his fellow man acting improperly is obliged to rebuke etc.". And they said (Talmud Bavli, tractate Shabbat 54b): "Every person who has the possibility to reprimand his fellow citizens [for their wrongdoings] and does not do so [will be punished for them; if he has the possibility to reprimand] everybody, he will ultimately be punished for [the sins] of everybody".
One may not show favoritism to anybody where profanation of the Divine Name is involved. When one sees a person committing a sin which he can prevent either by word or by deed, the person giving rebuke must in all events first make sure that he himself is without any trace of sin or vice, so that he should be a practical example of what he requires of others by his word or deed. In this way, none will object to him as our Sages of blessed memory said (Talmud Bavli, tractate Arachin 16b): "Rabbi Tarfon says: I would be greatly surprised if there was somebody in this generation who was qualified to give rebuke, since if he were to say [by way of rebuke] "Remove the splinter from between your teeth", they would reply to him "remove the plank from between your eyes1." The words of the prophet (Tzephania 2, 1): "Arouse yourselves and mend your ways" were interpreted in this sense 2, since the words of the person who practices what he preaches are pleasant.
However, happy is the man who listens to all those who instruct him, teach him and rebuke him whoever they may be and pays heed to content [of the rebuke] and not to the person giving it, as it is written (Proverbs 12, 1): "One who welcomes chastisement loves knowledge; and one who hates rebuke is a fool." It is further brought down in the same passage of tractate Arachin that Rabbi Yochanan ben Nuri said: "I can call Heaven and Earth to testify upon myself that on numerous occasions Akiva ben Yoseph was hit as a result of my having complained about him before Rabban Shimon ben Gamliel and in spite of this he increased his friendship towards me in order to fulfill what is said (ibid. 9, 1): 'Do not rebuke the scoffer lest he hate you; instead, rebuke the wise man and he will love you [for it]'."
Go and learn this trait from the wolf, about which it is said that when it approaches a house or residence with the intention of finding prey, if on the way its foot strays from the level path and it stumbles, giving warning to the occupants, with the result that it has to flee for its life, it subsequently puts its paw in its mouth and bites it, as if it wants to punish and rebuke it for the bad turn it has done so that it will learn to beware in the future about keeping to the straight path as the wolf wishes.
We see that as long as the sinner is one's own friend or relative, our rebuke is more appropriate, as it is written (Proverbs 27, 6): "The wounds received from a friend are faithful; and the kisses from an enemy are abundant3." He states further (ibid. 27, 5): "An explicit rebuke is better than an ambivalent friendship".
Diogenes said: "One who wants to be a true friend will give reprove in private, since the person who rebukes mildly and discreetly will strengthen friendship, whereas one who chastises harshly and in public will acquire enmity.". Cato said: "If your friend continues to misbehave in your presence, you should continue to rebuke him". Plato said: "Beware of chastising your friend in front of others when he is angry".
The prophets, servants of the Lord, wrote in their books that one shouldn't refrain from rebuking one's contemporaries and fellow men who stray [from the right path], even if as a result he brings upon himself harm and humiliation, since this was the way of all the prophets. It is related in the Midrash Tana Deve Eliahu concerning Isaiah of blessed memory: "I was once sitting in the great synagogue of Jerusalem and a student approached and asked me just as a son would ask his father why Isaiah was different from all the other prophets in that he prophesied all the pleasant things and consolations [which would befall] the Jewish People. I replied: 'My son, it is because he accepted the yoke of Heaven joyously, as it is said (Isaiah 6, 8-10): And I heard the voice of the Lord […] 'Go and say' […] Let the heart of this people thicken4 […]'." The explanation why he merited this is that even though the Lord revealed to him that the people would not listen to him and would harden their hearts, this did not prevent him from going joyfully to rebuke them and he ultimately suffered martyrdom, as it is said (Talmud Bavli, tractate Yebamot 49b) that [king] Menashe killed him, as he sent his men to pursue him: Isaiah uttered the Divine Name and was hidden in a cedar tree; they brought the tree and they sawed it and when they reached Isaiah's mouth, his soul departed and he died. As he said in his book (Isaiah 50, 6): "I offered my back to the strikers and my cheeks to those who pluck out [my beard]; I have never hid my face from insults or spit".
Similarly, Jeremiah was imprisoned and thrown into a dungeon where he was beaten and wounded as he said (Jeremiah 11, 19): "Like a tame sheep I was led to slaughter etc." (ibid 15, 10): "I lent to nobody and nobody lent to me and everybody curses me etc.".
In every generation there were several prophets and righteous men who were not deterred by insults and blows from rebuking their fellow men. How great is the merit of the one who brings back others from evil ways, since the merit of the community stems from him and his righteousness is eternal.
Notes of the translators:
 The first expression was meant as a figurative term of rebuke, and the response in the same tone expresses the apparent unfitness of the rebuker to give rebuke.
 We assume that the prophet was above reproach and therefore he was entitled to reprimand.
 The Malbim explains that a person's enemy will refrain from rebuking his wrongdoing and on the contrary he will encourage him out his hatred to continue in his evil ways.
 These verses describe Isaiah's willful acceptance of his prophetic mission to the Jewish People. For some reason, the Hebrew edition points inaccurately to another chapter of Isaiah.
The whole book Tzemach Tzadik in Hebrew (printed in Rashi characters) can be downloaded in PDF format and read online at Hebrewbooks.org. Chapter 16 about rebuke is found at pages 33-36.