The Power of the Levites: Rectification of the Left
The power to rectify the body of the demon comes from the appointment of the Levites after the Giving of the Torah. To be inducted into the holy service surrounding the Tabernacle, the Levites first needed to be purified, as described in the Torah. This purification took the entire development of the left axis of the sefirot (and the left dimension of reality), all the way down to its end, and transformed it into holiness. This is the ability of the Levites, exclusively. It was not given to the priests. This is another reason why, in the future, the Levites will become the priests, the meaning of the phrase, “the priests—the Levites.”
Part of the task entrusted to the Levites is to convert reality by rectifying evil and transforming it into good. Prior to the Giving of the Torah, this was impossible. Even the patriarch Isaac, who is considered to be the archetypal figure associated with the left axis of the sefirot, could not perform this transformation, for he lived before the Giving of the Torah. What was the role of the Levites and why did they merit this special power?
In the wilderness, the Levites carried the Ark of the Covenant on their shoulders. This is a very great privilege. In the Temple, when the ark no longer left the Sanctuary and the primary focus of their service is song. Song consists of both longing and joy, as explained at length in the writings of Chabad. Now, the Levite corresponds to the beinoni (the intermediate person, who is “judged” by both his animal soul and his Divine soul, as described in the Tanya) and thus, the service of the Levites is the service of the beinoni. 
A most important correspondence that we implement in our contemplation of the nature of the tzaddik, the beinoni, and the rasha (the righteous, the intermediate, and the wicked types of individuals) is that of time. The righteous individual’s consciousness is focused on the future; the consciousness of the wicked type of individual is focused on the past. The consciousness of the beinoni, the intermediate individual is focused on serving God in the present moment. He lives and acts in the present, which requires him to be joyous in every situation.
In the Bible, the word for “present” (הוה) also means “to break.” Someone unfamiliar with Chasidic thought would suppose that we should not associate being “broken” with the service of God. However, this is contrary to what Chasidut teaches. Even though the primary mode of the psyche of the individual serving God should be happiness in every situation; accepting all that happens with joy, still, the intermediate individual also feels broken at every moment. A person who lives in the present really does have a broken heart. He is broken by the fact that he still has not seen Mashiach with his own eyes (unlike the righteous individual whose consciousness is in the future and who already lives with Mashiach in a truly enlightened reality). It may be that the intermediate individual is even able to live with Mashiach. But his heart is broken because there are others – perhaps a country at the end of the world – that do not. This is a topic the Lubavitcher Rebbe explained in depth in the last essay he distributed “V’atah teztaveh”).
The broken heart is a receptacle. There is a well-known Chasidic aphorism that states, “There is no vessel more whole than a broken heart.” If an individual is attached to the impure husks, having a broken heart is dangerous; the person is broken and his psyche can shatter, God forbid. But when a person is attached to the sacred, to holiness, then a broken heart becomes the ultimate vessel for revealing the secrets of the Torah, as the sages state, “secrets of the Torah are given only to he whose heart is worried [i.e., broken] within.” The broken heart is the sense of existential lowliness that the intermediate individual is encouraged to promote. Hence, it is the receptacle into which the joy flowing from above can be poured. As a matter of fact, the heart of the intermediate individual who focuses on serving God in the present moment contains both the cry of anguish over the present concealment of Godliness and the delight and joy pouring in from above and revealing Godliness at the same time. There are two opposite states of the heart that a person can feel simultaneously. This is one of the foundations of the service of the beinoni in the Tanya. Joy is the light and weeping is the vessel that can contain it. Both of them are in the present. This is the service of the Levite at the present moment.
The service of rectification and transformation of evil also occurs in the heart. In our present context, it means taking all the demons and the negative inclinations they represent and transforming them into holiness through this particular service of the Levites, the service of the beinoni, through contemplation that creates and arouses love in the soul. The primary time to perform this contemplation is when we say Shema Yisrael, which leads to a state of, “And you shall love Havayah your God with all of your heart and with all your soul and with all your being.” The demon or the primary negative inclination we need to rectify is the craving of the animal soul. If we can use craving to love God, we can attain the level described by the sages as “’Very good’ [referring to how God summed up all of Creation at the end of the sixth day]: this is the evil inclination.” Thus, we advance from a state of “with all of your heart”—a state in which the heart contains both the vessel of lowliness and the light of joy to the state of “with all your being,” whereby all our faculties have been transformed and sweetened.
. Numbers 8:5 and on.
. Sha’ar HaGilgulim, hakdamah 35; Sha’ar HaPesukim on Ezekiel 20.
. Ezekiel 44:15.
. See at length in Sefer HaNigunim and more.
. See the preface to Rabbi Ginsburgh’s Hebrew books, Shechinah Beineihem footnote 1 (p. 10); Lev LaDa’at, footnote 10 in Perek BeAvodat HaShem; Nefesh Bri’ah chs. 4 and 28.
. See Ezekiel 7:26 (in the commentaries there); Proverbs 17:4; Psalms 57:2 and 91:3 (in Metzudat Tziyon).
. Tanya, Iggeret HaKodesh 11.
. VeAtah Tetzaveh 5741, distributed as Kuntres Purim Katan 5752.
. Kol Mevasser part 1 on Psalms 147 (in the name of Rebbe Bunim of Parshischa). See Kuntres HaTefillah, ch. 12.
. Chagigah 13a.
. Explained at length in Ma’amar HaShiflut VeHasimchah, by Rebbe Isaac of Homil.
. Tanya ch. 34; Iggeret HaTeshuvah.
. Deuteronomy 6:5.
. Berachot 9:45.
. Bereishit Rabbah 9:5 and ff.