Having established that only philosophical natures can be rulers of the kallipolis, Socrates and co. proceed to both list and rule out qualities that would either characterize or exemplify a person from becoming a philosopher. A philosopher is quick to learn, has a good memory, is courageous, high-minded and virtuous. A philosopher is also born one. A person cannot become a philosopher–they must be so naturally inclined.
Educating a philosophical nature to suit their idea of a philosopher-king is of crucial importance, for if such a person were to be raised in the incorrect environment, their nature would allow them to be simultaneously influential and destructive. These developing philosophers, then, must not be exposed to the corrupting influence of the public, the majority–a majority Socrates deems non-philosophic.
The majority holds false conceptions of what philosophers and wisdom and knowledge should be–indeed, they are unable to ever truly know because of the nature that defines them. Instead, they labor under the falsehoods told to them by false philosophers–sophists– who take to philosophy “like prisoners escaping from jail to take refuge in a temple” (495d.) for the prestige and admiration it would grant them.
A philosopher-king is possible, though rare. It would be rare to find a person who retains all the qualities of a philosopher (having been raised properly) yet still choose a proper and stable life. Socrates reminds his listeners that it is not the small qualities that are significant–it is only important that they have the most important virtue.
Pleasure is ruled out, as well as knowledge (as it could only be knowledge of the good,) and thus it is this good that is the most important. The form of this good is not explicitly revealed to us by Socrates; he claims he has knowledge of it, but cannot explain it (or we cannot understand it) and closes off the book with an analogy of the book to the sun (as discussed in Jill’s lecture.)