In the Republic, Socrates argues that justice pays. In a just human being, the parts of the soul are in "harmony." Reason rules, spirit is reason's ally, and appetite is held in check. In this way, the three parts of the soul in a just human being are organized so that action is in accordance with knowledge of what the good life is. This knowledge belongs to reason. In this part of the soul, there is the knowledge that existence in the body is temporary, that the body and its needs are distractions, and that the good most of all resides in "contemplation" (θεωρία).
Crater Plato, north of the Mare Imbrium ("Sea of Rains")
The Unjust Life is not Worth Living
Glaucon ask Socrates to show that just life is better, and he sets a very high bar. He asks Socrates to show that a just human being who suffers what are popularly understood as great misfortunes is still better off, even if the comparison is to an unjust human being who suffers none of these so-called misfortunes but instead is showered in what are popularly understood as the good things in life.
"We must take away his reputation, for a reputation for justice would bring him honor and rewards, so that it wouldn't be clear whether he is just for sake of justice itself or for those honors and rewards. We must strip him of everything except justice and make his situation the opposite of the unjust person's. Though he does no injustice, he must have the greatest reputation for it, so that his justice may be tested. ... [T]hose who praise injustice at the expense of justice will say that a just person in such circumstances is worse because he will be whipped, stretched on a rack, chained, blinded with fire, and, at the end, when he has suffered every kind of ill, he'll be impaled..." (Republic II.361b-362a).
"Socrates, I think that from this point on our inquiry looks ridiculous now that justice and injustice have been shown to be as we have described them. Even if one has every kind of food and drink, lots of money, and every sort of power to rule, life is thought not to be lived if the nature of the body is ruined. So if someone can do whatever he wishes, except what will free him from vice and injustice and make him have justice and virtue (δικαιοσύνην δὲ καὶ ἀρετὴν), how can it be worth living if the nature by which we live is ruined and in turmoil (Republic IV.445a-b).
Despite Glaucon's suggestion that further inquiry would be "ridiculous," it is not at all clear that Socrates has shown that the just life is better. Part of the problem is that Socrates has not explained in any real detail what it is to be "happy" (εὐδαίμων). He thinks that "contemplation" (θεωρία) makes a substantial contribution. This much seems clear, but Socrates never explains how this contribution is weighed in comparison with other things traditionally thought to contribute to or detract from "happiness" (εὐδαιμονία).
The truest pleasures are proper to the parts
Socrates argues that the just life is the most pleasurable and thus is happier than the unjust life.
Socrates says that there is pleasure for each part of the soul, that the objects of desire approved by reason are "most true," and that the life of reason is the "sweetest." A few lines later he provides more detail. He argues that a part of the soul will get the "truest" pleasure when it gets what is "proper" to it, that when the part of the soul with reason dominates, all parts of the soul get their "truest" pleasures because the knowledge in the part with reason directs action, and that when the appetitive or spirited parts dominate, the parts of the soul do not get their "truest" pleasures.
"The three parts of the soul have, it appears to me, three kinds of pleasure, one peculiar to each, and similarly three appetites and controls" (Republic IX.580d).
"But since the tests are experience and wisdom and reason (ἐμπειρίᾳ καὶ φρονήσει καὶ λόγῳ), what follows?
Of necessity, that the things approved by the lover of wisdom and the lover of reason (ὁ φιλόσοφός τε καὶ ὁ φιλόλογος) are most true (ἀληθέστατα).
Then of the three kinds of pleasure, the pleasure of that part of the soul whereby we learn is the sweetest, and the life of the man in whom that part dominates is the most pleasurable (ἡ τούτου τοῦ μέρους τῆς ψυχῆς ᾧ μανθάνομεν ἡδίστη ἂν εἴη, καὶ ἐν ᾧ ἡμῶν τοῦτο ἄρχει, ὁ τούτου βίος ἥδιστος;)?
How coud it be otherwise" (Republic IX.582e-583a)?
"[T]o be filled with what befits (προσηκόντων) nature is pleasure..." (Republic IX.585d).
"Let us confidently assert that those desires of even the profit-loving and honor-loving parts of the soul, which follow knowledge and reason (ἐπιστήμῃ καὶ λόγῳ) and pursue with their help those pleasures which intelligence (φρόνιμον) prescribes, will attain the truest (ἀληθεστάτας) pleasures possible for them, since they are following the truth. These pleasures are proper (οἰκείας) to them, if that which is best for each thing may be said to be most proper to it. So if the whole soul follows the wisdom-loving part (φιλοσόφῳ) and there is no internal dissension, then each part will be able to fulfill its own task and be just in other respects, and also each will reap its own pleasures, the best and the truest as far as possible. And when one of the other two [parts of the soul] gets the mastery the result for it is that it does not find its own proper pleasure and constrains the others to pursue an alien pleasure and not the true" (Republic IX.586d-587a).
Why the lovers of wisdom rule in a just city
In a just city, the rulers are lovers of wisdom. How can this be true if, as Socrates claims, justice pays? Wouldn't the lovers of wisdom be better off abandoning their posts as rulers so that they can spend more of their time in "contemplation" since this is the activity, not ruling, in which happiness consists most of all?
Socrates says that the rulers rule out of "necessity." "Each ruler will spend much of his time in the love of wisdom (φιλοσοφίᾳ), but, when his turn comes, he labors in politics and for city's sake, not as if he were doing something fine (καλόν), but as a necessity (ἀναγκαῖον)" (Republic VII.540b).
Why is the "necessity" that makes the lovers of wisdom rule? The answer not completely clear, but maybe the "necessity" stems from the fact that ruling is the way for the rulers to maximize their happiness. They may think that each taking his turn in ruling will maximize the time each spends in "contemplation" and hence the satisfaction he takes in his life. In this case, each thinks that if he were not to take his turn in ruling, the city would become unjust and that in these circumstances he would be forced to spend less time in "contemplation."
"And so the man proposes the penalty of death. Well, then, what shall I propose as an alternative? Clearly that which I deserve, shall I not? And what do I deserve to suffer or to pay, because in my life I did not keep quiet, but neglecting what most men care for--money-making and property, and military offices, and public speaking, and the various offices and plots and parties that come up in the state--and thinking that I was really too honorable to engage in those activities and live, refrained from those things by which I should have been of no use to you or to myself, and devoted myself to conferring upon each citizen individually what I regard as the greatest benefit? For I tried to persuade each of you to care for himself and his own perfection in goodness and wisdom (βέλτιστος καὶ φρονιμώτατος) rather than for any of his belongings, and for the state itself rather than for its interests, and to follow the same method in his care for other things. What, then, does such a man as I deserve? Some good thing, men of Athens, if I must propose something truly in accordance with my deserts; and the good thing should be such as is fitting for me. Now what is fitting for a poor man who is your benefactor, and who needs leisure to exhort you? There is nothing, men of Athens, so fitting as that such a man be given his meals in the prytaneum. That is much more appropriate for me than for any of you who has won a race at the Olympic games with a pair of horses or a four-in-hand. For he makes you seem to be happy (εὐδαίμονας), whereas I make you happy in reality; and he is not at all in need of sustenance, but I am needy. So if I must propose a penalty in accordance with my deserts, I propose maintenance in the prytaneum" (Apology 36b-37a).
Contemplation is the Reward
A just human being arranges his life so that there is time for "contemplation" (θεωρία). It is in this way that justice pays and that just life is better than the unjust life.