In his dialogue Republic, Plato uses Socrates to argue for justice that covers both the just person and the just City State. Justice is a proper, harmonious relationship between the warring parts of the person or city. Hence, Plato's definition of justice is that justice is the having and doing of what is one's own. A just man is a man in just the right place, doing his best and giving the precise equivalent of what he has received. This applies both at the individual level and at the universal level. A person's soul has three parts – reason, spirit and desire. Similarly, a city has three parts – Socrates uses the parable of the chariot to illustrate his point: a chariot works as a whole because the two horses' power is directed by the charioteer. Lovers of wisdom – philosophers, in one sense of the term – should rule because only they understand what is good. If one is ill, one goes to a medic rather than a farmer, because the medic is expert in the subject of health. Similarly, one should trust one's city to an expert in the subject of the good, not to a mere politician who tries to gain power by giving people what they want, rather than what's good for them. Socrates uses the parable of the ship to illustrate this point: the unjust city is like a ship in open ocean, crewed by a powerful but drunken captain (the common people), a group of untrustworthy advisors who try to manipulate the captain into giving them power over the ship's course (the politicians), and a navigator (the philosopher) who is the only one who knows how to get the ship to port. For Socrates, the only way the ship will reach its destination – the good – is if the navigator takes charge.
Advocates of divine command theory argue that justice, and indeed the whole of morality, is the authoritative command of God. Murder is wrong and must be punished, for instance, because God says it so. Some versions of the theory assert that God must be obeyed because of the nature of his relationship with humanity, others assert that God must be obeyed because he is goodness itself, and thus doing what he says would be best for everyone.
A meditation on the Divine command theory by Plato can be found in his dialogue, Euthyphro. Called the Euthyphro dilemma, it goes as follows: "Is what is morally good commanded by God because it is morally good, or is it morally good because it is commanded by God?" The implication is that if the latter is true, then justice is arbitrary; if the former is true, then morality exists on a higher order than God, who becomes little more than a passer-on of moral knowledge.
Some philosophers have deemed this to be a false dilemma. that goodness is the very nature of God, and there is necessarily reflected in His commands.. Another response, popularized in two contexts by Immanuel Kant and C. S. Lewis, is that it is deductively valid to argue that the existence of an objective morality implies the existence of God and vice versa.
For advocates of the theory that justice is part of natural law (e.g., John Locke), it involves the system of consequences that naturally derives from any action or choice. In this, it is similar to the laws of physics: in the same way as the Third of Newton's laws of Motion requires that for every action there must be an equal and opposite reaction, justice requires according individuals or groups what they actually deserve, merit, or are entitled to. Justice, on this account, is a universal and absolute concept: laws, principles, religions, etc., are merely attempts to codify that concept, sometimes with results that entirely contradict the true nature of justice.