Final answer:
Thrasymachus defines justice as the advantage of the stronger, implying that it's an instrument for those in power. Socrates challenges this, leading Thrasymachus to concede rulers may legislate contrary to their interests, which undermines his initial definition.
Step-by-step explanation:
Thrasymachus, a character in Plato's The Republic, initially offers the definition of justice as nothing more than the advantage of the stronger. To him, rulers make laws to serve their own interests, so obeying those laws (what is conventionally deemed 'just') is effectively serving the interests of those in power. Thrasymachus sees justice as an instrument of power and not inherently valuable.
However, under scrutiny from Socrates' questioning, Thrasymachus's view shifts. Outlined by Socrates, justice is presented as a virtue that is beneficial to individuals and societies, leading to a harmonious state. It goes beyond personal advantage or obedience to law; it is the specialization and fulfillment of one's role in society, contributing to the overall good. Thrasymachus is pressured into acknowledging that rulers may unintentionally legislate in a way that is not to their advantage, hence contradicting his initial assertion that justice is simply the advantage of the stronger.