Classic commentary on Pirkei Avos, page 196-197.
4:13 Rav Eliezer ben Yaakov says: One who performs one mitzvah acquires for himself one defender, and one who transgresses one sin acquires for himself one prosecutor; repentance and good deeds are like a shield before the punishments.
4:13 A shield before the punishments. A shield only protects against weapons that are facing in one direction. If a person is surrounded by archers on all sides, he needs to surround himself with several shields in order to effectively defend himself. “If a person commits a sin with a particular limb, he should strive to perform a mitzvah with that same limb in order to create a shield” (Midrash Rabba Vayikra 21:5). Similarly, “My eyes shed streams of water, because they did not keep your Torah” (Tehillim 119:136). The eyes sinned, and therefore they needed to shed tears in order to rectify the sin.
Hashem tailors the punishment to the sin, measure for measure. When man suffers, he must carefully analyze his misfortune in order to understand why he experienced that particular punishment. Once he discovers which sin is the cause, he can rectify that sin and his suffering will abate. If he rectifies a different sin, he certainly has acted properly, but he is like a soldier who aims his shield in the wrong direction. It will not prevent the arrows from finding their mark. So, too, the punishment will continue.
“If a man experiences suffering, he should examine his deeds. If he cannot find any misdeeds, he should assume that he is being punished for bitul Torah, neglecting his Torah study” (Berachos 5a). Rashi explains that if he cannot find a sin which would fit this suffering, he should assume that it comes from bitul Torah.
“A healing for all his flesh” (Mishlei 4:22). 15 Torah study is different from all other mitzvos in that it can provide life and healing to the entire body. 16 Logically,the punishment for neglecting Torah study could affect any part of the body. If he cannot find any particular sin that would parallel this suffering, he should blame it on bitul Torah. This is because the punishment for bitul Torah could manifest itself in any way.
Alternatively, a shield against punishment can mean that even after the punishment begins, the observance of a mitzvah will arrest man’s suffering. 17
14.The verse does not say, “I did not keep your Torah.” Rather, it says, “They did not keep your Torah.” Eyes are the agents of sin, and their transgression is expiated through tears (Sha’arei Teshuvah 1:15).
15. If someone’s entire body is racked with pain, he should study Torah. As it says, “A healing for all his flesh” (Eruvin 54a).
16.Other mitzvos correspond to one of the various body parts, and performing a mitzvah will give life to its parallel limb (Zohar, quoted in Nefesh HaChaim 4:29). Rav Chaim Vital discusses a similar concept in respect to the future Resurrection of the Dead. Every limb earns the right to be resurrected by virtue of the person having performed the mitzvah which corresponds to that limb. If a person neglects a mitzvah, he will be resurrected without that limb (Sha’ar HaKedushah, quoted in preface to Mishnah Berurah, vol. 3). This seems to contradict the idea that punishment is meted out on the limb that committed the sin, not the limb which corresponds to that particular sin. Perhaps we can draw a distinction between the system of punishment and the mechanism through which man maintains himself and prepares for resurrection.
17. The first approach assumes that mitzvah observance only helps prevent punishment. Here, Rav Chaim suggests that the mitzvah observance can nullify punishment even after it has commenced.
Ruach Chaim, published by Targum Press, INC., copyright by Chanoch Levi.