This essay argues that the form Geoffrey Chaucer devises for the Canterbury Tales rests on a recursive and iterative corrective process based on grammatical emendation that was tied, by a long-standing analogy, to moral reform. The Tale of Melibee makes this process most explicit and suggests both the ambitions and the dangers of this artistic and moral project. On the one hand, it is in the Melibee that the logic of the corrective process can be seen most clearly, as Prudence makes correction a principle of her prose; the tale portrays a slow, incremental repetition that only gradually brings about change. In that way, the tale displays the ambitions of the project. On the other hand, its dangers are clear enough, because the tale is notoriously unsatisfactory. Chaucer, however, deliberately stages those dangers in the Melibee and contrasts the dangers with a solution. While in the Melibee that incremental repetition illustrates literary pitfalls, in the Tales it becomes a means for literary innovation: the certainty of error and the corruption of discourse provide Chaucer an artistic method, one that evades moral clarity but provides the occasion for ongoing intellectual, artistic, and moral exercise. This account of Chaucer's moral poetics suggests that debates over the moral bearing of his poetry are unavoidable by design, but also irresolvable by design.