James Adam, The Republic of Plato

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ξυμφέρον γέ τι. There is here a hint of the main purpose of the Republic, which is to prove that δίκαιον is ξυμφέρον in the truest sense for the individual and the state.

339B - 341A Now that the meaning of the definition has been explained, Socrates proceeds to attack it. Even if we assume that rulers seek their own advantage, yet they often err, and enact laws to their own disadvantage: therefore, as it is just for subjects to obey their rulers, Justice will sometimes consist in doing what is not the interest of the stronger. Socrates reiterates this objection and is supported by Polemarchus. It is urged by Clitophon that Thrasymachus meant by ‘the interest of the stronger’ what was thought—whether rightly or wrongly— by the stronger to be to their interest. Thrasymachus declines to avail himself of this suggestion, and explains that, strictly speaking, rulers, qua rulers, cannot err. This statement he supports by arguing from the analogy of medical practitioners and others, pleading that his earlier concession was but a popular way of expressing the fact that rulers seem to err. Therefore the original definition was strictly correct. Justice is the interest of the stronger, since rulers make laws in their own interest, and, qua rulers, are infallible.

On the reasoning of Thrasymachus in these two chapters see 341 A note

οὐ -- μέντοι. “In interrogationibus haec particula” (μέντοι） “ita cum οὐ negatione coniungitur, ut gravissima sententiae vox intercedat, quo modo aliquis eis quae ex altero quaerit summam veritatis ingerit speciem” (Hoefer de part. Plat. p. 34). μέντοι is simply ‘of course,’ ‘surely’: ‘surely you regard it as just to obey the rulers, do you not?’ The idiom is frequent in Plato. The other examples of it (cited by Stallbaum) in the Republic are infra 346 A, VII 521 D, IX 581 A, 584 A, X 596 E.

πότερον δὲ ἀναμάρτητοι κτλ. The reasoning echoes that of 334 C above.