The ninth chapter of the Tanya maps the battlefield inside every human being.
The animal soul—the nefesh (the vital soul) habehamit (נפש הבהמית)—lives in the left ventricle of the heart. This is the seat of desire: lust, pride, anger, and every craving for worldly pleasure. From the heart, these passions spread throughout the body and rise to the brain, where they generate thoughts and schemes to satisfy them.
The divine soul—the nefesh ha'elokit (נפש האלוקית)—lives in the brain. Its home base is the intellect. From the brain, it extends downward into the right side of the heart, where it manifests as the burning love of God, the yearning to cleave to the Infinite.
Rabbi Schneur Zalman uses a verse from (Genesis 25:23) to frame the conflict: "One nation shall prevail over the other." The body is a "small city" (Ecclesiastes 9:14). Two kings are fighting over it. Each wants to rule its inhabitants—the limbs, the mouth, the eyes—according to his will.
The divine soul wants the body to study Torah, pray, perform commandments, and contemplate God's greatness. The animal soul wants pleasure, status, comfort, and distraction. They cannot coexist peacefully because their goals are mutually exclusive. When one gains territory, the other loses it.
The key insight: the divine soul's stronghold is the brain, and the animal soul's stronghold is the heart. This means the war is fought on the bridge between thinking and feeling. When a person uses intellectual contemplation of God's greatness to generate love and awe, the divine soul advances into the heart. When desires and passions cloud the mind, the animal soul conquers the brain.
The Tanya says this war is not a defect. It is the entire purpose of human existence. The body was designed as a contested space. The struggle itself—the daily, hourly effort to keep the divine soul in command—is what creates spiritual light. A city at peace has no need for soldiers. A city under siege produces heroes.