[Though the third section of sefer “Shmiras HaLashon” includes much important and beneficial information which encourages proper speech], nevertheless, the gentle reader should know that even if the book finds favor in his eyes, with the Help of HaShem Yisbarach, he should not think that it is sufficient for him to know only this third volume, for knowledge of the first two volumes are also necessary for “the man who desires life”. As it says in the Midrash (Yalkut Shimoni on Mishlei, 1:929): “‘To know wisdom and mussar….’ (Mishlei: 1; 2) If a person possesses wisdom, [behold, he learns mussar, and] if a person does not possess wisdom, he is incapable of learning mussar.” The meaning of this passage is obvious: If someone is mistaken regarding the nature of the law, no mussar will benefit him. Take, for example, someone who does not consider one of his [prohibited] business dealings as thievery. How would it help for him to learn all the areas of mussar that would inspire him with the severity of the sin of thievery, when he believes that [his dealings] do not involve theft at all? The same holds true [regarding] all areas of Torah. Therefore, one needs to learn the laws of the Torah so that he will know what is forbidden and what is permitted. He must also learn the concepts of mussar which bring a person to the fear of HaShem Yisbarach, so that he will motivate himself to fulfill the [precepts of the] Torah, and [specifically] the positive precept (Devarim: 10; 20): “You shall fear HaShem your G-d.” The same is true in the matter of [Lashon HaRa]: How will one benefit from all the mussar in the world concerning the severity of the prohibition involving Lashon HaRa and Rechilus prohibitions, once he has rationalized [his behavior by] saying that it is not Lashon HaRa at all, or that the Torah does not include [speaking against] “A person like that!” in the prohibition of Lashon HaRa. Therefore, a person must know what falls under the category of Lashon HaRa according to the halacha. [Additionally], in order to enable himself to overpower his [evil] inclination and fulfill that which he learned, he should see to it to learn mussar, it being the [discussion of] the magnitude of the punishment and reward concerning this matter [of guarding one's tongue].
[Correspondingly], there is another benefit in knowing the laws [of Lashon HaRa], whereby one will not come to be overly strict upon himself and forbid [himself from speaking against] an individual regarding whom it is a mitzvah to deride. [It would be a mitzvah to damage the reputation of someone [who] spreads heresy wherever such derision would] prevent the individual from deceiving the public with his heretical views (see Chofetz Chaim, section 8, paragraph 5 in the first section and in Be’air Mayim Chaim). [Knowledge of the laws of Lashon HaRa will also prevent one from being overly strict and forbidding himself from speaking negatively where there is] some other [extenuating] benefit [in speaking negatively] (see the introduction to sefer Chofetz Chaim).
 “Mussar” is moral reproof.
 The text from Yalkut Shimoni states, as follows:
““To know wisdom and mussar...” (Mishlei: 1; 2) - If there is mussar, why [is there] wisdom, and if there is wisdom, why [is there] mussar? Rather, if a person possesses wisdom, behold, he learns mussar, and if a person does not possess wisdom, he is not capable of learning mussar. ”
“Another explanation [of the aforementioned quote from “Mishlei”]: “To know wisdom and mussar...” - If a person possesses wisdom, behold, the words of Torah are passed (mussarin - related to the word “mussar”) into his possession, and if he does not possess wisdom, the words of Torah are not passed into his possession.”