At this point, Plato is perhaps alluding to his first trip to Syracuse when he still had hopes of helping his friend, Dion, to persuade the young king, Dionysius II, to become a friend to philosophy and to enlighten his fellow citizens. Thus, in actuality, Plato might have, as he had hoped, produced in Dionysius II an enlightened despot, a king-turned-philosopher. But Plato's plan failed (see the Life and Background section).
When Socrates is here speaking of Plato's idea of "Goodness," Plato's meaning is "Goodness itself"; it is the supreme Form, inherent, timeless, essential; hence, the reflexive, "Goodness itself." Goodness is embodied not only in the cardinal virtues, but also in all of the universe. Earlier for Plato (and for us), Goodness might be achieved through the exercise of the virtues, resulting in the good and happy life (embracing courage, justice, temperance, wisdom). We now are to see Goodness itself manifested in the moral universe and in the physical universe (the beauty of the heavenly bodies and the order of them). We are to see this supreme Goodness itself as a manifestation of a divine Reason at work in the universe. This apprehension of a divine Reason at work permits us to see how the universe works; it leads to our "seeing" knowledge (the Forms), and the universe is thus illuminated. As illumination, Goodness itself is analogous to the sun, which sheds light upon vision and upon things made visible and is the source of all mortal life.
Socrates never in this dialogue, nor in any dialogue, defines Goodness itself. But Socrates does say that the knowledge of it may come to one in a kind of revelation after a long course of philosophical study (Jowett translation 540 A). And we know that Plato says, in the letter he wrote to Dion's friends and family, that he never wrote down a definition for Goodness itself (Letter VII 341 c, Harward translation).
We may briefly set forth the Analogy of the Sun thus: For sight, the sun is the source of light, and so makes objects visible and allows the eye to see; for Knowledge, Goodness is the source of Truth, and so makes the Forms intelligible and allows the mind to know.
