Thrasymachus continues to bluster and to engage in persiflage (whistle-talk). He argues that most people are "good" in appearance only; they do "right" things or try to pursue dike (the way things ought to be) only because they are ignorant, or stupid, or afraid of the punishment of the law. Strong men and intelligent men have the courage to do wrong; they can out-think simpler citizens and overpower weaker ones, weaker in whatever sense. Injustice (adikia) is the best course of action; the unjust man can take advantage of his fellows in every instance; he can cheat on his taxes, rob the public coffers and defraud the public, juggle books in a position of trust, and so on. And if one steals, Thrasymachus says, one ought to steal big. The more power, the better: The tyrant's life is the good life. At this point, Thrasymachus would like to leave the debate.
Socrates says that Thrasymachus is wrong on three counts: that the unjust man is more knowledgeable than the just, that injustice is a source of strength; and that injustice brings happiness.
In his argument at this point, Socrates again employs analogies, in this case the physician and the flute-player. We notice, Socrates says, that it is the ignorant man who always attempts home-remedies; always the man ignorant of music who attempts to outdo the musician and thereby shows his ignorance of the art.
Next, Socrates reminds Thrasymachus that even thieves have to trust one another and to show it by a fair division of their ill-gotten gain. That is, they too have to practice a kind of justice; otherwise, a gang of thieves would break up and their little "state" would degenerate into disunity, chaos, unhappiness. Unjust men, at whatever level of their practicing injustice, degenerate from an assumed strength to weakness.
Socrates' next argument advances analogies of the pruning hook, the eye, the ear, and the soul, all of which possess their several essences, what we may call their essential functions, or virtues. The eye sees, the ear hears, the pruning knife cuts well. These are their several virtues. What of man and his virtue in this instance? Man's virtue herein is his justice; it enables him to live well in harmony with others and to be happy. Only justice can bring happiness. Injustice at whatever level brings chaos, discord, unhappiness. In thus producing happiness, justice may be said to be more profitable than injustice.
At this point Thrasymachus quits the debate.
Beginning with his theory that might makes right, Thrasymachus is now advocating that injustice is better than justice; injustice is better for the individual. Thrasymachus is arguing that crime pays. Thrasymachus herein is arguing a kind of situational ethics; he is praising the benefits of amorality, and he here attempts to stand the entire argument on its head.
At the same time, we may find fault with Socrates' argument from analogy. Socrates is arguing that a man who prescribes medicine for himself has a fool for a physician, but we might object that a given man's ignorance in this instance may be said to be inconclusive; much the same is true of the flute-player analogy. The comparisons attempted here may not agree in sufficient points.
Socrates then argues that it follows that there must be a kind of honor among criminals, that in order to retain some sort of communal strength, they must practice a kind of honor. But Thrasymachus seems to have been arguing for man as an isolato, a self-sustained creature who does not require any sense of community.
Socrates' third rebuttal is also rather vague; the analogies he seeks to advance are not very clear, and it is difficult to perceive their essential similarities as being readily similar to the essence of the good man and his pursuit of justice.
Plato is probably not attempting to argue conclusively at this point; he has at this juncture in the Republic noticed that he is going to be required to extend his definition, argue more examples, adopt further analogies in order to amplify his argument and bring it to a close. As many readers and students over the centuries have remarked, Book I of the Republic may be viewed as an introduction to the conversation in its entirety.
So we are left more or less in the dark in our ideas of "the good life" and "happiness" and "justice" thus far in the proceedings. For Thrasymachus, these concepts seem to come to fruition in a power-grab motivated by simple greed. For Socrates, the attainment of these things seems to involve a deeper philosophical impact (ethical, perhaps spiritual choices).
And we have not yet defined "justice."
lyre a small stringed instrument of the harp family, used by the ancient Greeks to accompany singers and reciters.
end i.e., purpose, the object for the sake of which a thing exists or is made.
epicure a person who is especially fond of luxury and sensual pleasure; especially (and here), one with sensitive and discriminating tastes in food or wine. (The English word epicure is derived from the name of third-century B.C. Greek philosopher Epicurus; thus its use in translations of Plato is anachronistic.)