Plato focused his philosophical studies mainly on two virtues: the love in the book "The banquet", and justice in the book "The Republic". In the book "The Republic", he founded the existence of the polis in the virtue of justice and wrote a reflection: "The greatest injustice is to seem just without being." With this reflection comes the question of what is justice and virtue.
We practice the virtues because we naturally want to be happy. Among the virtues, the Justice has a prominent role. Aristotle defined Justice as "give and receive proportionaly". Maybe, the Justice, is the virtue that invoke more the man, "the desire for justice" keeps the friendship between men.
But every virtue can hide a vice, in particular the rigor can be pass as justice.
In a sense we can see the equivalent of virtues in other fields of man. We could say that the equivalent in the psychology of virtue is the concept of mask. Justice puts a facade, an appearence printed in the written laws. No law or human code establishes that the neighbor matters.
When the neighbor no matters justice is a right of those who have power and money. When the other no matters the injustice prevails and as Plato says in his book The Republic comes the greatest injustice, "to seem just without being." This problem leads to another, "the opression"; the oppression to the neighbor. The oppression in a sense is general injustice.
Saint Augustine takes up the problem of justice but from the perspective of love. True justice is born of the heart of men, the love for God and the neighbor. Without Charity there is no justice said Saint Augustine. Without the good love, without the love to work there can be no justice. The foundation of all justice is the most sublime love, love for God and the neighbor.