Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
Any one who has common sense will remember that the bewilderments of the eyes are of two kinds, and arise from two causes, either from coming out of the light or from going into the light, which is true of the mind's eye, quite as much as of the bodily eye; and he who remembers this when he sees any one whose vision is perplexed and weak, will not be too ready to laugh; he will first ask whether that soul of man has come out of the brighter life, and is unable to see because unaccustomed to the dark, or having turned from darkness to the day is dazzled by excess of light. And he will count the one happy in his condition and state of being, and he will pity the other; or, if he have a mind to laugh at the soul which comes from below into the light, there will be more reason in this than in the laugh which greets him who returns from above out of the light into the den. —Plato, The Republic
Have you ever walked from a brightly lit room into a darkened one? How about vice versa? Plato is pretty sure that you're going to have trouble adjusting in either situation, which of course calls for some serious reflection.
Let's remix it, now: say you just went through a major surgery to radically improve your intelligence, like our buddy Charlie. That's a pretty huge development that you can only truly understand if you went through it, right? And the epigraph is all about transitioning from one state to another and reflecting on that moment of transformation.
Ironically, Charlie doesn't have his light bulb moment immediately after surgery. It's only after he truly contemplates his intelligence, toward the end, when he sees a "holy light" (17.284) that shows him truth. While the epigraph suggests that truth is different for everyone, it could be an understanding of what it means to be human.