James Adam, The Republic of Plato

In the first place, the definition is made more precise by representing justice as an art, whose business it is to benefit friends and injure foes (332 C, D). The question is then raised—how does the art of justice do good to friends and harm to foes? By the analogy of other arts Polemarchus is induced to say that Justice benefits friends and harms enemies (1) by fighting with them and against them in time of war, and (2) in connexion with partnerships concerned with money in time of peace (332 D—333 B). The explanation of Simonides' saying is now complete.

Socrates first directs his attack against (2). In cases where money has to be used, it is not justice, but some other art, that is useful for the required purpose: in other words justice is (in time of peace) useful only in dealing with useless or unused money and other unused objects: which is an unworthy view of the art (333 B— 333 E). Further the analogy of the other arts shews that the art of justice, if it is the art of keeping money safe, is also the art of stealing money—always provided that it does so for the benefit of friends and the injury of foes (333 E—334 B). Polemarchus, in bewilderment, reiterates his definition in the old form, and Socrates thereupon starts a fresh line of argument. By ‘friends’ and ‘foes’ Polemarchus means those who seem to us good and bad, not those who are so. But as bad men often seem to us good and good men bad, Justice will often consist in benefiting bad men, and harming good, i.e. in wronging those who do no wrong; or conversely, if we refuse to accept this conclusion, and hold that it is just to benefit the just and hurt the unjust, it will often be just to hurt friends and benefit enemies, viz. when our friends are bad, and our enemies good (334 C—334 E).

Polemarchus hereupon amends his explanation of ‘friend’ and ‘enemy’ into ‘him who both seems and is good,’ and ‘him who both seems and is bad’: and the definition now becomes, ‘It is just to benefit a friend if he is good, and injure an enemy if he is bad (335 A).’

To this amended definition Socrates now addresses himself. He first proves by the analogy of the other arts that to hurt a human being is to make him worse in respect of human excellence, i.e. Justice, in other words to make him more unjust, and afterwards by means of similar analogical reasoning, that no one can be made more unjust by one who is just. Simonides' saying, if Polemarchus has explained it aright, was more worthy of a tyrant than of him (335 A—336 A).