“It is the duty of us, the founders, then,” said I, “to compel the best natures to attain the knowledge which we pronounced the greatest, and to win to the vision of the good, to scale that ascent, and when they have reached the heights and taken an adequate view, we must not allow what is now permitted.” “What is that?” “That they should linger there,” I said, “and refuse to go down again among those bondsmen and share their labors and honors, whether they are of less or of greater worth.” “Do you mean to say that we must do them this wrong, and compel them to live an inferior life when the better is in their power?”
“You have again forgotten, my friend,” said I, “that the law is not concerned with the special happiness of any class in the state, but is trying to produce this condition in the city as a whole, harmonizing and adapting the citizens to one another by persuasion and compulsion, and requiring them to impart to one another any benefit which they are severally able to bestow upon the community, and that it itself creates such men in the state, not that it may allow each to take what course pleases him, but with a view to using them for the binding together of the commonwealth...we shall not be wronging, either, the philosophers who arise among us, but that we can justify our action when we constrain them to take charge of the other citizens and be their guardians. For we will say to them that it is natural that men of similar quality who spring up in other cities should not share in the labors there. For they grow up spontaneously from no volition of the government in the several states, and it is justice that the self-grown, indebted to none for its breeding, should not be zealous either to pay to anyone the price of its nurture. But you we have engendered for yourselves and the rest of the city to be, as it were, king-bees and leaders in the hive. You have received a better and more complete education than the others, and you are more capable of sharing both ways of life. Down you must go then, each in his turn, to the habitation of the others and accustom yourselves to the observation of the obscure things there. For once habituated you will discern them infinitely better than the dwellers there, and you will know what each of the ‘idols’ is and whereof it is a semblance, because you have seen the reality of the beautiful, the just and the good. So our city will be governed by us and you with waking minds, and not, as most cities now which are inhabited and ruled darkly as in a dream by men who fight one another for shadows and wrangle for office as if that were a great good, when the truth is that the city in which those who are to rule are least eager to hold office must needs be best administered and most free from dissension...--Plato Republic 519C-520D.
Philosophy is a form of addictive escapism (a view echoed by Adam Smith). And so, according to Socrates, force is required to compel philosophers to return to their station and to focus on the more practical and less exciting affairs of ruling the best city. His justification relies on the distinction between those-would-be-philosophers born in a bad city and those-would-be-philosophers born in the best city. The distinction between them is not a natural one; they are, in fact, of the same natural type.
Socrates assumes that would-be-philosophers can be born anywhere.* Rather the difference between them is a matter of moral (bad) luck: the philosophers -- like Socrates -- who are born in a bad city do not owe their philosophical development to their home city. And, in turn, they need not rule--an argument he develops more fully (recall) in Book 9, 592ab. While the focus is rightly on the role of philosopher-kings in the ideal city, this also has an interesting implication for one's interpretation of Socrates's life; he suggests there was no obligation for him to rule in Athens and, more important, it would have been unnatural for him to do so. This can be taken as an oblique reference to the implied charge that he was implicated in the rule of 30 tyrants. His response is de facto that he would have felt not so urge to join in.
There are two parts to his justification for forcing philosophers in the best city to rule. The first is a kind of familiar consequentialist argument in which it is permitted (even required) to compel a (relatively) small class of citizens to sacrifice their private happiness or benefits for the flourishing of the whole. What I find more interesting is the second part, which is a constraint on or limitation of this consequentialist argument: namely that one can only be so compelled if one has benefited from the city's institutions first. So, in a badly-ordered city the consequentialist argument has no force. That is, the underlying intuition seems to be that one has no obligations to serve a political order if that political order has not benefited you first.
That is, Socrates is relying here on a kind of tacit contract between state and citizen: the state provides benefits to its citizens and in return it can require service from them. The details are very murky because even in a bad ordered city (see, for example, Crito 50-1)** a state may well be providing some benefits to its would-be-philosophers, and it is also unclear what the proportions are between the benefits offered by the state and how much it can force in return. So, one wishes that Glaucon had pressed Socrates on the required distinctions to make this argument work.
This is not to say there is no detail. In particular, Socrates claims that in the best city, philosophers have been bred "to be, as it were, king-bees and leaders in the hive.+ You have received a better and more complete education than the others, and you are more capable of sharing both ways of life." So, it looks as if the second part relies on the idea that (i) a well ordered city can compel X from a class of citizens (of would be philosopher kings) if X was part of its purpose in educating the would-be-philosopher-kings. And (ii) that this class of citizens is capable of doing X. This seems to rely on a version of ought implies can.
We can combine this with (iii) that X enhances the flourishing of the city as a whole. I take it that (i-iii) is a considerable constraint on what can and cannot be demanded by the best state from its citizens.
*There are a few passages where he seems to suggest that some geographic areas are more hospitable to philosophical development, but I do not think this undermines his suggestion that the (relatively rare) philosophical type(s) can be born anywhere. His demographic commitment seems to be that in any population there will be a (hierarchically valued) mix of human types a few of which will always be would-be-philosophers.
**I have in mind the passage in which he explains to Crito why he will obey the Athenian verdict against his life, and he imagines a dialogue between the laws of Athens and himself:
What then if the laws should say, “Socrates, is this the agreement you made with us, or did you agree to abide by the verdicts pronounced by the state?...In the first place, did we not bring you forth? Is it not through us that your father married your mother and begat you? Now tell us, have you any fault to find with those of us who are the laws of marriage?...Or with those that have to do with the nurture of the child after he is born and with his education which you, like others, received?...Well then, when you were born and nurtured and educated, could you say to begin with that you were not our offspring and our slave, you yourself and your ancestors? And if this is so, do you think right as between you and us rests on a basis of equality, so that whatever we undertake to do to you it is right for you to retaliate? There was no such equality of right between you and your father or your master, if you had one, so that whatever treatment you received you might return it, answering them if you were reviled, or striking back if you were struck, and the like; and do you think that it will be proper for you to act so toward your country and the laws, so that if we undertake to destroy you, thinking it is right, you will undertake in return to destroy us laws and your country, so far as you are able, and will say that in doing this you are doing right, you who really care for virtue?... whoever of you stays here, seeing how we administer justice and how we govern the state in other respects, has thereby entered into an agreement with us to do what we command.--Crito 50-51.
It is an open question if the principles he relies on here -- in the context of a badly ordered state -- carry over to the best city.
+Mandeville develops the idea in surprising ways.