In the middle dialogues, Plato casts the good life in terms of the "contemplation" (θεωρία) the soul enjoys in its disincarnate state.

According to this understanding of the good life, a human being is an immortal soul temporarily located in a mortal body. In its disincarnate state, the soul is free from practical concerns and the need to exercise reason to meet them. Its existence is blessed and completely characterized by knowledge of the forms, the things the "What is it?" question is about. The good life for a human being is one in which he or she engages in the love of wisdom to regain, as much as is humanly possible, the existence in contemplation of the forms the soul enjoyed prior to incarnation.

This understanding of the good life highlights a problem

The suggestion in the early dialogues was that the competency involved in living a good life was a matter of having justice and the traditional virtues. Yet, if the good life for a human being is a life in imitation of the existence of the soul enjoys in its natural and disincarnate state, it is unclear how this understanding of wisdom and the traditional virtues can be true. It seems that an unjust life would be better because it would allow one to spend more time in in contemplation of forms and thus to have a better life than someone whose life is burdened with the demands of justice.

In the Republic, Plato seems to explore a possible solution this problem. He has Socrates argue that justice pays.

Book I of the Republic

The Opening Conversation (Cephalus, Polemarchus, Thrasymachus)

The Republic divides into ten books. Book I is in the style of definitional dialogue. Socrates' interlocutors are Cephalus, Polemarchus, and Thrasymachus. The discussion is about justice, what it is and whether it pays. This discussion is preliminary to the Theory of Justice that Socrates presents in the subsequent books of the Republic.

1. Cephalus

Cephalus has a common view about justice and happiness. He is rich, near the end of his life, and the conversation turns to how he has benefited from his wealth (Republic I.330d).

He suggests that wealth is good because it removes the need to act unjustly (Republic I.331a-b). The rich man has no need to risk punishment, either in this world or the next. Justice opposes happiness and tempts a human being to act unjustly to satisfy his desires. Wealth is good because it undermines this temptation and renders such unjust action unnecessary.

Socrates thinks that Cephalus is mistaken. Cephalus suggests that justice is a burden a human being is sometimes better off without. Socrates believes that the just life is always better than the unjust life and that injustice is never in one's interest. So, to determine the truth of the matter, he asks Cephalus a question about what justice is (Republic I.331c).

But speaking of this very thing, justice, are we to affirm thus without qualification that it is truth-telling and paying back what one has received from anyone, or may these very actions sometimes be just and sometimes unjust? I mean, for example, as everyone I presume would admit, if one took over weapons from a friend who was in his right mind and then the lender should go mad and demand them back, that we ought not to return them in that case and that he who did so return them would not be acting justly—nor yet would he who chose to speak nothing but the truth to one who was in that state.
You are right, Socrates.
Then this is not the definition (ὅρος) of justice: to tell the truth and return what one has received (Republic I.331c-d).

Cephalus drops out now, but Plato has indicated in a rough way what justice is about. It has to do with paying debts, telling the truth, and so on.

2. Polemarchus

Polemarchus takes his father's place once the conversation has taken a more serious turn (Republic I.331d).

Polemarchus appeals to the poet Simonides (who died about the time of Socrates' birth) to say what justice is (Republic I.331e). Polemarchus says that it justice is giving each man his due, that to one's enemies what is due is harm, and that to one's friends what is due is benefit.

This exchange brings into sharper focus the point of justice. To evaluate the traditional conception of justice (paying owns debts, telling the truth, and so on), it is necessary to know the point of justice. Polemarchus introduces the idea that it is to give human beings what is due to them. If justice is what is appropriate in connection with human beings, then they are due what is appropriate for them. Polemarchus is confused about what this is. He cannot defend his answers because what is appropriate for human beings is good for them, both friends and enemies alike.

3. Thrasymachus

Thrasymachus jumps into the conversation once Polemarchus drops out (Republic I.336b).

"[D]o you yourself answer and tell us what you say the just is. And don't you be telling me that it is that which ought to be, or the beneficial or the profitable or the gainful or the advantageous, but express clearly and precisely whatever you say. For I won't take from you any such drivel as that" (Republic I.336d; cf. Clitophon 409b-c).

