Justice stops being a list of right acts and becomes an architecture.
The dialogue begins among familiar candidates for justice: repayment, loyalty, reputation, advantage, conventional conduct. Plato’s city–soul construction changes the kind of answer the inquiry can accept. Justice becomes an ordering relation among differentiated functions rather than a catalogue of externally correct behaviors.
That structure then generates consequences. Education, appetite, reason, political form, degeneration and character become intelligible through relations established by the same architecture. The city is therefore not merely an analogy placed beside the person; it is part of a mechanism for discovering what justice would have to be.
The construction is speculative, but it is not unconstrained stipulation. Objections, rival definitions, psychological consequences and political degeneration repeatedly force the proposed structure to carry explanatory weight beyond the point at which it was introduced.