James Adam, The Republic of Plato

οἶμαι γοῦν κτλ. No one who does not know the ἰδέα τοῦ ἀγαθοῦ can possibly know in what respect or how far particular δίκαια (such as, for example, a particular νόμιμον περὶ δικαίου: see V 479 D note) are good, because it is the παρουσία τοῦ ἀγαθοῦ in them which makes them good (505 A note). And no one who is ignorant ὅπῃ ποτὲ ἀγαθά ἐστιν can possibly defend δίκαια (such as for example the δίκαια ἐπιτηδεύματα of Plato's city), because he is at the mercy of anyone who attempts to shew that they are bad. Nor, until we know how far particular δίκαια are good (πρότερον), can we adequately know these δίκαια themselves, i.e. know which of these really is δίκαιον and which not, for we do not know how they stand in relation to the ultimate source of all justice, viz. the Idea of the Good. It is this which, in the last resort, is the true ‘measure of all things’ (Laws 716 C ff., where Plato employs the language of theology). Hence the supreme necessity for our Guardians to know the ἰδέα τοῦ ἀγαθοῦ. Bekker first restored the true reading of this passage, which had been strangely mutilated in earlier editions. Stallbaum reads μηδ᾽ ἂν ἕνα with q, but instances of the future with ἄν—see on 492 C—should not be wilfully multiplied.