Thrasymachus is not satisfied with how the conversation has gone. He wants an analysis of the underlying facts and gets closer to the truth about what justice is, at least as Plato understands it. Thrasymachus thinks (despite his rant) that justice is what is in one's interest, but he is confused about what that is. He seems to have the common idea that the good life is a life of satisfying one's desires. He tells Socrates that "justice is nothing other than the advantage of the stronger" (Republic I.338c), that the rulers are the strong, that they use the power of the state to satisfy their desires, and that "justice is the same in every city, being that which supervenes on the advantage of the established rulers" (Republic I.338e-339a)

The conversation turns to a new question: whether justice is better than injustice (Republic I.343b; cf. Republic I.347e).

Socrates and Thrasymachus disagree about whether the just life is better than the unjust life Thrasymachus thinks that the unjust life is better. He is convinced that injustice helps a person get the most he can for himself. Thrasymachus, however, is unable to defend his views about justice against Socrates' questioning.

"For I tell you, Thrasymachus, I am not convinced, and do not think that injustice is more profitable than justice, not even if one gives it free scope and does not hinder it" (Republic I.345a).

Justice and the Function of the Soul

Socrates gets Thrasymachus to admit that the "soul" (ψυχή) has a "function" (ἔργον). They agree that this function is "to take care of things, to rule, to deliberate" (τὸ ἐπιμελεῖσθαι καὶ ἄρχειν καὶ βουλεύεσθαι). They agreed, earlier in the conversation at Republic I.350c-d, that "justice is the virtue (ἀρετήν) of the soul."

"The soul, has it a work which you couldn't accomplish with anything else in the world, as for example, management, rule, deliberation, and the like, is there anything else than soul to which you could rightly assign these and say that they were its peculiar work?
Nothing else.
And again life? Shall we say that too is the function of the soul?
Most certainly.
And do we not also say that there is a virtue of the soul?
We do.
Will the soul ever accomplish its own work well if deprived of its own virtue, or is this impossible?
It is impossible.
Of necessity, then, a bad soul will govern and manage things badly while the good soul will in all these things do well.
Of necessity.
And did we not agree that the excellence or virtue of soul is justice and its defect injustice?
Yes, we did" (Republic I.353d-e).

Given this much, Socrates goes on to argue that injustice is never more profitable than justice.

"The just soul and the just man then will live well and the unjust badly?
So it appears by your reasoning, Socrates.
But surely he who lives well is blessed and happy (μακάριός τε καὶ εὐδαίμων), and he who does not the contrary.
Of course.
Then the just is happy and the unjust miserable.
So be it, Socrates.
But it surely does not pay to be miserable, but to be happy.
Of course not.
Never, then, most worshipful Thrasymachus (Θρασύμαχε), can injustice be more profitable (λυσιτελέστερον) than justice" (Republic I.353e-354a).

(The word λυσιτελέστερον ("more profitable") is the comparative of λυσιτελής, which is a compound formed from λύω ("free") and τέλος ("end") So the issues is which life frees the end more, the just life or the unjust life. The answer depends on what the end is. Thrasymachus seems to think it is get whatever one happens to desire.)

Book II of the Republic

Socrates refutes Thrasymachus, but the answers to the two questions about justice (what justice is and whether the just life is better) have not been settled (Republic I.354b-c). Book I of the Republic thus ends in perplexity (like the early dialogues devoted to the search for a definition), but Plato's older brother, Glaucon, is not content to let such an important matter go without further discussion. He tell Socrates that he is "eager to hear the nature of each, of justice and injustice, and what effect its presence has upon the soul (ψυχῇ)" (Republic II.358b).

The common view about justice and whether the just life is better

To press his case, Glaucon outlines a view of justice and the value of the just life that many accept but that he himself wonders about. "They say that to do wrong is by nature good, to be wronged is bad, but the suffering of injury so far exceeds in badness the good of inflicting it that when men do wrong and are wronged by one another and taste of both, those who lack the power to avoid the one and take the other determine that it is for their profit to make a compact with one another neither to commit nor to suffer wrong; and that this is the beginning of legislation and covenants between men, and that they name the commandment of the law the lawful and the just, and that this is the genesis and being of justice (γένεσίν τε καὶ οὐσίαν δικαιοσύνης)" (Republic II.358e-359a).

"Advantage (πλεονεξίαν) is what all of nature naturally pursues as good..." (Republic II.359c).

