DEUT1423 We are forbidden to strike others. When someone receives lashes in court, the Torah prohibits striking him beyond the prescribed amount. This prohibition also applies to a layman who strikes his fellow man. Even someone who deserves lashes in court is not to be hit needlessly; all the more so anyone else. (Chinuch 595, Choshen Mishpot 420:1; Shaarey Tshuvah 3:77). 1) You have no right to hit someone even if he starts an argument with you and curses you. If, however, he hits you first, you are permitted to hit him back to save yourself. Nevertheless, you have no right to hit him after he stops hitting you unless he's going to hit you again. Even when you may hit someone to protect yourself, you should not hit him more than is necessary. (Shulchan Aruch Harav, vol. 6). 2) If you see A hitting B, it is a mitzvah to hit A to stop him if that is your only option. If, however, you are able to stop A with any other method, you are obliged to do so. (ibid). 3) If someone tries to steal something from you, you are permitted to hit him to stop him. (ibid). 4) It is forbidden to hit someone even if he gives you permission to do so. (ibid). 5) A parent is permitted to hit his child to train him to behave properly, but not otherwise. (ibid). 6) You violate this prohibition if you maliciously throw an object at someone, even if the object is light and does not cause damage. (Rabbi Yosef Dov Soloveitchik, Rosh Yeshivah of Brisk in Jerusalem).