If Socrates is to convince Glaucon that justice and injustice in the soul is like justice and injustice in the city-state, then he must prove that there are parts of the soul analogous to the three classes of the city-state. Socrates believes he can do this by using the Principle of Opposites. But having made his point he runs into another question: Is each part of the soul an entity (a "humunclus") that has its own parts?