Plato was not a neutral observer of the time and culture in which he lived. On the contrary, he was highly critical of what he considered the decadent and corrupt state of Athens. He saw the political system being undermined by men motivated by self-satisfaction and greed, by amateur self-interested meddling in important matters of state and government, and by excessive individualism in every aspect of life. The remedy for all this, in his view, was a society based on the principles of 'justice', and he outlined the nature of such an ideal society in his Republic. The question of what exactly justice is, is thus a centrally important one to Plato and large sections of The Republic are devoted to this issue.
There were many ideas about justice before Plato, and he uses Book I of his Republic to consider some of the prevailing notions. The discussion follows the classic pattern of the Socratic dialogue: a question is posed ('What is justice?'), several answers are proposed in turn, and each is questioned and criticised in an attempt to define what justice really is. The elderly, wealthy Cephalus suggests that justice is a matter of being honest to all and repaying one's debts -- a definition that Socrates argues is insufficient, for while returning borrowed arms to a lunatic would be repaying a debt it would hardly be a just act: 'everyone would surely say that if a man takes weapons from a friend when the latter is of sound mind, and the friend demands them back when he is mad, one shouldn't give back such things, and the man who gave them back would not be just' (331c). The next speaker is Polemarchus, Cephalus's heir, who suggests that returning weapons to a madman would not be 'fitting', and justice is doing what is fitting to people, so that each receives 'just what's owed to them' (332b), whether it be good for friends or harm for enemies. Socrates's response to this is to ask who is best placed to give a good to a friend: for example, would Polemarchus accept the good of medicine from a just person or from a physician, or the good of food from a just person or from a farmer? Polemarchus chooses the professional in each case, which leads Socrates to ask him, in that case, what use a just person is? Polemarchus responds that he is useful when goods are being stored and need to be guarded, which leads Socrates to suggest that justice is apparently at its most useful when goods are at their most useless, i.e. when they are being stored rather than used: 'is justice useless in the use of each and useful in its uselessness' (333d)? Furthermore, is not the person standing guard over the goods the person best placed to take them, thus making the just person a thief; is a person necessarily able to judge between true friends and enemies, ensuring that good only goes to the former; and can the just person ever deliberately harm a person, for example by forcing an unjust person to be just against their nature? With Polemarchus dealt with, Thrasymachus intervenes to suggest that justice is whatever the strongest decide it is, and that the strong decide that whatever is in their best interest is just. Socrates dismisses this by pointing out that the strong are no more able than anybody else to understand what their best interest is, which means that whatever they come up with cannot be justice, since justice is a good thing.
The views propounded by Cephalus and Polemarchus, and criticized by Plato through the comments of Socrates, reflect prevailing Greek morality. Plato finds them unsatisfactory because of their emphasis on individualism; they seek to regulate justice on the basis of relationships between individuals, based on flawed individual perceptions and self-interested individual morality. Plato sees this as ignoring the needs of society as a whole and transforming what should be a system designed for the common good into one intended to provide benefits to the individual. Similarly Thrasymachus's argument that justice is whatever the strong say it is fails to take into account that any art -- whether medicine, pottery, or governance -- has as its main interest its subject rather than itself, so that just as the doctor 'prescribes with a view not to his own interest but that of his patient' (342d) so no ruler 'exercises his authority, whatever its sphere, with his own interest in view, but that of the subject of his skill' (342e). A society that considered justice only from the viewpoint of the individual would be incapable of achieving anything, because each individual would have an interpretation of justice that would be inconsistent with every other, argument and hatred would break out, and co-operation would be impossible:
Injustice, then, seems to have the following results, whether it occurs in a state or family or army or in anything else: it renders it incapable of any common action because of factions and quarrels, and sets it at variance with itself and with its opponents and with whatever is just. (351d-e)
A further theorization of justice that Plato sets out to demolish is that proposed by Glaucon, who argues that justice is not innately to do with morality or with good but developed as a means of protecting the weak from the strong. Human society created justice, not for its own sake but for what its members could get out of it -- their own security. This is in a sense a forerunner of the 'social contract' argument by which people create civil society to overcome the condition of anarchy that will exist if the stronger are at liberty constantly to impose their will upon the weaker.
It is according to nature a good thing to inflict wrong or injury, and a bad thing to suffer it, but ... The disadvantages of suffering it exceed the advantages of inflicting it; after a taste of both, therefore, men decide that, as they can't evade the one and achieve the other, it will pay to make a compact with each other by which they forego both. They accordingly proceed to make laws and mutual agreements, and what the law lays down they call lawful and right. This is the origin and nature of justice. (358e-359a)
Plato's criticism of this theorization, as with those that have preceded it, is that it is based upon individualism, and sees justice as an external element, an accomplishment or a convention, rather than arising from within the human soul and motivated by the desire of the soul to act in accord with its nature.
To act in accord with one's nature is to do what one is best suited to do, as well as one can do it. This is the basis of the Greek notion of a craft or a skill. Plato believed that one of the things that ailed Athens was the tendency of people to meddle with things that were none of their concern and which they did not understand, and when he discusses the ideal city-state in The Republic one of the cardinal principles is that each member of society and each group within society confines itself to whatever is its proper concern. This is the fundamental principle behind the creation of the 'Guardians' -- the professionalization of governance and its restriction to the wise and qualified. The significance of this for justice is, to Plato, obvious: 'we have often heard it said and often said ourselves that justice consists in minding your own business and not interfering with other people' (433a-b). In considering the fundamental qualities that must be possessed by the ideal state, Plato has identified discipline, courage and wisdom, and concludes these three alone are not enough and that there must be a further quality that 'makes it possible for them to come into being in our state and preserves them by its continued presence' (433b) and that this quality is justice.
Now, if we were asked to judge which of these qualities by its presence contributed most to the goodness of our state, we should find it a difficult decision to make. Is it the agreement between rulers and subjects? Is it the retention by our soldiers of law-abiding judgement about what is and is not to be feared? Is it the wisdom and watchfulness of our Guardians? Or is the greatest contribution to its excellence made by the quality which makes each individual -- child or woman, slave, free man or artisan, ruler or subject -- get on with his own job and not interfere with other people. (433c-d)
Thus the ideal state has a harmonious form in which each individual and each class does what it is best suited by nature to do and leaves others to do what they are best suited to do. To damage this harmony is to damage the state and…