The divine soul has ten holy faculties and three garments—thought, speech, and action—through which it connects to God via the 613 commandments. But there is another soul inside you too. The nefesh (the vital soul) habehamit (נפש הבהמית), the animal soul, has its own ten faculties and its own three garments. And they are dark mirrors of the divine ones.
Chapter four of the Tanya explains the architecture of the animal soul. It has its own intellect—chochmah, binah, and da'at (Knowledge)—but these serve the passions rather than commanding them. Where the divine soul's intellect rules its emotions, the animal soul's intellect is a servant to desire. It reasons backward from what it wants, constructing justifications after the fact.
Rabbi Schneur Zalman makes a striking observation: this is why children love worthless things. A child's intellect is too immature to appreciate what is truly valuable, so the child's emotions attach to whatever is nearest and shiniest. Adults whose animal soul dominates their intellect operate the same way—their mind, however sharp, serves their appetites.
The seven emotional attributes of the animal soul—its loves, fears, ambitions, and pleasures—flow from the four elements that compose it: fire (pride and anger), water (desire for pleasure), air (frivolity and boasting), and earth (laziness and depression). Every negative trait a person experiences can be traced to one of these four roots.
When a person thinks, speaks, or acts according to these impure faculties, the Tanya says, they are wearing the "impure garments" of the animal soul. These garments wrap around the ten dark faculties the way clothing wraps around a body. And the Tanya's term for everything done under their influence is devastating: "vanity and a striving after wind" (Ecclesiastes 1:14). Not evil in the dramatic sense. Just empty. Every thought, word, and deed not directed toward God is, in the Tanya's framework, ultimately hollow.