James Adam, The Republic of Plato

ἦν. The past does not exclude the present: cf. IV 436 C note

τίς -- πολιτεία: i.e. what the best constitution is. Adimantus was about to ask whether the ἀρίστη πολιτεία is not the one which they have described. The reply is yes, provided that the position and status of the Rulers is made clearer. As it stands it is not the best: cf. VIII 543 E καλλίω ἔτι ἔχων (imperfect participle) εἰπεῖν πόλιν τε καὶ ἄνδρα (the Philosopher's City and the Philosopher), where see note.

ὅτι δεήσοι κτλ.: ‘that there would always have to be present in the city a certain factor possessed of a reasoned theory of the constitution, identical with that possessed by you, the legislator, when you made the laws.’ The rulers must understand the constitution and not merely accept it on the legislator's authority, if the spirit of the original legislator is to survive his death. ὀρθὴ δόξα is not enough; in order to fill the place of the founder of the city they require ἐπιστήμη. Plato confesses that he did not make this clear enough before (οὐχ ἱκανῶς ἐδηλώθη), and his confession is most true. In one passage (IV 429 C note), indeed, he seems expressly to imply that the Rulers do not fill the legislator's shoes. But there are also some hints or traces of the later view: see on III 414 A, IV 423 E (to which, perhaps, ἐρρήθη is intended to refer, although the reference is hardly justified), and 442 C. Cf. 502 D, 503 A, 504 D notes