Chapter ten of the Tanya defines the difference between two kinds of righteous people, and the gap between them is enormous.
The "completely righteous" person—the tzaddik (a righteous person) gamur (צדיק גמור)—has converted all evil within himself into actual good. He does not merely suppress his dark impulses. He has transformed them. The desires that once pulled him toward worldly pleasure now pull him toward God with equal intensity. He has, in the Tanya's language, "completely divested himself of the filthy garments." He looks at physical pleasures and feels nothing but revulsion, because his love of God is so overwhelming that everything else tastes like ash.
The "incompletely righteous" person—the tzaddik she'eino gamur (צדיק שאינו גמור)—has fought the same war but not finished it. He has expelled evil from his heart and subdued it. He believes it is gone. But the Tanya says: if it were truly gone, it would have been converted into good. The fact that it was merely expelled, not transformed, means a residue remains. The incompletely righteous person still carries a trace of attraction to worldly things, so small he is not even aware of it.
Rabbi Schneur Zalman explains the mechanism. Complete righteousness requires absolute hatred of evil—"I hate them with the utmost hatred; I regard them as my own enemies" (Psalms 139:22). This hatred must be proportional to love of God. If your love of God is infinite, your hatred of evil will be absolute, and the evil will be fully transformed. If your love is great but not infinite, some sliver of tolerance for worldly pleasure will remain.
The practical implication is both humbling and clarifying. Most people who consider themselves righteous are, at best, incompletely righteous. They have won the war but not destroyed the enemy. The enemy is in hiding, dormant, forgotten—but not dead. The Tanya does not consider this a failure. It considers it reality. Only the rarest souls achieve total transformation. For everyone else, the war continues underground.