A translated excerpt from Rabbi Zuriel’s Otzrote Hatorah, Korach (1)
In summary: “Take fast hold of Mussar, let her not go; keep her; for she is thy life” (Proverbs 4:13). And they said concerning this: “A parable of one who is cast into the midst of the water, the captain extended the rope and said to him, ‘Hold this in your hand and do not let it go. For if you let it go, you have no life.’ Thus said the Holy One, blessed be He, to Israel: ‘Take fast hold of Mussar, let her not go'” (Bamidbar Rabbah, end of Parashat Shelach). And indeed, “fools despise wisdom and mussar” (Proverbs 1:7). The verse divides them into two, to teach us that there is a “wise man” like Korach, but his heart is consumed by jealousy and love of authority. And this is what Rabbi Yisrael Salanter answered when he was asked by someone what to study, since he was busy with his livelihood all day. When he asked a certain rabbi, he answered him to study only practical Halacha (law). But one Rosh Yeshiva answered him to study only Gemara, because that is the base of everything. And a rabbi wise in secrets answered him to study only Kabbalah, because that is the peak of the Torah’s loftiness. And another rabbi answered him to study only Scripture, because that is what we heard from Sinai from the mouth of Hashem. When they neglect ‘Mussar’, then there is no guarantee in the Torah itself that it will save a person from going down to the pit. And the Ramchal spoke much about this (in his introduction to “Mesillat Yesharim”). “Behold, the fear of Hashem, that is wisdom,” only it alone is wisdom, and the rest of the occupations of the Torah will not compare to it (see there for his proofs).