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.
This is America—a town of a few thousand, in a region of wheat and corn and dairies and little groves. The town is, in our tale, called 'Gopher Prairie, Minnesota.' But its Main Street is the continuation of Main Streets everywhere. The story would be the same in Ohio or Montana, in Kansas or Kentucky or Illinois, and not very differently would it be told Up York State or in the Carolina hills.
Main Street is the climax of civilization. That this Ford Car might stand in front of the Bon Ton Store […]
Such is our comfortable tradition and sure faith. Would he not betray himself an alien cynic who should otherwise portray Main Street, or distress the citizens by speculating whether there may not be other faiths?
Before you get into the meat of Main Street, Sinclair Lewis wants you to read a passage where he describes the small town of Gopher Prairie and the type of people who live there. He also makes a point of saying that the story of Gopher Prairie could be the same for any small town in America. He tells us that the folks of this town have good solid morals and will never bother to think about anything not familiar to them.
Of course, Sinclair Lewis is exaggerating here. It's almost as if he put this epigraph into the book to make sure his readers understand that he's criticizing towns like Gopher Prairie for being closed-minded and hypocritical. His overblown language is also supposed to mock the self-assured confidence that many small-town folk have about their special little towns and the great people who live in them.
Lewis even goes so far as to say that all of Western history has happened so that small towns like Gopher Prairie can exist. The funny thing is that many of the characters in this book actually believe these things. Lewis, on the other hand, can only do a facepalm and shake his head at them.