Final answer:
Socrates chooses to explain justice in the context of a city rather than a soul to show how the behavior of individuals within a society reflects justice.
Step-by-step explanation:
Socrates chooses to explain justice in the context of a city rather than a soul because he believes that the behavior of individuals within a society can serve as a reflection of justice. He argues that if a just city is more successful than an unjust one, it follows that a just individual will also be more successful than an unjust individual.
By using the analogy of a just city, Socrates can demonstrate how the principles of justice can be applied to the larger society and guide individual behavior.
Socrates does not include justice as a virtue within the city, suggesting that justice does not exist within the human soul either, rather it is the result of a "well-ordered" soul. A result of this conception of justice separates people into three types; that of the soldier, that of the producer, and that of a ruler.
He proposes to look for justice in the city first and then to proceed by analogy to find justice in the individual (368c-369a). This approach will allow for a clearer judgment on the question of whether the just person is happier than the unjust person.