Similar views about justice and the good life in the Protagoras and Gorgias

On the view of the many Glaucon outlines, justice is a convention that provides individuals with space to work out for themselves what the good life is. The only restriction is that these lives must be consistent with the convention is justice and that regulates group living. This view of justice is a variation on the views Protagoras and Callicles suggest in the Protagoras and Gorgias.

"Thus far provided, men dwelt separately in the beginning, and cities (πόλεις) there were none; so that they were being destroyed by the wild beasts, since these were in all ways stronger than they; and although their skill in handiwork was a sufficient aid in respect of food, in their warfare with the beasts it was defective; for as yet they had no civic (πολιτικὴν) art, which includes the art of war. So they sought to band themselves together and secure their lives by founding cities. Now as often as they were banded together they did wrong to one another through the lack of civic art (ἠδίκουν ἀλλήλους ἅτε οὐκ ἔχοντες τὴν πολιτικὴν τέχνην), and thus they began to be scattered again and to perish. So Zeus, fearing that our race was in danger of utter destruction, sent Hermes to bring respect and right (αἰδῶ τε καὶ δίκην) among men, to the end that there should be regulation of cities and friendly ties to draw them together. Then Hermes asked Zeus in what manner then was he to give men right and respect: 'Am I to deal them out as the arts have been dealt? That dealing was done in such a way that one man possessing medical art is able to treat many ordinary men, and so with the other craftsmen. Am I to place among men right and respect in this way also, or deal them out to all?' 'To all,' replied Zeus; 'let all have their share: for cities cannot be formed if only a few have a share of these as of other arts. And make thereto a law of my ordaining, that he who cannot partake of respect and right shall die the death as a public pest.' Hence it comes about, Socrates, that people in cities, and especially in Athens, consider it the concern of a few to advise on cases of artistic excellence or good craftsmanship, and if anyone outside the few gives advice they disallow it, as you say, and not without reason, as I think: but when they meet for a consultation on civic art, where they should be guided throughout by justice (δικαιοσύνης) and good sense (σωφροσύνης), they naturally allow advice from everybody, since it is held that everyone should partake of this excellence (ἀρετῆς), or else that states cannot be" (Protagoras 322a-323a).

"The makers of the laws (τοὺς νόμους) are the weaker sort of men, and the more numerous. So it is with a view to themselves and their own interest that they make their laws and distribute their praises and censures; and to terrorize the stronger sort of folk who are able to get an advantage (πλέον), and to prevent them from getting one over them, they tell them, that such aggrandizement is foul and unjust (αἰσχρὸν καὶ ἄδικον), and that wrongdoing is just this endeavor to get the advantage of one's neighbors: for I expect they are well content to see themselves on an equality, when they are so inferior. So this is why by convention (νόμῳ) it is termed unjust and foul to aim at an advantage over the majority, and why they call it wrongdoing: but nature (φύσις), in my opinion, herself proclaims the fact that it is right for the better to have advantage of the worse, and the abler of the feebler. It is obvious in many cases that this is so, not only in the animal world, but in the states and races, collectively, of men--that right has been decided to consist in the sway and advantage of the stronger over the weaker" (Gorigias 483b-d).

Perseus Digital Library:
Plato's, Republic
Henry George Liddell, Robert Scott, A Greek-English Lexicon:
δικαιοσύνη, dikaiosynē, noun, "justice,"

"[Once conception of Ethics]--equivalent to the qualified term 'Private Ethics,' which is sometimes preferred-- [is] a study of the Good or Wellbeing of man, so far as this is attainable by the rational activity of individuals as such. ... [Another is] concerned primarily with the general rules of Duty or Right Action sometimes called the Moral Code viewed as absolutely binding on every man, and properly to be obeyed by him without regard to his personal interests ; the relation of duty to the agent s private happiness being regarded as a matter of secondary concern from an ethical point of view" (Henry Sidwick, Outlines of the History of Ethics for English Readers, 5th edition. (Macmillan and Company, 1902), 3, 6).

Θρασύμαχε, Thrasymache, proper name (from θρασύμαχος, thrasymachos, adjective,"bold in battle"),
ὅρος, horos, noun, "boundary,"
πόλις, polis, noun, "city"