"The Custom-House” is a stand-alone section of the novel. It resembles more a tract or a personal essay than an introduction to a piece of fiction, but it offers plenty of insights that will support the rest of The Scarlet Letter. For one thing, we gain a sense of why the narrator feels the need to tell the story. As a man of youth and vigor, he feels somewhat at odds with the Puritan nature of his society. He himself seems to feel a deep resentment for the strict fidelity to rules and values that would deem his whole personality, and his ambition to write, as frivolous or even sinful.
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