Philosophers by nature
It is reasonably assumed that all philosophers in Plato's Republic are the same, and yet, arguably, they are not. The philosophers whom I call the "philosopher by nature," are those Socrates limns in Book 5 and for the first part of Book 6 (through 502c) [henceforth, "the philosophers of Book 6"]. These philosophers, by their nature, love and delight in truth and knowledge concerning "what is." Even in their youth, these true philosophers are not finicky about what they study; rather, they are willing to taste every kind of learning, and they approach learning insatiably with joy and with gusto. Genuine philosophers love as ardently as other lovers do; they differ only in the object of their delight: whereas non-philosophers revel in sights, sounds, arts, opinions, the many beautiful sounds, colors, and shapes, and all that the crafts fashion from such things-things subject to flux and change, to coming into existence and perishing, and to variation in accordance with subjective perspective-philosophers prefer truth and knowledge and the beautiful itself, things that are real and stable and the same always. Moreover, they are blessed with an array of moral, intellectual, and personal virtues. Justice, moderation, and courage-the moral virtues-are found in philosophers as a result of their being drawn to the higher realm of being and their consequent indifference to worldly goods. Their intellectual virtues (the ability to learn quickly and to retain what is learned) and their personal charm and grace are prerequisites for their sort of philosophic engagement and so are also necessary components of their nature. In addition, the decent philosophers of Book 6 would rule willingly for the sake of justice-that is, for the sake of improving people's souls with respect to justice-were it not for the harm they would likely incur, harm to their souls and perhaps even death, at the hands of those in power. Were it not for those forbidding conditions, they would not need to be coerced to rule.
Philosophers by design
In Book 6 at 502c, Socrates begins anew and introduces, without calling attention to his doing so, a second set of philosophers [henceforth, "the philosophers of Book 7"]. These are the "philosophers by design," who blend in their nature sharp wits and mulish constancy, intellectual ability with the qualities of a soldier. These philosophers-and not the philosophers by nature-are the counterparts of the prisoner who is released from the Cave in the famous Allegory of the Cave in Book 7. Thus, they neither recognize anything better or higher than the visible realm nor want to know of any such thing. They don't resent their fetters, their attachments to the things of this world; they don't yearn to be free. They don't leave the visible world of their own accord, fueled by their own longings; someone-the founders-releases them. Like the prisoners who are freed, they ascend only reluctantly ("distressed and annoyed"); they, too, have to be dragged. The dragging continues for both prisoners and philosophers as they travel "along the rough, steep, upward way." And once
see the Form of the Good, at the completion of an arduous and coercive educational regimen, they wish to remain in the intelligible realm, having nothing but disdain for human affairs. By nature, these philosophers are appetitive. Their souls are naturally yoked to the realm of Becoming by the vulgar pleasures it affords, by such things as eating and drinking. These pleasures-and not the delights of beholding the "true things"-are what entice them. There is no mention of intellectual eros in Book 7-not at the end once the philosophers have seen the Good, and certainly not while they are still on the path to it. It is in fact not until these philosophers actually see the Forms that they display a marked preference for the higher realm (and so have to be "compelled" to go back down to the Cave), and not until they see the very highest Form that they feel as if they "have emigrated to a colony on the Isles of the Blessed while they are still alive." Unlike the philosophers of Book 6, those of Book 7 are driven to philosophic heights, not by their own eros, but involuntarily, by the coercive measures taken by the founders of Callipolis. Coercion replaces the erotic impulse and is required in its absence. Although once they are in the light they are eager to remain there, it is nevertheless the case that until they get there they don't want to go. Moreover, even at the final stage of their ascent they have no desire to see the Good, but have to be forced to do so. Book 7's philosophers are thus oddly unlike any others: the usual worry is that those who love "what is" may nevertheless fail to attain it; with respect to Rep. 7's philosophers, however, the concern is that though they attain it-they actually see the Good-they never quite love it. They are sharp-sighted, to be sure, and they prefer the intelligible realm to the sensible and regard themselves as happy once they are in it, but it is not their love for the Good that propels them upward or keeps them there. Lacking genuine love of wisdom, the philosophers of Rep. 7 miss out, too, on the full complement of moral virtues that comes with that kind of love. And as the allegory of the Cave makes plain, these philosophers have an unabashed disinclination to rule.
Socrates himself is not described in the Republic in a direct way. Yet he represents a third kind of philosopher who stands in marked contrast to the other two. He most closely resembles the philosophers by nature of Book 6, but whereas those worthy philosophers, recognizing the threat to their lives and to the purity of their souls posed by the corruption that surrounds them, retreat from the world and "stand aside under a little wall," Socrates, confronting the very same circumstances, chooses, if not the life of politics per se, a certain form of political life, a life that he calls being "a busybody in private." In a class by himself, only Socrates manages to carve out for himself a way to serve justice and so to serve the god. Not content to protect his own soul's purity, Socrates descends into the trenches, seeking to improve souls one by one.