Defoe wrote Moll Flanders at a time when there was still little precedent for the novel as a genre, and he accordingly felt compelled to justify his book by presenting it as a true story. He stages his novel therefore as the memoir of a person who, though fictional, is a composite of real people who experienced real events in Defoe's London. (Of course, part of the comic effect stems from the fact that no one person could have experienced all that Moll does.) He draws on the established conventions of the rogue biography--a genre that presented the lives and escapades of real criminals in semi-fictionalized and entertaining ways. Moll Flanders concerns itself above all with the practical, day-to-day exigencies of a woman who enjoys no long-standing social stability or financial security, allowing the accumulation of factual detail to stand as evidence for the writing's truthfulness, if not its literal truth. His language, which is also Moll's throughout, is plain and un-literary. The prose is not allusive, ornamental, or metaphoric, relying rather on the combination of journalistic accuracy and a strong personal voice for their effects of authenticity.

Defoe emphasizes in his Preface to the novel that the tale is meant to convey a serious moral. But the novel itself, which details its heroine's scandalous sexual and criminal adventures, keeps moralizing (particularly traditional Christian moralizing) to a minimum. Her immoral actions have no real consequences, and the narrative tends to excuse her behavior by referring it to material necessity. If Moll Flanders is surprisingly unmoralizing, Defoe's indulgent attitude toward his heroine accords with the reaction of most readers. E.M. Forster called the book "a masterpiece of characterization," and it is a testimony to the psychological nuance of her character, as well as to its liveliness, that we like Moll more than we censure her. Defoe creates in Moll a character of limitless interest, in spite of her unconcealed ethical shortcomings. His vision is one that values the personal qualities of self-reliance and perseverance, and that dignifies human labor, even when it takes the form of crime.

Defoe's own attitude toward his character and her escapades is less than clear, as is his final verdict on the questions and conflicts her life story raises. What emerges unequivocally in the novel is Defoe's fascination with moral ambiguity, and with the isolated life of the individual human being. Moll Flanders illustrates unflinchingly the kinds of motives that rise to the surface in human life under hardship and duress, and the frankness with which Moll discusses her own motivations is an appeal to their universality. The book therefore generates a conflict between an absolute Christian morality on the one hand and the conditional ethics of measurement and pragmatism that govern the business world, as well as the human struggle for survival, on the other.