Chapter twenty-two of the Tanya confronts a paradox: if God's speech never separates from God, and if that speech is what sustains all of creation, then how can evil exist at all?
Rabbi Schneur Zalman introduces the concept of panim (פנים), "face" or "inner will," and achorayim (אחוריים), "back" or "reluctant will." God sustains the holy through His "face"—with desire, delight, and direct intention. God sustains the unholy through His "back"—reluctantly, indirectly, the way a person throws something to an enemy over his shoulder without looking.
This is why the forces of impurity are called elohim acherim (אלהים אחרים), "other gods." The word acherim ("other") shares a root with achorayim ("back"). They receive their life force not from God's inner desire but from the most external, reluctant expression of divine energy. It is enough to sustain them but not enough to truly enliven them.
This explains why the sitra achara—the "other side"—is associated with death and defilement. It receives only the faintest trace of divine light. It survives, but barely. It exists in what the Tanya calls a state of "actual exile"—divine sparks trapped in shells that suppress and conceal them.
And this, the Tanya says, is the ultimate purpose of Torah and mitzvot (commandments). The Torah was given to Israel specifically to extract those trapped sparks—to rescue the divine light imprisoned in the shells and restore it to holiness. Every commandment performed, every word of Torah studied, pulls another spark free. Every sin pushes sparks deeper into captivity.
The Torah "employs human language" precisely because it had to descend into the physical world—the world most saturated with shells and concealment—to reach the sparks trapped there. The Torah did not remain in heaven because heaven had no sparks to rescue. The mission was always here, in the lowest world, where the light is hardest to find and the extraction is most valuable.