For the allegory, the Cave corresponds to the realm of belief; the World of Day corresponds to the realm of knowledge. The sun stands for the Form of Goodness itself. If the prisoner were to be returned to the Cave, his old fellows would not believe his experiences, since they have always been imprisoned in their world, the Cave.
Thus, allegorically, we must release the prisoners from their Cave: We must give the Guardians the experience of education so that they can become the philosopher-kings of the Ideal State, because they will be able to know the Forms and, finally, Goodness itself.
But it is not enough that the prisoner, freed, now possesses knowledge. He must be returned to the Cave to enlighten his erstwhile fellows about the knowledge he now perceives.
Glaucon objects: He argues that for the enlightened prisoner to return to the Cave would make him unhappy. It would be a lot of work to lead his fellows into the light of a kind of new dawn of knowledge. Socrates here reminds us, again, that the business of rulers is not to make themselves happy; their happiness is to be realized in the happiness of every citizen in the Ideal State.
