Remember that the Sachs/Grote problem is that Plato's argument in Republic II-IV changes the subject. What is at issue is whether to be just in the ordinary sense makes someone happy. But Plato argues that to be just in a special, newly defined sense makes someone happy. Plato therefore needs to connect the two notions of justice and argue:
(A) Anyone who has internal (psychological, Platonic) justice also has ordinary (pratical, vulgar) justice.Eric Brown attempts a solution (at least for claim (A)) in his recent, stimulating article, "Minding the Gap in Plato's Republic" (Philosophical Studies, 2004, 275-302). The solution is also encapsulated in Brown's contribution to SEP. Brown's view, in brief, is that the 'gap' between these two notions of justice is meant to be filled by Plato in his account of early childhood education in books II and III:
(B) Anyone who has ordinary (practical, vulgar) justice also has internal (psychological, Platonic) justice.
This brings us to Books Two and Three, where Socrates offers a long discussion of how to educate the guardians for the ideal city. This education is most notable for its carefully censored "reading list;" the young guardians-to-be will not be exposed to inappropriate images of gods and human beings. But Socrates is remarkably optimistic about the results of a sufficiently careful education. Well-trained guardians will "praise fine things, be pleased by them, receive them into his soul, and, being nurtured by them, become fine and good," and each will "rightly object to what is shameful, hating it while he's still young and unable to grasp the reason" (401e4-402a2). Note that Socrates has the young guardians not only responding to good things as honorable (with spirited attitudes), but also becoming fine and good. Moreover, Socrates is confident that the spirited guardians are stably good: when he is describing the possibility of civic courage in Book Four, he suggests that proper education can stain the spirited part of the soul with the right dispositions so deeply that they will be preserved "through everything" (429b8, 429c8, 430b2-3).
This optimism suggests that the motivations to do what is right are acquired early in moral education, built into a soul that might become, eventually, perfectly just. And this in turn suggests one reason why Socrates might have skipped the question of why the psychologically just can be relied upon to do what is right. Socrates might be assuming that anyone who is psychologically just must have been raised well, and that anyone who is raised well can be relied upon to do what is right. So understood, early childhood education, and not knowledge of the forms, is the driving force that links psychological justice and just action (SEP, "Ethics and Politics in Plato").
The resolution is ingenious, not least for maintaining that the 'gap' in the Republic should be bridged by looking to material that occurs before book IV.
Nonetheless, Brown's resolution seems open to an obvious difficulty. According to Brown, Socrates is presupposing that "anyone who is raised well can be relied upon to do what is right." However, what is it to be 'raised well'? If you say: "It is to be raised in such a way that you are just in the ordinary way, so that you become as Socrates, not Thrasymachus, recommends in bk. I", then that begs the question.
Grant that it is possible to train a child so that he or she turns out (given additional education, etc.), as an adult, to be disposed not to betray, steal, commit adultery, etc. Grant even that one can be trained to prefer these things for their own sake. But what is at issue is whether that sort of upbringing is what it is to be raised well--that is, must children be raised in that way, if they are to be happy? That is what Thrasymachus would presumably dispute.
Suppose you say, "But in being raised to have a harmonious soul, the future guardians are thereby raised to do what is ordinarily right"--then someone might say in response (echoing Sachs), "But why couldn't someone be raised to have a harmonious soul, and at the same time be raised to steal, betray, commit adultery, etc., in those circumstances in which he could do so without getting caught?" Once again, the question is begged: it remains unclear why there is not an alternative, Thrasymachean way of being raised 'well', which imparts psychic harmony, while at the same time teaching a child to reach out unhesitatingly for advantages, when he can do so without punishment or loss of a good reputation.
What we need is an argument that there is only one way of being raised well, and that such an education imparts, at once, both internal justice and ordinary justice. But Plato presupposes this in books II-III, he does not argue for it. So the 'gap' remains unbridged by this approach.