Plato presents the enigmatic and ambiguous formula, "each one doing his own" as his definition of social justice. I will search for the sense that he establishes for that definition: to show how he thinks he has established that that unlikely formula is in fact a reasonable definition of social justice, and to analyze what it means. Plato's theory of justice has its primary sources in sophistic thinking, in particular to the contractarian approach to political philosophy.
Lee, Edward Nichols, "Plato's Theory of Social Justice in Republic II-IV" (1981). The Society for Ancient Greek Philosophy Newsletter. 120.