Plato's Republic: The Role of Glaucon and Adeimantus's Challenge

In Book II of Republic, Glaucon and Adeimantus (two of Socrates' interlocutors) demand a defense of justice "for its own sake". This challenge is the foundation of the philosophical project of Plato's Republic. This demand requires Socrates to look beyond the superficial, commonplace defenses of justice and embark on a deep dive into its nature, both in the soul and ideal city. 

Socrates notes that common understandings of justice are based on external rewards (good reputation, divine favor, social standing, favorable afterlife, etc.). Glaucon and Adeimantus argue that if justice is pursued for its consequences, then injustice would be preferable, as demonstrated by the Ring of Gyges myth. This challenge forces Socrates to argue that justice is inherently good regardless of external rewards. Every aspect of this city is designed to be analogous for the functioning of justice in the soul.  

By questioning whether the just life is more preferable than the unjust one, Glaucon and Adeimantus have transformed the discussion into searching for the best way to live. If injustice truly is better when one can get away with it, then reason dictates being unjust. This pushes Socrates to develop his own system for proving justice's worth and serves as the foundation for the rest of Republic.